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RIFT VALLEY INSTITUTE OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF ELECTRICAL AND ELECTRONIC

ENGINEERING

(CEE)

INDUSTRIAL MACHINE CONTROL

(IMC)

Lecture Notes

By

Dr. Cliff Orori Mosiori

©2015

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Contents INTRODUCTION ................................................................................................................................... 1

ELECTRIC MACHINES ........................................................................................................................ 1

Electric Motor ..................................................................................................................................... 1

Principle and Working of Electric Motor .................................................................................................. 2

TOPIC ONE ........................................................................................................................................ 2

D.C MACHINES................................................................................................................................. 2

Principles of D.C machines .................................................................................................................. 2

Construction of D.C machines ............................................................................................................. 2

Frame .................................................................................................................................................. 3

Main poles ........................................................................................................................................... 2

Armature ............................................................................................................................................. 2

Field windings ..................................................................................................................................... 2

Commutator ........................................................................................................................................ 3

Brush and brush holders ...................................................................................................................... 4

Generator E.M.F Equation ................................................................................................................... 1

Methods of Excitation.......................................................................................................................... 1

Torque and power ................................................................................................................................ 2

Voltage and current ............................................................................................................................. 2

Generator Characteristics ..................................................................................................................... 2

Characteristics Series of DC generator ................................................................................................. 6

Characteristics Shunt DC generator...................................................................................................... 7

Characteristics compound generator..................................................................................................... 8

The Efficiency of the DC Motor Increases by: ................................................................................... 10

Motor Characteristics......................................................................................................................... 10

Torque Speed Curves ......................................................................................................................... 10

Direct on line starter ............................................................................................................................ 1

TOPIC TWO: AC MACHINES ............................................................................................................... 2

Induction Motor................................................................................................................................... 2

Principle of operation and comparison to synchronous motors ............................................................. 2

Construction ........................................................................................................................................ 2

Types of rotors .................................................................................................................................... 2

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Slip ring rotor ...................................................................................................................................... 2

Solid core rotor .................................................................................................................................... 3

Starting of induction motors................................................................................................................. 3

Direct-on-line starting .......................................................................................................................... 3

Wye-Delta starters ............................................................................................................................... 4

Variable-frequency drives .................................................................................................................... 4

Resistance starters ............................................................................................................................... 4

Series Reactor starters.......................................................................................................................... 5

Single Phase induction motor ............................................................................................................... 5

Rotating magnetic field ........................................................................................................................ 6

Description of magnetic field ............................................................................................................... 6

Permanent-split capacitor motor........................................................................................................... 1

Capacitor-start induction motor............................................................................................................ 2

Capacitor-run induction motor ............................................................................................................. 2

Resistance split-phase induction motor ................................................................................................ 3

THREE PHASE INDUCTION MOTOR ................................................................................................. 1

Working Principle of Three Phase Induction Motor ................................................................................. 1

Production of Rotating Magnetic Field ............................................................................................. 1

What is the operating principle of a 3ph induction motor? .................................................................... 2

Production of a rotating magnetic field ................................................................................................ 3

Production of magnetic flux ................................................................................................................. 3

SPEED CONTROL OF THREE PHASE INDUCTION MOTOR ............................................................ 5

The Speed of Induction Motor is changed from Both Stator and Rotor Side .............................. 2

Speed Control from Stator Side .................................................................................................... 2

Speed Control from Rotor Side ..................................................................................................... 4

Electric Motor Controls ....................................................................................................................... 2

Motor Starting ..................................................................................................................................... 2

Motor Protection.................................................................................................................................. 5

Other Motor Protection Devices ........................................................................................................... 6

TOPIC THREE: CONTACTORS ............................................................................................................ 2

CONTACTORS .................................................................................................................................. 2

Applications of Contactors................................................................................................................... 2

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1. Lighting control ....................................................................................................................... 2

2. Magnetic starter ....................................................................................................................... 3

3. Vacuum contactor .................................................................................................................... 3

How Contactor Controls an Electric Motor .......................................................................................... 3

Topic four ............................................................................................................................................... 1

Preventive Maintenance........................................................................................................................... 1

Controlling Maintenance Hazards ........................................................................................................ 3

Instrumentation systems ......................................................................... Error! Bookmark not defined.

Assemble a simple instrumentation ......................................................... Error! Bookmark not defined.

Signal processing methods ...................................................................... Error! Bookmark not defined.

Data processing elements ........................................................................ Error! Bookmark not defined.

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INTRODUCTION

ELECTRIC MACHINES Electric Motor An electric motor is an electric machine that converts electrical energy into mechanical energy.

In normal motoring mode, most electric motors operate through the interaction between an

electric motor's magnetic field and winding currents to generate force within the motor.

In certain applications, such as in the

transportation industry with traction motors,

electric motors can operate in both motoring

and generating or braking modes to also

produce electrical energy from mechanical

energy.

Found in applications as diverse as industrial

fans, blowers and pumps, machine tools,

household appliances, power tools, and disk

drives, electric motors can be powered by

direct current (DC) sources, such as from

batteries, motor vehicles or rectifiers, or by

alternating current (AC) sources, such as

from the power grid, inverters or generators.

Small motors may be found in electric

watches. The largest of electric motors are

used for ship propulsion, pipeline

compression and pumped-storage

applications with ratings reaching 100

megawatts. Electric motors may be

classified by electric power source type,

internal construction, application, type of

motion output, and so on

Application of electric motors

revolutionized industry. Industrial processes

were no longer limited by power

transmission using line shafts, belts,

compressed air or hydraulic pressure.

Instead every machine could be equipped

with its own electric motor, providing easy

control at the point of use, and improving

power transmission efficiency. Electric

motors applied in agriculture eliminated

human and animal muscle power from such

tasks as handling grain or pumping water.

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Household uses of electric motors reduced

heavy labor in the home and made higher

standards of convenience, comfort and

safety possible.

Working Principle of Electric Motor

An electric motor is a device which converts

electrical energy into mechanical energy. A

common motor works on direct current. So,

it is also called DC motor. When a

rectangular coil carrying current is placed in

a magnetic field, a torque acts on the coil

which rotates it continuously. When the coil

rotates, the shaft attached to it also rotates

and thus it is able to do mechanical work.

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TOPIC ONE

D.C MACHINES

Principles of D.C machines

D.C machines are the electro mechanical energy converters which work from a D.C source and

generate mechanical power or convert mechanical power into a D.C power. In any electric

motor, operation is based on simple electromagnetism. A current-carrying conductor generates a

magnetic field; when this is then placed in an external magnetic field, it will experience a force

proportional to the current in the conductor, and to the strength of the external magnetic field.

Magnetic field in motor

As you are well aware of from playing with magnets as a kid, opposite (North and South)

polarities attract, while like polarities (North and North, South and South) repel. The internal

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configuration of a DC motor is designed to harness the magnetic interaction between a current-

carrying conductor and an external magnetic field to generate rotational motion.

Direction of rotation

Let's start by looking at a simple 2-pole DC

electric motor (here red represents a magnet

or winding with a "North" polarization,

while green represents a magnet or winding

with a "South" polarization). Every DC

motor has six basic parts -- axle, rotor

(armature), stator, commutator, field

magnet(s), and brushes. In most common

DC motors, the external magnetic field is

produced by high-strength permanent

magnets. The stator is the stationary part of

the motor -- this includes the motor casing,

as well as two or more permanent magnet

pole pieces. The rotor (together with the

axle, and attached commutator) rotate with

respect to the stator. The rotor consists of

windings (generally on a core), the windings

being electrically connected to the

commutator.

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Torque in motor

The geometry of the brushes, commutator

contacts, and rotor windings are such that

when power is applied, the polarities of the

energized winding and the stator magnet(s)

are misaligned, and the rotor will rotate until

it is almost aligned with the stator's field

magnets. As the rotor reaches alignment, the

brushes move to the next commutator

contacts, and energize the next winding.

Given our example two-pole motor, the

rotation reverses the direction of current

through the rotor winding, leading to a "flip"

of the rotor's magnetic field, driving it to

continue rotating.DC motors will always

have more than two poles. This avoids "dead

spots" in the commutator. With a two-pole

motor, there is a moment where the

commutator shorts out the power supply.

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This would be bad for the power supply,

waste energy, and damage motor

components as well. Yet another

disadvantage of such a simple motor is that

it would exhibit a high amount of torque

"ripple".

Current in Motor

Force in Motor

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You'll notice that one pole is fully energized

at a time (but two others are "partially"

energized). As each brush transitions from

one commutator contact to the next, one

coil's field will rapidly collapse, as the next

coil's field will rapidly charge up. You can

see that this is a direct result of the coil

windings' series wiring. There's probably no

better way to see how an average DC motor

is put together, than by just opening one up.

This is a basic 3-pole DC motor, with 2

brushes and three commutator contacts. The

use of an iron core armature is quite

common, and has a number of advantages.

First off, the iron core provides a strong,

rigid support for the windings -- a

particularly important consideration for

high-torque motors. The core also conducts

heat away from the rotor windings, allowing

the motor to be driven harder than might

otherwise be the case. Iron core construction

is also relatively inexpensive compared with

other construction types.

But iron core construction also has several

disadvantages. The iron armature has a

relatively high inertia which limits motor

acceleration. This construction also results

in high winding inductances which limit

brush and commutator life.

In small motors, an alternative design is

often used which features a 'coreless'

armature winding. This design depends upon

the coil wire itself for structural integrity. As

a result, the armature is hollow, and the

permanent magnet can be mounted inside

the rotor coil. Coreless DC motors have

much lower armature inductance than iron-

core motors of comparable size, extending

brush and commutator life. DC motors have

been used in industrial applications for

years. Coupled with a DC drive, DC motors

provide very precise control DC motors can

be used with conveyors, elevators, extruders,

marine applications, material handling,

paper, plastics, rubber, steel, and textile

applications to name a few.

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Figure 1 - DC motor in schematic form

Standard DC motors are readily available in

one of two main forms:

Wound-field - where the magnetic flux in

the motor is controlled by the current

flowing in a field or excitation winding,

usually located on the stator.

Permanent magnet - where the magnetic

flux in the motor is created by permanent

magnets which have a curved face to create

a constant air-gap to the conventional

armature, located on the rotor. These are

commonly used at powers up to

approximately 3 kW.

Torque in a DC motor is produced by the

product of the magnetic field created by the

field winding or magnets and the current

flowing in the armature winding. The action

of a mechanical commutator switches the

armature current from one winding to

another to maintain the relative position of

the current to the field, thereby producing

torque independent of rotor position.

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The circuit of a shunt-wound DC motor

(Fig. 2 below) shows the armature M, the

armature resistance Ra and the field

winding. The armature supply voltage Va is

supplied typically from a controlled thyristor

system and the field voltage Vf from a

separate bridge rectifier.

Figure 2 -Shunt wound DC motor

As the armature rotates, an electromotive

force (emf ) Ea is induced in the armature

circuit and is called the back-emf since it

opposes the applied voltage Va (according to

Lenz’s Law). The Ea is related to armature

speed and main field flux, φ by:

Ea = k1nφ (1)

where n is the speed of rotation, φ is the

field flux and k1 is a motor constant. From

Figure 1 it is seen that the terminal armature

voltage Va is given by:

Va = Ea + IaRa (2)

Multiplying each side of eqn 2 by Ia gives:

VaIa = EaIa + Ia2Ra (3)

(or total power supplied = power output +

armature losses). The interaction of the field

flux and armature flux produces an armature

torque as given in below equation 4.

Torque M = k2IfIa (4)

where k2 is a motor constant and If is the

field current. This confirms the straight-

forward and linear characteristic of the DC

motor and consideration of these simple

equations will show its controllability and

inherent stability. The characteristic of a

motor is represented by curves of speed

against input current or torque and its shape

can be derived from eqns 1 and 2:

k1nφ = Va – (IaRa) (5)

If the flux is held constant by holding the

field current constant in a properly

compensated motor then:

n = k2[Va – (IaRa)] (6)

From eqns 4 and 6, it follows that full

control of the DC motor can be achieved

through control of the field current and the

armature current. In the DC shunt wound

motor shown in Figure 2 these currents can

be controlled independently. Most industrial

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DC motor controllers or drives are voltage

fed and the current is controlled by

measuring the current and adjusting the

voltage to give the desired current. This

basic arrangement is shown in Figure 3.

The series DC motor shown in Figure 4 has

the field and armature windings connected

in series. In this case the field current and

armature current are equal and show

characteristically different performance

results, though still defined by eqns. 4 and 6.

In the shunt motor the field flux φ is only

slightly affected by armature current, and

the value of IaRa at full load rarely exceeds

5 per cent of Va, giving a torque - speed

curve shown as a in Figure 6, where speed

remains constant over a wide range of load

torque.

The compound-wound DC motor shown in

Figure 5 combines both shunt and series

characteristics. The shape of the torque–

speed characteristic is determined by the

resistance values of the shunt and series

fields. The slightly drooping characteristic

(curve b in Figure 6) has the advantage in

many applications of reducing the

mechanical effects of shock loading.

Figure 3 - Control structure for a shunt wound DC motor

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The series DC motor curve (in Figure 6) shows that the initial flux increases in proportion to

current, falling away due to magnetic saturation. In addition the armature circuit includes the

resistance of the field winding and the speed becomes roughly inversely proportional to the

current. If the load falls to a low value the speed increases dramatically, which may be

hazardous, so, the series motor should not normally be used where there is a possibility of load

loss.

Figure 4 - Schematic of series DC motor

But because it produces high values of torque at low speed and its characteristic is falling speed

with load increase, it is useful in applications such as traction and hoisting, and some mixing

duties where initial stiction is dominant. Under semiconductor converter control with speed

feedback from a tachogenerator, the shape of the speed–load curve is determined within the

controller. It has become standard to use a shunt DC motor with converter control even though

the speed-load curve, when under open-loop control is often slightly drooping. The power-speed

limit for the DC motor is approximately 3 × 106 kW rev/min, due to restrictions imposed by the

commutator.

Construction of D.C machines A D.C machine consists mainly of two part the stationary part called stator and the rotating part

called rotor. The stator consists of main poles used to produce magnetic flux ,commutating poles

or interpoles in between the main poles to avoid sparking at the commutator but in the case of

small machines sometimes the interpoles are avoided and finally the frame or yoke which forms

the supporting structure of the machine. The rotor consist of an armature a cylindrical metallic

body or core with slots in it to place armature windings or bars, a commutator and brush gears

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The magnetic flux path in a motor or generator is show below and it is called the magnetic

structure of generator or motor. The major parts can be identified as,

1. Frame

2. Yoke

3. Poles Institute of Technology Madras

4. Armature

5. Commutator and brush gear

6. Commutating poles

7. Compensating winding

8. Other mechanical parts

Frame Frame is the stationary part of a machine on

which the main poles and commutator poles

are bolted and it forms the supporting

structure by connecting the frame to the bed

plate. The ring shaped body portion of the

frame which makes the magnetic path for the

magnetic fluxes from the main poles and

interpoles is called Yoke.

Why we use cast steel instead of cast iron

for the construction of Yoke?

In early days Yoke was made up of cast iron

but now it is replaced by cast steel. This is

because cast iron is saturated by a flux density

of 0.8 Wb/sq.m whereas saturation with cast

iron steel is about 1.5 Wb/sq.m. So for the

same magnetic flux density the cross section

area needed for cast steel is less than cast iron

hence the weight of the machine too. If we use

cast iron there may be chances of blow holes

in it while casting.so now rolled steels are

developed and these have consistent magnetic

and mechanical properties.

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End Shields or Bearings

If the armature diameter does not exceed 35

to 45 cm then in addition to poles end shields

or frame head with bearing are attached to

the frame. If the armature diameter is greater

than 1m pedestals type bearings are mounted

on the machine bed plate outside the frame.

These bearings could be ball or roller type

but generally plain pedestals bearings are

employed. If the diameter of the armature is

large a brush holder yoke is generally fixed

to the frame.

Main poles Solid poles of fabricated steel with

separate/integral pole shoes are fastened to

the frame by means of bolts. Pole shoes are

generally laminated. Sometimes pole body

and pole shoe are formed from the same

laminations. The pole shoes are shaped so as

to have a slightly increased air gap at the tips.

Inter-poles are small additional poles located

in between the main poles. These can be

solid, or laminated just as the main poles.

These are also fastened to the yoke by bolts.

Sometimes the yoke may be slotted to receive

these poles. The inter poles could be of

tapered section or of uniform cross section.

These are also called as commutating poles

or com poles. The width of the tip of the com

pole can be about a rotor slot pitch.

Armature The armature is where the moving

conductors are located. The armature is

constructed by stacking laminated sheets of

silicon steel. Thickness of these lamination is

kept low to reduce eddy current losses. As

the laminations carry alternating flux the

choice of suitable material, insulation coating

on the laminations, stacking it etc are to be

done more carefully. The core is divided into

packets to facilitate ventilation. The winding

cannot be placed on the surface of the rotor

due to the mechanical forces coming on the

same. Open parallel sided equally spaced

slots are normally punched in the rotor

laminations. These slots house the armature

winding. Large sized machines employ a

spider on which the laminations are stacked

in segments. End plates are suitably shaped

so as to serve as ’Winding supporters’.

Armature construction process must ensure

provision of sufficient axial and radial ducts

to facilitate easy removal of heat from the

armature winding.

Field windings In the case of wound field machines (as

against permanent magnet excited machines)

the field winding takes the form of a

concentric coil wound around the main poles.

These carry the excitation current and

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produce the main field in the machine. Thus

the poles are created electromagnetically.

Two types of windings are generally

employed. In shunt winding large number of

turns of small section copper conductor isof

Technology Madras used. The resistance of

such winding would be an order of

magnitude larger than the armature winding

resistance. In the case of series winding a few

turns of heavy cross section conductor is

used. The resistance of such windings is low

and is comparable to armature resistance.

Some machines may have both the windings

on the poles. The total ampere turns required

to establish the necessary flux under the poles

is calculated from the magnetic circuit

calculations. The total mmf required is

divided equally between north and south

poles as the poles are produced in pairs. The

mmf required to be shared between shunt and

series windings are apportioned as per the

design requirements. As these work on the

same magnetic system they are in the form of

concentric coils. Mmf ’per pole’ is normally

used in these calculations. Armature winding

As mentioned earlier, if the armature coils

are wound on the surface ofthe armature,

such construction becomes mechanically

weak. The conductors may fly away when

the armature starts rotating. Hence the

armature windings are in general pre-formed,

taped and lowered into the open slots on the

armature. In the case of small machines, they

can be hand wound. The coils are prevented

from flying out due to the centrifugal forces

by means of bands of steel wire on the

surface of the rotor in small groves cut into it.

In the case of large machines slot wedges are

additionally used to restrain the coils from

flying away. The end portion of the windings

are taped at the free end and bound to the

winding carrier ring of the armature at the

commutator end. The armature must be

dynamically balanced to reduce the

centrifugal forces at the operating speeds.

Compensating winding One may find a bar

winding housed in the slots on the pole shoes.

This is mostly found in D.C machines of very

large rating. Such winding is called

compensating winding. In smaller machines,

they may be absent.

Commutator Commutator is the key element which made

the D.C machine of the present day possible.

It consists of copper segments tightly

fastened together with mica/micanite

insulating separators on an insulated base.

The whole commutator forms a rigid and

solid assembly of insulated copper strips and

can rotate at high speeds. Each com mutator

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segment is provided with a ’riser’ where the

ends of the armature coils get connected. The

surface of the commutator is machined and

surface is made concentric with the shaft and

the current collecting brushes rest on the

same. Under-cutting the mica insulators that

are between these commutator segments has

to be done periodi- cally to avoid fouling of

the surface of the commutator by mica when

the commutator gets worn out.

Brush and brush holders Brushes rest on the surface of the

commutator. Normally electro-graphite is

used as brush material. The actual

composition of the brush depends on the

peripheral speed of the commutator and the

working voltage. The hardness of the

graphite brush is selected to be lower than

that of the commutator. When the brush

wears out the graphite works as a solid

lubricant reducing frictional coefficient.

More number of relatively smaller width

brushes is preferred in place of large broad

brushes. The brush holders provide slots for

the brushes to be placed. The connection

Brush holder with a Brush and Positioning of

the brush on the commutator from the brush

is taken out by means of flexible pigtail. The

brushes are kept pressed on the commutator

with the help of springs. This is to ensure

proper contact between the brushes and the

commutator even under high speeds of

operation. Jumping of brushes must be

avoided to ensure arc free current collection

and to keep the brush contact drop low. Other

mechanical parts End covers, fan and shaft

bearings form other important me- chanical

parts. End covers are completely solid or

have opening for ventilation. They support

the bearings which are on the shaft. Proper

machining is to be ensured for easy

assembly. Fans can be external or internal. In

most machines the fan is on the non-

commutator end sucking the air from the

commutator end and throwing the same out.

Adequate quantity of hot air removal has to

be ensured.

Bearings Small machines employ ball

bearings at both ends. For larger machines

roller bearings are used especially at the

driving end. The bearings are mounted press-

fit on the shaft. They are housed inside the

end shield in such a manner that it is not

necessary to remove the bearings from the

shaft for dismantling.

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Generator E.M.F Equation Let Φ = flux/pole in weber Z = total number

of armture conductors = No.of slots x No.of

conductors/slot P = No.of generator poles A

= No.of parallel paths in armature N =

armature rotation in revolutions per minute

(r.p.m) E = e.m.f induced in any parallel path

in armature Generated e.m.f Eg = e.m.f

generated in any one of the parallel paths i.e

E. Average e.m.f geneated /conductor =

dΦ/dt volt (n=1) Now, flux cut/conductor in

one revolution dΦ = ΦP Wb No. of

revolutions/second = N/60 Time for one

revolution, dt = 60/N second Hence,

according to Faraday's Laws of

Electromagnetic Induction, E.M.F

generated/conductor is

For a simplex wave-wound generator No.

of parallel paths = 2 No. of conductors (in

series) in one path = Z/2 E.M.F.

generated/path is

For a simplex lap-wound generator

No.of parallel paths = P No.of conductors (in

series) in one path = Z/P

E.M.F.generated/path

In general generated e.m.f

where A = 2 - for simplex wave-winding A

= P - for simplex lap-winding

Methods of Excitation Various methods of excitation of the field windings are shown in Fig.

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Figure shows Field-circuit connections of dc machines: (a) separate excitation, (b) series, (c) shunt, (d) compound.

Consider first dc generators.

Separately-excited generators and Self-

excited generators: series generators, shunt

generators, compound generators.

With self-excited generators, residual

magnetism must be present in the machine

iron to get the self-excitation process started.

o N.B.: long- and short-shunt, cumulatively

and differentially compound.

Typical steady-state volt-ampere

characteristics are shown in Fig.7.5,

constant-speed operation being assumed.

The relation between the steady-state

generated emf Ea and the armature terminal

voltage Va is Va =Ea −IaRa (7.10)

Figure Volt-ampere characteristics of dc

generators. Any of the methods of excitation

used for generators can also be used for

motors.

Typical steady-state dc-motor speed-

torque characteristics are shown in Fig.7.6,

in which it is assumed that the motor

terminals are supplied from a constant-

voltage source.

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In a motor the relation between the emf

Ea generated in the armature and and the

armature terminal voltage Va is

Va=Ea+IaRa (7.11)

The application advantages of dc

machines lie in the variety of performance

characteristics offered by the possibilities of

shunt, series, and compound excitation.

Figure Speed-torque characteristics of dc

motors.

Torque and power The electromagnetic torque Tmech,

Tmech =KaΦdIa

The generated voltage, Ea

Ea =KaΦdωm

Voltage and current Va: the terminal voltage of the armature

winding Vt: the terminal voltage of the dc

machine, including the voltage drop across

the series connected field winding,

Va = Vt

if there is no series field winding Ra: the

resistance of armature, Rs: the resistance of

the series field,

Va =Ea ± IaRa

Vt=Ea ± Ia( Ra+Rs) IL

=Ia±If

Generator Characteristics The three most important characteristics or curves of a d.c generator are;

1. Open Circuit Characteristic (O.C.C.)

This curve shows the relation between the

generated e.m.f. at no-load (E0) and the field

current (If) at constant speed. It is also

known as magnetic characteristic or no-load

saturation curve. Its shape is practically the

same for all generators whether separately or

self-excited. The data for O.C.C. curve are

obtained experimentally by operating the

generator at no load and constant speed and

recording the change in terminal voltage as

the field current is varied.

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2. Internal or Total characteristic (E/Ia)

This curve shows the relation between the

generated e.m.f. on load (E) and the

armature current (Ia). The e.m.f. E is less

than E0 due to the demagnetizing effect of

armature reaction. Therefore, this curve will

lie below the open circuit characteristic

(O.C.C.). The internal characteristic is of

interest chiefly to the designer. It cannot be

obtained directly by experiment. It is

because a voltmeter cannot read the e.m.f.

generated on load due to the voltage drop in

armature resistance. The internal

characteristic can be obtained from external

characteristic if winding resistances are

known because armature reaction effect is

included in both characteristics.

3. External characteristic (V/IL)

This curve shows the relation between the

terminal voltage (V) and load current (IL).

The terminal voltage V will be less than E

due to voltage drop in the armature circuit.

Therefore, this curve will lie below the

internal characteristic. This characteristic is

very important in determining the suitability

of a generator for a given purpose. It can be

obtained by making simultaneous;

1. No-load saturation characteristic

(E0/If)

It is also know as Magnetic

characteristic or Open circuit

Characteristic (O.C.C). It shows the

relation between the no-load

generated e.m.f in armature, E0 and

the field or exciting current If at a

given fixed speed. It is just the

magnetisation curve for the material

of the electromagnets. Its shape is

practically the same for all

generators whether separately-

excited or self-excited.

A typical no load saturation curve is shown

in Figure. It has generator output voltage

plotted against field current. The lower

straight line portion of the curve represents

the air gap because the magnetic parts are

not saturated. When the magnetic parts start

to saturate, the curve bends over until

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3

complete saturation is reached. Then the

curve becomes a straight line again.

2. Separately-excited Generator

The No-load saturation curve of a separately

excited generator will be as shown in the

above figure. It is obvous that when If is

increased from its initial small value, the

flux and hence generated e.m.f Eg increase

directly as current so long as the poles are

unsaturated. This is represented by straight

portion in figure. But as the flux density

increases, the poles become saturated, so a

greater increase If is required to produce a

given increase in voltage than on the lower

part of the curve. That is why the upper

portion of the curve bends.

The O.C.C curve for self-excited generators

whether shunt or series wound is shown in

above figure. Due to the residual magnetism

in the poles, some e.m.f (= OA) is generated

even when If =0.Hence, the curve starts a

little way up. The slight curvature at the

lower end is due to magnetic inertia. It is

seen that the first part of the curve is

practically straight. This is due to fact that at

low flux densities reluctance of iron path

being negligible, total reluctance is given by

the air gap reluctance which is constant.

Hence, the flux and consequently, the

generated e.m.f is directly proportional to

the exciting current. However, at high flux

densities, where μ is small, iron path

reluctance becomes appreciable and straight

relation between E and If no longer holds

good. In other words, after point B,

saturation of pole starts. However, the initial

slope of the curve is determined by air-gap

width. O.C.C for higher speed would lie

above this curve and for lower speed, would

lie below it.

Separately-excited Generator Let us

consider a separately-excited generator

giving its rated no-load voltage of E0 for a

certain constant field current. If there were

no armature reaction and armature voltage

drop, then this voltage would have remained

constant as shown in figure by the horizontal

line 1. But when the generator is loaded, the

voltage falls due to these two causes,

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4

thereby giving slightly dropping

characteristics. If we subtract from E0 the

values of voltage drops due to armature

reaction for different loads, then we get the

value of E-the e.m.f actually induced in the

armature under load conditions. Curve 2 is

plotted in this way and is known as the

internal characteristic.

Series Generator

In this generator, because field windings are

in series with the armature, they carry full

armature current Ia. As Ia is increased, flux

and hence generated e.m.f. is also increased

as shown by the curve. Curve Oa is the

O.C.C. The extra exciting current necessary

to neutralize the weakening effect of

armature reaction at full load is given by the

horizontal distance ab. Hence, point b is on

the internal characteristic. 3. External

characteristic (V/I) It is also referred to as

performance characteristic or sometimes

voltage-regulating curve. It gives relation

between the terminal voltage V and the load

current I. This curve lies below the internal

characteristic because it takes in to account

the voltage drop over the armature circuit

resistance.

The values of V are obtained by subtracting

IaRa from corresponding values of E. This

characteristic is of great importance in

judging the suitability of a generator for a

particular purpose. It may be obtained in two

ways (i) by making simultaneous

measurements with a suitable voltmeter and

an ammeter on a loaded generator or (ii)

graphically from the O.C.C provided the

armature and field resistances are known

and also if the demagnetizing effect or the

armature reaction is known.

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5

Figure above shows the external

characteristic curves for generators with

various types of excitation. If a generator,

which is separately excited, is driven at

constant speed and has a fixed field current,

the output voltage will decrease with

increased load current as shown. This

decrease is due to the armature resistance

and armature reaction effects. If the field

flux remained constant, the generated

voltage would tend to remain constant and

the output voltage would be equal to the

generated voltage minus the IR drop of the

armature circuit.

However, the demagnetizing component of

armature reactions tends to decrease the

flux, thus adding an additional factor, which

decreases the output voltage. In a shunt

excited generator, it can be seen that the

output voltage decreases faster than with

separate excitation. This is due to the fact

that, since the output voltage is reduced

because of the armature reaction effect and

armature IR drop, the field voltage is also

reduced which further reduces the flux. It

can also be seen that beyond a certain

critical value, the shunt generator shows a

reversal in trend of current values with

decreasing voltages. This point of maximum

current output is known as the breakdown

point. At the short circuit condition, the only

flux available to produce current is the

residual magnetism of the armature.

To build up the voltage on a series

generator, the external circuit must be

connected and its resistance reduced to a

comparatively low value. Since the armature

is in series with the field, load current must

be flowing to obtain flux in the field. As the

voltage and current rise the load resistance

may be increased to its normal value. As the

external characteristic curve shows, the

voltage output starts at zero, reaches a peak,

and then falls back to zero.

The combination of a shunt field and a series

field gives the best external characteristic as

illustrated in Figure. The voltage drop,

which occurs in the shunt machine, is

compensated for by the voltage rise, which

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6

occurs in the series machine. The addition of

a sufficient number of series turns offsets

the armature IR drop and armature reaction

effect, resulting in a flat-compound

generator, which has a nearly constant

voltage. If more series turns are added, the

voltage may rise with load and the machine

is known as an over-compound generator.

The speed of a D.C machine operated as a

generator is fixed by the prime mover.

For general-purpose operation, the prime

mover is equipped with a speed governor so

that the speed of the generator is practically

constant. Under such condition, the

generator performance deals primarily with

the relation between excitation, terminal

voltage and load. These relations can be best

exhibited graphically by means of curves

known as generator characteristics. These

characteristics show at a glance the

behaviour of the generator under different

load conditions.

Characteristics Series of DC generator Fig. shows the connections of a series

wound generator. Since there is only one

current (that which flows through the whole

machine), the load current is the same as the

exciting current.

(i) O.C.C. Curve 1 shows the open circuit

characteristic (O.C.C.) of a series generator.

It can be obtained experimentally by

disconnecting the field winding from the

machine and exciting it from a separate D.C

source as discussed in Sec. (3.2).

(ii) Internal characteristic Curve 2 shows

the total or internal characteristic of a series

generator. It gives the relation between the

generated e.m.f. E. on load and armature

current. Due to armature reaction, the flux in

the machine will be less than the flux at no

load. Hence, e.m.f. E generated under load

conditions will be less than the e.m.f. EO

generated under no load conditions.

Consequently, internal characteristic curve

generated under no load conditions.

Consequently, internal characteristic curve

lies below the O.C.C. curve; the difference

between them representing the effect of

armature reaction [See Fig. 3.7 (ii)].

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7

(iii)External characteristic Curve 3 shows

the external characteristic of a series

generator. It gives the relation between

terminal voltage and load current IL.

V= E-Ia(Ra+Rse)

Therefore, external characteristic curve will

lie below internal characteristic curve by an

amount equal to ohmic drop [i.e.,

Ia(Ra+Rse)] in the machine as shown in Fig.

(3.7) (ii). The internal and external

characteristics of a D.C series generator can

be plotted from one another as shown in Fig.

(3.8). Suppose we are given the internal

characteristic of the generator. Let the line

OC represent the resistance of the whole

machine i.e. Ra+Rse.If the load current is

OB, drop in the machine is AB i.e.

AB = Ohmic drop in the machine =

OB(Ra+Rse)

Now raise a perpendicular from point B and

mark a point b on this line such that ab =

AB. Then point b will lie on the external

characteristic of the generator. Following

similar procedure, other points of external

characteristic can be located. It is easy to see

that we can also plot internal characteristic

from the external characteristic.

Characteristics Shunt DC generator Fig (3.9) (i) shows the connections of a

shunt wound generator. The armature

current Ia splits up into two parts; a small

fraction Ish flowing through shunt field

winding while the major part IL goes to the

external load.

I. O.C.C. The O.C.C. of a shunt

generator is similar in shape to that of a

series generator as shown in Fig. (3.9)

(ii). The line OA represents the shunt

field circuit resistance. When the

generator is run at normal speed, it will

build up a voltage OM. At no-load, the

terminal voltage of the generator will be

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8

constant (= OM) represented by the

horizontal dotted line MC.

II) Internal characteristic When the

generator is loaded, flux per pole is reduced

due to armature reaction. Therefore, e.m.f. E

generated on load is less than the e.m.f.

generated at no load.As a result, the internal

characteristic (E/Ia) drops down slightly as

shown in Fig.(3.9) (ii).

(iii)External characteristic Curve 2 shows

the external characteristic of a shunt

generator. It gives the relation between

terminal voltage V and load current IL. V =

E – IaRa = E -(IL +Ish)Ra Therefore,

external characteristic curve will lie below

the internal characteristic curve by an

amount equal to drop in the armature circuit

[i.e., (IL +Ish)Ra ] as shown in Fig. (3.9)

(ii). Note. It may be seen from the external

characteristic that change in terminal voltage

from no-load to full load is small. The

terminal voltage can always be maintained

constant by adjusting the field rheostat R

automatically

External Resistance for Shunt Generator

f the load resistance across the terminals of a

shunt generator is decreased, then load

current increase? However, there is a limit to

the increase in load current with the

decrease of load resistance. Any decrease of

load resistance beyond this point, instead of

increasing the current, ultimately results in

reduced current. Consequently, the external

characteristic turns back (dotted curve) as

shown in Fig. (3.10). The tangent OA to the

curve represents the minimum external

resistance required to excite the shunt

generator on load and is called critical

external resistance. If the resistance of the

external circuit is less than the critical

external resistance (represented by tangent

OA in Fig. 3.10), the machine will refuse to

excite or will de-excite if already running

This means that external resistance is so low

as virtually to short circuit the machine and

so doing away with its excitation.

Note. There are two critical resistances for a

shunt generator viz., (i) critical field

resistance (ii) critical external resistance. For

the shunt generator to build up voltage, the

former should not be exceeded and the latter

must not be gone below

Characteristics compound generator In a compound generator, both series and

shunt excitation are combined as shown in

Fig. (3.13). The shunt winding can be

connected either across the armature only

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9

(short-shunt connection S) or across

armature plus series field (long-shunt

connection G). The compound generator can

be cumulatively compounded or

differentially compounded generator. The

latter is rarely used in practice. Therefore,

we shall discuss the characteristics of

cumulatively compounded generator. It may

be noted that external characteristics of long

and short shunt compound generators are

almost identical.

External characteristic

Fig. (3.14) shows the external characteristics

of a cumulatively compounded generator.

The series excitation aids the shunt

excitation. The degree of compounding

depends upon the increase in series

excitation with the increase in load current.

(i).If series winding turns are so adjusted

that with the increase in load current the

terminal voltage increases, it is called over-

compounded generator. In such a case, as

the load current increases, the series field

m.m.f. increases and tends to increase the

flux and hence the generated voltage. The

increase in generated voltage is greater than

the IaRa drop so that instead of decreasing,

the terminal voltage increases as shown by

curve A in Fig. (3.14).

(ii) If series winding turns are so

adjusted that with the increase in load

current, the terminal voltage

substantially remains constant, it is

called flat-compounded generator. The

series winding of such a machine has

lesser number of turns than the one in

over-compounded machine and,

therefore, does not increase the flux as

much for a given load current.

Consequently, the full-load voltage is

nearly equal to the no-load voltage as

indicated by curve B in Fig (3.14).

(iii) If series field winding has lesser

number of turns than for a flat

compounded machine, the terminal

voltage falls with increase in load

current as indicated by curve C m Fig.

(3.14). Such a machine is called under-

compounded generator.

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10

Voltage Regulation

The change in terminal voltage of a

generator between full and no load (at

constant speed) is called the voltage

regulation, usually expressed as a percentage

of the voltage at full-load. % Voltage

regulation= [(VNL-VFL)/VFL] × 100 where

VNL = Terminal voltage of generator at no

load VFL = Terminal voltage of generator at

full load Note that voltage regulation of a

generator is determined with field circuit

and speed held constant. If the voltage

regulation of a generator is 10%, it means

that terminal voltage increases 10% as the

load is changed from full load to no load.

The Efficiency of the DC Motor Increases

by:

Increasing number of turns in the coil

Increasing the strength of the current

Increasing X-section area of of the coil

Increasing the strength of the radial

magnetic field

Motor Characteristics

Torque Speed Curves In order to effectively design with D.C

motors, it is necessary to understand their

characteristic curves. For every motor, there

is a specific Torque/Speed curve and Power

curve.

The graph above shows a torque/speed curve

of a typical D.C motor. Note that torque is

inversely proportional to the speed of the

output shaft. In other words, there is a

tradeoff between how much torque a motor

delivers, and how fast the output shaft spins.

Motor characteristics are frequently given as

two points on this graph:

The stall torque,, represents the point on

the graph at which the torque is a maximum,

but the shaft is not rotating.

The no load speed,, is the maximum

output speed of the motor (when no torque is

applied to the output shaft).

The linear model of a D.C motor

torque/speed curve is a very good

approximation. The torque/speed curves

shown below are actual curves for the green

maxon motor (pictured at right) used by

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11

students in 2.007. One is a plot of empirical

data, and the other was plotted mechanically

using a device developed at MIT. Note that

the characteristic torque/speed curve for this

motor is quite linear.

This is generally true as long as the curve

represents the direct output of the motor, or

a simple gear reduced output. If the

specifications are given as two points, it is

safe to assume a linear curve.

Recall that earlier we defined power as the

product of torque and angular velocity. This

corresponds to the area of a rectangle under

the torque/speed curve with one corner at

the origin and another corner at a point on

the curve. Due to the linear inverse

relationship between torque and speed, the

maximum power occurs at the point where τ

= ½ , and = ½ .

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12

By substituting equations 3. and 4. into equation 2 above, we see that the power curves for a D.C

motor with respect to both speed and torque are quadratics, as shown in equations 5. and 6. From

these equations, we again find that maximum output power occurs at = ½, and = ½ respectively.

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1

Direct on line starter In electrical engineering, a direct on line

(DOL) or across the line starter starts

electric motors by applying the full line

voltage to the motor terminals. This is the

simplest type of motor starter. A DOL motor

starter also contain protection devices, and

in some cases, condition monitoring.

Smaller sizes of direct on-line starters are

manually operated; larger sizes use an

electromechanical contactor (relay) to

switch the motor circuit. Solid-state direct

on line starters also exist.

A direct on line starter can be used if the

high inrush current of the motor does not

cause excessive voltage drop in the supply

circuit. The maximum size of a motor

allowed on a direct on line starter may be

limited by the supply utility for this reason.

For example, a utility may require rural

customers to use reduced-voltage starters for

motors larger than 10 kW.

DOL starting is sometimes used to start

small water pumps, compressors, fans and

conveyor belts. In the case of an

asynchronous motor, such as the 3-phase

squirrel-cage motor, the motor will draw a

high starting current until it has run up to

full speed. This starting current is commonly

around six times the full load current, but

may as high as 12 times the full load current.

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2

TOPIC TWO

AC MACHINES

Induction Motor An induction motor or asynchronous motor is a type of alternating current motor where power

is supplied to the rotor by means of electromagnetic induction.

An electric motor converts electrical power

to mechanical power in its rotor (rotating

part). There are several ways to supply

power to the rotor. In a DC motor this power

is supplied to the armature directly from a

DC source, while in an induction motor this

power is induced in the rotating device. An

induction motor is sometimes called a

rotating transformer because the stator

(stationary part) is essentially the primary

side of the transformer and the rotor

(rotating part) is the secondary side.

Unlike the normal transformer which

changes the current by using time varying

flux, induction motors use rotating magnetic

fields to transform the voltage. The primary

side's current creates an electromagnetic

field which interacts with the secondary

side's electromagnetic field to produce a

resultant torque, thereby transforming the

electrical energy into mechanical energy.

Induction motors are widely used, especially

polyphase induction motors, which are

frequently used in industrial drives.

Induction motors are now the preferred

choice for industrial motors due to their

rugged construction, absence of brushes

(which are required in most DC motors)

and—thanks to modern power electronics—

the ability to control the speed of the motor.

Principle of operation and comparison to synchronous motors 3-phase power supply provides a rotating

magnetic field in an induction motor. The

basic difference between an induction motor

and a synchronous AC motor is that in the

latter a current is supplied into the rotor

which in turn creates a magnetic field

around the rotor. The rotating magnetic field

of the stator will impose an electromagnetic

torque on the still magnetic field of the rotor

causing it to move (about a shaft) and

rotation of the rotor is produced. It is called

synchronous because at steady state the

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2

speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

By way of contrast, the induction motor

does not have any direct supply onto the

rotor; instead, a secondary current is induced

in the rotor. To achieve this, stator windings

are arranged around the rotor so that when

energized with a polyphase supply they

create a rotating magnetic field pattern

which sweeps past the rotor. This changing

magnetic field pattern induces current in the

rotor conductors. These currents interact

with the rotating magnetic field created by

the stator and in effect cause a rotational

motion on the rotor. However, for these

currents to be induced, the speed of the

physical rotor must be less than the speed of

the rotating magnetic field in the stator, or

else the magnetic field will not be moving

relative to the rotor conductors and no

currents will be induced. If by some chance

this happens, the rotor typically slows

slightly until a current is re-induced and then

the rotor continues as before. This difference

between the speed of the rotor and speed of

the rotating magnetic field in the stator is

called slip. It is unitless and is the ratio

between the relative speed of the magnetic

field as seen by the rotor (the slip speed) to

the speed of the rotating stator field. Due to

this an induction motor is sometimes

referred to as an asynchronous machine.

Construction The stator consists of wound 'poles' that

carry the supply current to induce a

magnetic field that penetrates the rotor. In a

very simple motor, there would be a single

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2

projecting piece of the stator (a salient pole)

for each pole, with windings around it; in

fact, to optimize the distribution of the

magnetic field, the windings are distributed

in many slots located around the stator, but

the magnetic field still has the same number

of north-south alternations. The number of

'poles' can vary between motor types but the

poles are always in pairs (i.e. 2, 4, 6, etc.).

Induction motors are most commonly built

to run on single-phase or three-phase power,

but two-phase motors also exist. In theory,

two-phase and more than three phase

induction motors are possible; many single-

phase motors having two windings and

requiring a capacitor can actually be viewed

as two-phase motors, since the capacitor

generates a second power phase 90 degrees

from the single-phase supply and feeds it to

a separate motor winding. Single-phase

power is more widely available in residential

buildings, but cannot produce a rotating

field in the motor (the field merely oscillates

back and forth), so single-phase induction

motors must incorporate some kind of

starting mechanism to produce a rotating

field. They would, using the simplified

analogy of salient poles, have one salient

pole per pole number; a four-pole motor

would have four salient poles. Three-phase

motors have three salient poles per pole

number, so a four-pole motor would have

twelve salient poles. This allows the motor

to produce a rotating field, allowing the

motor to start with no extra equipment and

run more efficiently than a similar single-

phase motor.

Types of rotors There are three types of rotor:

Squirrel-cage rotor

The most common rotor is a squirrel-cage

rotor. It is made up of bars of either solid

copper (most common) or aluminum that

span the length of the rotor, and those solid

copper or aluminum strips can be shorted or

connected by a ring or sometimes not, i.e.

the rotor can be closed or semi closed type.

The rotor bars in squirrel-cage induction

motors are not straight, but have some skew

to reduce noise and harmonics.

Slip ring rotor

A slip ring rotor replaces the bars of the

squirrel-cage rotor with windings that are

connected to slip rings. When these slip

rings are shorted, the rotor behaves similarly

to a squirrel-cage rotor; they can also be

connected to resistors to produce a high-

resistance rotor circuit, which can be

beneficial in starting.

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3

Solid core rotor A rotor can be made from solid mild steel.

The induced current causes the rotation.

Speed control

The synchronous rotational speed of the

rotor (i.e. the theoretical unloaded speed

with no slip) is controlled by the number of

pole pairs (number of windings in the stator)

and by the frequency of the supply voltage.

Under load, the induction motor's speed

varies according to size of the load. As the

load is increased the speed of the motor

decreases increasing the slip which increases

the rotor's field strength to bear the extra

load.

Before the development of economical

semiconductor power electronics, it was

difficult to vary the frequency to the motor

and induction motors were mainly used in

fixed speed applications. As an induction

motor has no brushes and is easy to control,

many older DC motors are now being

replaced with induction motors and

accompanying inverters in industrial

applications.

Starting of induction motors

Direct-on-line starting The simplest way to start a three-phase

induction motor is to connect its terminals to

the line. This method is often called "direct

on line" and abbreviated DOL. In an

induction motor, the magnitude of the

induced emf in the rotor circuit is

proportional to the stator field and the slip

speed of the motor, and the rotor current

depends on this emf. When the motor is

started, the rotor speed is zero. The

synchronous speed is constant, based on the

frequency of the supplied AC voltage.

So the slip speed is equal to the synchronous

speed, the slip ratio is 1, and the induced

emf in the rotor is large. As a result, a very

high current flows through the rotor. This is

similar to a transformer with the secondary

coil short circuited, which causes the

primary coil to draw a high current from the

mains. When an induction motor starts

DOL, a very high current is drawn by the

stator, in the order of 5 to 9 times the full

load current. This high current can, in some

motors, damage the windings; in addition,

because it causes heavy line voltage drop,

other appliances connected to the same line

may be affected by the voltage fluctuation.

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4

To avoid such effects, several other

strategies are employed for starting motors.

Wye-Delta starters An induction motor's windings can be

connected to a 3-phase AC line in two

different ways:

i. wye in U.S, star in Europe,

where the windings are

connected from phases of the

supply to the neutral;

ii. delta (sometimes mesh in

Europe), where the windings are

connected between phases of the

supply.

A delta connection of the machine winding

results in a higher voltage at each winding

compared to a wye connection. A wye-delta

starter initially connects the motor in wye,

which produces a lower starting current than

delta, then switches to delta when the motor

has reached a set speed.

Disadvantages of this method over DOL

starting are:

i. Lower starting torque, which

may be a serious issue with

pumps or any devices with

significant breakaway torque

ii. Increased complexity, as more

contactors and some sort of

speed switch or timers are

needed

iii. Two shocks to the motor (one for

the initial start and another when

the motor switches from wye to

delta)

Variable-frequency drives Variable-frequency drives (VFD) can be of

considerable use in starting as well as

running motors. A VFD can easily start a

motor at a lower frequency than the AC line,

as well as a lower voltage, so that the motor

starts with full rated torque and with no

inrush of current. The rotor circuit's

impedance increases with slip frequency,

which is equal to supply frequency for a

stationary rotor, so running at a lower

frequency actually increases torque.

Resistance starters This method is used with slip ring motors

where the rotor poles can be accessed by

way of the slip rings. Using brushes,

variable power resistors are connected in

series with the poles. During start-up the

resistance is large and then reduced to zero

at full speed. At start-up the resistance

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5

directly reduces the rotor current and so

rotor heating is reduced. Another important

advantage is the start-up torque can be

controlled. As well, the resistors generate a

phase shift in the field resulting in the

magnetic force acting on the rotor having a

favorable angle.

Series Reactor starters In series reactor starter technology, an

impedance in the form of a reactor is

introduced in series with the motor

terminals, which as a result reduces the

motor terminal voltage resulting in a

reduction of the starting current; the

impedance of the reactor, a function of the

current passing through it, gradually reduces

as the motor accelerates, and at 95 % speed

the reactors are bypassed by a suitable

bypass method which enables the motor to

run at full voltage and full speed. Air core

series reactor starters or a series reactor soft

starter is the most common and

recommended method for fixed speed motor

starting. The applicable standards are [IEC

289] AND [IS 5553 (PART 3)].

Single Phase induction motor In a single phase induction motor, it is

necessary to provide a starting circuit to start

rotation of the rotor. If this is not done,

rotation may be commenced by manually

giving a slight turn to the rotor. The single

phase induction motor may rotate in either

direction and it is only the starting circuit

which determines rotational direction.

For small motors of a few watts the start

rotation is done by means of a single turn of

heavy copper wire around one corner of the

pole. The current induced in the single turn

is out of phase with the supply current and

so causes an out-of-phase component in the

magnetic field, which imparts to the field

sufficient rotational character to start the

motor. Starting torque is very low and

efficiency is also reduced.

Such shaded-pole motors are typically used

in low-power applications with low or zero

starting torque requirements, such as desk

fans and record players. Larger motors are

provided with a second stator winding which

is fed with an out-of-phase current to create

a rotating magnetic field. The out-of-phase

current may be derived by feeding the

winding through a capacitor, or it may

derive from the winding having different

values of inductance and resistance from the

main winding.

In some designs the second winding is

disconnected once the motor is up to speed,

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6

usually either by means of a switch operated

by centrifugal force acting on weights on the

motor shaft, or by a positive temperature

coefficient thermistor which after a few

seconds of operation heats up and increases

its resistance to a high value, reducing the

current through the second winding to an

insignificant level. Other designs keep the

second winding continuously energized

during running, which improves torque.

Control of speed in induction motor can be

obtained in 3 ways:

1. Scalar control

2. Vector control

3. Direct torque control

Rotating magnetic field

Description of magnetic field A symmetric rotating magnetic field can be

produced with as few as three coils. The

three coils will have to be driven by a

symmetric 3-phase AC sine current system,

thus each phase will be shifted 120 degrees

in phase from the others. For the purpose of

this example, the magnetic field is taken to

be the linear function of the coil's current

Sine wave current in each of the coils

produces sine varying magnetic field on the

rotation axis. Magnetic fields add as vectors.

Vector sum of the magnetic field vectors of

the stator coils produces a single rotating

vector of resulting rotating magnetic field.

The result of adding three 120-degrees

phased sine waves on the axis of the motor

is a single rotating vector. The rotor has a

constant magnetic field. The N pole of the

rotor will move toward the S pole of the

magnetic field of the stator, and vice versa.

This magneto-mechanical attraction creates

a force which will drive rotor to follow the

rotating magnetic field in a synchronous

manner.

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7

A permanent magnet in such a field will

rotate so as to maintain its alignment with

the external field. This effect was utilized in

early alternating current electric motors. A

rotating magnetic field can be constructed

using two orthogonal coils with a 90 degree

phase difference in their AC currents.

However, in practice such a system would

be supplied through a three-wire

arrangement with unequal currents. This

inequality would cause serious problems in

the standardization of the conductor size.

In order to overcome this, three-phase

systems are used where the three currents

are equal in magnitude and have a 120

degree phase difference. Three similar coils

having mutual geometrical angles of 120

degrees will create the rotating magnetic

field in this case. The ability of the three

phase system to create the rotating field

utilized in electric motors is one of the main

reasons why three phase systems dominate

in the world electric power supply systems.

Rotating magnetic fields are also used in

induction motors. Because magnets degrade

with time, induction motors use short-

circuited rotors (instead of a magnet) which

follow the rotating magnetic field of a multi-

coiled stator.

In these motors, the short circuited turns of

the rotor develop eddy currents in the

rotating field of stator which in turn move

the rotor by Lorentz force. These types of

motors are not usually synchronous, but

instead necessarily involve a degree of 'slip'

in order that the current may be produced

due to the relative movement of the field and

the rotor.

3-φmotor runs from 1-φ power, but does not start

.

1

The single coil of a single phase induction

motor does not produce a rotating magnetic

field, but a pulsating field reaching

maximum intensity at 0o and 180o

electrical. Another view is that the single

coil excited by a single phase current

produces two counter rotating magnetic field

phasors, coinciding twice per revolution at

0o and 180o. When the phasors rotate to 90o

and -90o they cancel in figure b. At 45o and

-45o (figure c) they are partially additive

along the +x axis and cancel along the y

axis. An analogous situation exists in figure

d. The sum of these two phasors is a phasor

stationary in space, but alternating polarity

in time. Thus, no starting torque is

developed.

However, if the rotor is rotated forward at a

bit less than the synchronous speed, it will

develop maximum torque at 10% slip with

respect to the forward rotating phasor. Less

torque will be developed above or below

10% slip. The rotor will see 200% - 10%

slip with respect to the counter rotating

magnetic field phasor. Little torque (see

torque vs slip curve) other than a double

freqency ripple is developed from the

counter rotating phasor. Thus, the single

phase coil will develop torque, once the

rotor is started. If the rotor is started in the

reverse direction, it will develop a similar

large torque as it nears the speed of the

backward rotating phasor. Single phase

induction motors have a copper or aluminum

squirrel cage embedded in a cylinder of steel

laminations, typical of poly-phase induction

motors.

Permanent-split capacitor motor One way to solve the single phase problem

is to build a 2-phase motor, deriving 2-phase

power from single phase. This requires a

motor with two windings spaced apart 90o

electrical, fed with two phases of current

displaced 90o in time. This is called a

permanent-split capacitor motor in Figure

below,

Permanent-split capacitor induction motor.

This type of motor suffers increased current

magnitude and backward time shift as the

motor comes up to speed, with torque

pulsations at full speed. The solution is to

keep the capacitor (impedance) small to

minimize losses. The losses are less than for

.

2

a shaded pole motor. This motor

configuration works well up to 1/4

horsepower (200watt), though, usually

applied to smaller motors. The direction of

the motor is easily reversed by switching the

capacitor in series with the other winding.

Capacitor-start induction motor In Figure below a larger capacitor may be

used to start a single phase induction motor

via the auxiliary winding if it is switched out

by a centrifugal switch once the motor is up

to speed. Moreover, the auxiliary winding

may be many more turns of heavier wire

than used in a resistance split-phase motor to

mitigate excessive temperature rise. The

result is that more starting torque is

available for heavy loads like air

conditioning compressors. This motor

configuration works so well that it is

available in multi-horsepower (multi-

kilowatt) sizes.

Capacitor-start induction motor.

Capacitor-run induction motor A variation of the capacitor-start motor

(Figure below) is to start the motor with a

relatively large capacitor for high starting

torque, but leave a smaller value capacitor in

place after starting to improve running

characteristics while not drawing excessive

current. The additional complexity of the

capacitor-run motor is justified for larger

size motors.

Capacitor-run motor induction motor

A motor starting capacitor may be a double-

anode non-polar electrolytic capacitor which

could be two + to + (or - to -) series

connected polarized electrolytic capacitors.

Such AC rated electrolytic capacitors have

such high losses that they can only be used

for intermittent duty (1 second on, 60

seconds off) like motor starting. A capacitor

for motor running must not be of electrolytic

construction, but a lower loss polymer type.

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3

Resistance split-phase induction motor If an auxiliary winding of much fewer turns

of smaller wire is placed at 90o electrical to

the main winding, it can start a single phase

induction motor. (Figure below) With lower

inductance and higher resistance, the current

will experience less phase shift than the

main winding. About 30o of phase

difference may be obtained. This coil

produces a moderate starting torque, which

is disconnected by a centrifugal switch at

3/4 of synchronous speed. This simple (no

capacitor) arrangement serves well for

motors up to 1/3 horsepower (250 watts)

driving easily started loads.

This motor has more starting torque than a

shaded pole motor (next section), but not as

much as a two phase motor built from the

same parts. The current density in the

auxiliary winding is so high during starting

that the consequent rapid temperature rise

precludes frequent restarting or slow starting

loads.

Resistance split-phase motor induction motor

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TOPIC FOUR

THREE PHASE INDUCTION MOTOR

Working Principle of Three Phase Induction Motor

An electrical motor is such an electromechanical device which converts electrical energy into a

mechanical energy.

In case of three phase AC operation, most

widely used motor is Three phase induction

motor as this type of motor does not require

any starting device or we can say they are

self starting induction motor. For better

understanding the principle of three phase

induction motor, the basic constructional

feature of this motor must be known to us.

This Motor consists of two major parts:

Stator: Stator of three phase induction

motor is made up of numbers of slots to

construct a 3 phase winding circuit which is

connected to 3 phase AC source. The three

phase winding are arranged in such a

manner in the slots that they produce a

rotating magnetic field after AC is given to

them.

Rotor: Rotor of three phase induction

motor consists of cylindrical laminated core

with parallel slots that can carry conductors.

Conductors are heavy copper or aluminum

bars which fits in each slots & they are short

circuited by the end rings. The slots are not

exactly made parallel to the axis of the shaft

but are slotted a little skewed because this

arrangement reduces magnetic humming

noise & can avoid stalling of motor.

Production of Rotating Magnetic Field

The stator of the motor consists of

overlapping winding offset by an electrical

angle of 120°. When the primary winding or

the stator is connected to a 3 phase AC

source, it establishes a rotating magnetic

field which rotates at the synchronous speed.

Secrets behind the rotation:

According to Faraday’s law an e.m.f

induced in any circuit is due to the rate of

change of magnetic flux linkage through the

circuit. As the rotor winding in an induction

motor are either closed through an external

resistance or directly shorted by end ring,

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2

and cut the stator rotating magnetic field, an

e.m.f is induced in the rotor copper bar and

due to this e.m.f a current flows through the

rotor conductor.

Here the relative velocity between the

rotating flux and static rotor conductor is the

cause of electric current generation; hence as

per Lenz's law the rotor will rotate in the

same direction to reduce the cause i.e. the

relative velocity.

Thus from the working principle of three

phase induction motor it may observed that

the rotor speed should not reach the

synchronous speed produced by the stator. If

the speeds equals, there would be no such

relative velocity, so no emf induction in the

rotor, & no current would be flowing, and

therefore no torque would be generated.

Consequently the rotor cannot reach at the

synchronous speed. The difference between

the stator (synchronous speed) and rotor

speeds is called the slip. The rotation of the

magnetic field in an induction motor has the

advantage that no electrical connections

need to be made to the rotor.

Thus the three phase induction motor is:

• Self-starting.

• Less armature reaction and brush sparking

because of the absence of commutators and

brushes that may cause sparks.

• Robust in construction.

• Economical.

• Easier to maintain.

What is the operating principle of a 3ph

induction motor?

An electric motor converts electrical energy

into a mechanical energy which is then

supplied to different types of loads. A.C.

motors operate on an A.C. supply, and they

are classified into synchronous, single phase

and 3 phase induction, and special purpose

motors. Out of all types, 3 phase induction

motors are most widely used for industrial

applications mainly because they do not

require a starting device.

A 3 phase induction motor derives its

name from the fact that the rotor current is

induced by the magnetic field, instead of

electrical connections. The operating

principle of a 3 phase induction motor is

based on the production of rmf.

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3

Production of a rotating magnetic field

The stator of an induction motor consists of

a number of overlapping windings offset by

an electrical angle of 120°. When the

primary winding or stator is connected to a

three phase alternating current supply, it

establishes a rotating magnetic field which

rotates at a synchronous speed. The

direction of rotation of the motor depends on

the phase sequence of supply lines, and the

order in which these lines are connected to

the stator. Thus interchanging the

connection of any two primary terminals to

the supply will reverse the direction of

rotation.

The number of poles and the frequency of

the applied voltage determine the

synchronous speed of rotation in the motor’s

stator. Motors are commonly configured to

have 2, 4, 6 or 8 poles. The synchronous

speed, a term given to the speed at which the

field produced by primary currents will

rotate, is determined by the following

expression. Synchronous speed of rotation =

(120* supply frequency)/Number of poles

on the stator.

Production of magnetic flux

A rotating magnetic field in the stator is the

first part of operation. To produce a torque

and thus rotate, the rotors must be carrying

some current. In induction motors, this

current comes from the rotor conductors.

The revolving magnetic field produced in

the stator cuts across the conductive bars of

the rotor and induces an emf. The rotor

windings in an induction motor are either

closed through an external resistance or

directly shorted. Therefore, the emf induced

in the rotor causes current to flow in a

direction opposite to that of the revolving

magnetic field in the stator, and leads to a

twisting motion or torque in the rotor.

As a consequence, the rotor speed will not

reach the synchronous speed of the rmf in

the stator. If the speeds match, there would

be no emf induced in the rotor, no current

would be flowing, and therefore no torque

would be generated. The difference between

the stator (synchronous speed) and rotor

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4

speeds is called the slip. The rotation of the

magnetic field in an induction motor has the

advantage that no electrical connections

need to be made to the rotor.

What results is a motor that is:

Self-starting

Explosion proofed (because of the

absence of slip rings or commutators

and brushes that may cause sparks)

Robust in construction

Inexpensive

Easier to maintain

Production of rotating magnetic field in a

three phase induction motor

In a three-phase system, three circuit

conductors carry three alternating currents

(of the same frequency) which reach their

instantaneous peak values at one third of a

cycle from each other. Taking one current as

the reference, the other two currents are

delayed in time by one third and two thirds

of one cycle of the electric current. This

delay between phases causes an effect of

giving constant power transfer over each

cycle of the current and also makes it

possible to produce a rotating magnetic field

in an electric motor.

The sum of the currents is always zero and

each line returns the current from the other

two. Thus a three-phase system can operate

with only three wires.[3] Three-phase

systems may also have a fourth wire,

particularly in low-voltage distribution,

which is the neutral wire. The neutral allows

three separate single-phase supplies to be

provided at a constant voltage and is

commonly used for supplying groups of

domestic properties which are each single-

phase loads. The connections are arranged

so that as far as possible in each group equal

power is drawn from each phase. Further up

the supply chain in high-voltage distribution

the currents are usually well balanced and it

is therefore normal to omit the neutral wire.

Three-phase has properties that make it very

desirable in electric power systems:

The phase currents tend to cancel out

one another, summing to zero in the

case of a linear balanced load. This

makes it possible to reduce the size

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5

of the neutral conductor because it

carries little to no current; all the

phase conductors carry the same

current and so can be the same size,

for a balanced load.

Power transfer into a linear balanced

load is constant, which helps to

reduce generator and motor

vibrations.

Three-phase systems can produce a

rotating magnetic field with a

specified direction and constant

magnitude, which simplifies the

design of electric motors.

Most household loads are single-phase. In

North American residences, three-phase

power might feed a multiple-unit apartment

block, but the household loads are connected

only as single phase. In lower-density areas,

only a single phase might be used for

distribution. Some large European

appliances may be powered by three-phase

power, such as electric stoves and clothes

dryers.

Wiring for the three phases is typically

identified by color codes which vary by

country. Connection of the phases in the

right order is required to ensure the intended

direction of rotation of three-phase motors.

For example, pumps and fans may not work

in reverse. Maintaining the identity of

phases is required if there is any possibility

two sources can be connected at the same

time; a direct interconnection between two

different phases is a short-circuit.

SPEED CONTROL OF THREE PHASE INDUCTION MOTOR

A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to

control its speed.

The speed control of induction motor is

done at the cost of decrease in efficiency and

low electrical power factor. Before

discussing the methods to control the speed

of three phase induction motor one should

know the basic formulas of speed and torque

of three phase induction motor as the

methods of speed control depends upon

these formulas. Synchronous speed

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2

Where f = frequency and P is the number of

poles

• The speed of induction motor is given by,

Where N is the speed of rotor of induction

motor, Ns is the synchronous speed, S is the

slip.

• The torque produced by three phase

induction motor is given by,

When rotor is at sand-still slip, s is one. So

the equation of torque is,

Where E2 is the rotor emf

Ns is the synchronous speed

R2 is the rotor resistance

X2 is the rotor inductive

reactance

The Speed of Induction Motor is changed from Both Stator and Rotor Side

The speed control of three phase induction

motor from stator side are further classified

as:

1. V / f control or frequency control

2. changing the number of stator poles

3. controlling supply voltage

4. adding rheostat in the stator circuit

The speed controls of three phase induction

motor from rotor side are further classified

as:

1. Adding external resistance on rotor side

2. Cascade control method

3. Injecting slip frequency emf into rotor

side

Speed Control from Stator Side

1. V / f control or frequency control -

Whenever three phase supply is given to

three phase induction motor rotating

magnetic field is produced which rotates at

synchronous speed given by

In three phase induction motor emf is

induced by induction similar to that of

transformer which is given by

.

2

Where K is the winding constant, T is the

number of turns per phase and f is

frequency. Now if we change frequency

synchronous speed changes but with

decrease in frequency flux will increase and

this change in value of flux causes saturation

of rotor and stator cores which will further

cause increase in no load current of the

motor . So, its important to maintain flux, φ

constant and it is only possible if we change

voltage i.e if we decrease frequency flux

increases but at the same time if we decrease

voltage flux will also decease causing no

change in flux and hence it remains

constant. So, here we are keeping the ratio

of V/ f as constant. Hence its name is V/ f

method. For controlling the speed of three

phase induction motor by V/ f method we

have to supply variable voltage and

frequency which is easily obtained by using

converter and inverter set.

2. Controlling supply voltage: The torque

produced by running three phase induction

motor is given by;

In low slip region (sX)2 is very very small as

compared to R2 . So, it can be neglected. So

torque becomes;

Since rotor resistance, R2 is constant so the

equation of torque further reduces to

We know that rotor induced emf E2 ∝ V.

So, T ∝ sV2.

From the equation above it is clear that if we

decrease supply voltage torque will also

decrease. But for supplying the same load,

the torque must remains the same and it is

only possible if we increase the slip and if

the slip increases the motor will run at

reduced speed . This method of speed

control is rarely used because small change

in speed requires large reduction in voltage,

and hence the current drawn by motor

increases, which cause over heating of

induction motor.

3. Changing the number of stator poles : The

stator poles can be changed by two methods

• Multiple stator winding

method

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3

• Pole amplitude modulation

method (PAM)

• Multiple stator winding method –

In this method of speed control of three

phase induction motor, the stator is provided

by two separate winding . These two stator

windings are electrically isolated from each

other and are wound for two different pole

numbers. Using switching arrangement, at a

time, supply is given to one winding only

and hence speed control is possible.

Disadvantages of this method are that the

smooth speed control is not possible. This

method is more costly and less efficient as

two different stator winding are required.

This method of speed control can only be

applied for squirrel cage motor

• Pole amplitude modulation method

(PAM) –

In this method of speed control of three

phase induction motor the original

sinusoidal mmf wave is modulated by

another sinusoidal mmf wave having

different number of poles.

Let f1(θ) be the original mmf wave of

induction motor whose speed is to be

controlled f2(θ) be the modulation mmf

wave P1 be the number of poles of induction

motor whose speed is to be controlled P2 be

the number of poles of modulation wave

After modulation resultant mmf wave;

So we get, resultant mmf wave

Therefore the resultant mmf wave will have

two different number of poles

Therefore by changing the number of poles

we can easily change the speed of three

phase induction motor

4. Adding rheostat in the stator circuit - In

this method of speed control of three phase

induction motor rheostat is added in the

.

4

stator circuit due to this voltage gets

dropped .In case of three phase induction

motor torque produced is given by T ∝ sV22.

If we decrease supply voltage torque will

also decrease. But for supplying the same

load , the torque must remains the same and

it is only possible if we increase the slip and

if the slip increase motor will run reduced

speed.

Speed Control from Rotor Side

1. Adding external resistance on rotor

side – In this method of speed control of

three phase induction motor external

resistance are added on rotor side. The

equation of torque for three phase induction

motor is

The three phase induction motor operates in

low slip region .In low slip region term

(sX)2 becomes very very small as compared

to R2. So, it can be neglected . and also E2 is

constant. So the equation of torque after

simplification becomes,

Now if we increase rotor resistance, R2

torque decreases but to supply the same load

torque must remains constant. So, we

increase slip, which will further results in

decrease in rotor speed. Thus by adding

additional resistance in rotor circuit we can

decrease the speed of three phase induction

motor.

The main advantage of this method is that

with addition of external resistance starting

torque increases but this method of speed

control of three phase induction motor also

suffers from some disadvantages:

.The speed above the normal value is not

possible

• Large speed change requires large value of

resistance and if such large value of

resistance is added in the circuit it will cause

large copper loss and hence reduction in

efficiency

• Presence of resistance causes more losses

• This method cannot be used for squirrel

cage induction motor

2. Cascade control method – In this

method of speed control of three phase

induction motor, the two three phase

induction motor are connected on common

shaft and hence called cascaded motor. One

motor is the called the main motor and

another motor is called the auxiliary motor.

The three phase supply is given to the stator

.

5

of the main motor while the auxiliary motor

is derived at a slip frequency from the slip

ring of main motor.

Let NS1 be the synchronous speed of main

motor; NS2 be the synchronous speed of

auxiliary motor; P1 be the number of poles

of the main motor; P2 be the number of

poles of the auxiliary motor; F is the supply

frequency; F1 is the frequency of rotor

induced emf of main motor

N is the speed of set and it remains same for

both the main and auxiliary motor as both

the motors are mounted on common shaft

S1 is the slip of main motor;

The auxiliary motor is supplied with same

frequency as the main motor i.e

Now put the value of;

;

Now at no load , the speed of auxiliary rotor

is almost same as its synchronous speed i.e;

N = NS2

Now rearrange the above equation and find

out the value of N, we get,

This cascaded set of two motors will now

run at new speed having number of poles (P1

+ P2). In the above method the torque

produced by the main and auxiliary motor

will act in same direction, resulting in

number of poles (P1 + P2). Such type of

cascading is called cumulative cascading.

There is one more type of cascading in

which the torque produced by the main

motor is in opposite direction to that of

auxiliary motor. Such type of is called

differential cascading; resulting in speed

corresponds to number of poles (P1 - P2).In

this method of speed control of three phase

.

6

induction motor, four different speeds can

be obtained;

1. when only main induction motor work,

having speed corresponds to

NS1 = 120 F / P1

2. when only auxiliary induction motor

work, having speed corresponds to

NS2 = 120 F / P2

3. when cumulative cascading is done, then

the complete set runs at a speed of

N = 120F / (P1 + P2)

4. when differential cascading is done, then

the complete set runs at a speed of

N = 120F / (P1 - P2)

3. Injecting slip frequency emf into rotor

side - when the speed control of three phase

induction motor is done by adding resistance

in rotor circuit, some part of power called,

the slip power is lost as I2R losses.

Therefore the efficiency of three phase

induction motor is reduced by this method

of speed control. This slip power loss can be

recovered and supplied back in order to

improve the overall efficiency of three phase

induction motor and this scheme of

recovering the power is called slip power

recovery scheme and this is done by

connecting an external source of emf of slip

frequency to the rotor circuit. The injected

emf can either oppose the rotor induced emf

or aids the rotor induced emf. If it oppose

the rotor induced emf, the total rotor

resistance increases and hence speed

decreases and if the injected emf aids the

main rotor emf the total resistance decreases

and hence speed increases. Therefore by

injecting induced emf in rotor circuit the

speed can be easily controlled. The main

advantage of this type of speed control of

three phase induction motor is that wide

range of speed control is possible whether

its above normal or below normal speed.

.

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TOPIC FIVE

MOTOR ENCLOSURES

The enclosures of electrical motors are standardized by NEMA (National Electrical

Manufacturers Association) as:

1. Open Drip Proof (ODP)

An open motor in which the ventilating

openings are so constructed that drops of

liquid or solid particles falling on it, at any

angle not greater than 15 degrees for the

vertical, cannot enter either directly or by

striking and running along a surface of the

motor. Designed for reasonably dry, clean,

and well ventilated (usually indoors) areas.

Outdoor installations require the motor to be

protected with a cover that does not restrict

the flow of air to the motor. Ventilation

openings in shield and/or frame prevent

drops of liquid from falling into motor

within up to 15 degree angle from vertical.

Designed for reasonably dry, clean, and well

ventilated (usually indoors) areas. Outdoors

installation requires the motor to be

protected with a cover that does not restrict

the flow of air to the motor.

2. Totally Enclosed Fan Cooled

(TEFC)

A motor so enclosed as to prevent the free

exchange of air between the inside and

outside of the case, but not sufficiently

enclosed to be termed air-tight, and dust

does not enter in sufficient quantity to

interfere with satisfactory operation.

Cooling is by means of an external fan as an

integral part of the motor. The fan provides

cooling by blowing air on the outside of the

motor. Suitable where the motor is exposed

to dirt or dampness. Not suited in very moist

humid or hazardous (explosive) locations.

Same as TENV with an external fan as an

integral part of the motor. The fan provides

cooling by blowing air on the outside of the

motor.

3. Totally Enclosed Non Ventilated

(TENV)

A motor so enclosed as to prevent the free

exchange of air between the inside and

outside of the case but not sufficiently

.

2

enclosed to be termed air-tight, and dust

does not enter in sufficient quantity to

interfere with satisfactory operation.

Cooling is only by convection and radiation

from the enclosure. Suitable where the

motor is exposed to dirt or dampness. Not

suited in very moist humid or hazardous

(explosive) locations. No ventilation

openings, enclosed to prevent free exchange

of air (not airtight). No external cooling fan,

relies on convection cooling. Suitable where

the motor is exposed to dirt or dampness.

Not suited in very moist humid or hazardous

(explosive) air.

4. Totally Enclosed Air Over (TEAO)

A motor so enclosed as to prevent the free

exchange of air between the inside and

outside of the case. A dust-tight enclosure

used on fan and blower motors for shaft

mounted fans or belt driven fans. The motor

must be mounted within the airflow of the

fan for cooling. Dust-tight fan and blower

motors for shaft mounted fans or belt driven

fans. The motors mounted within the airflow

of the fan.

5. Totally Enclosed Blower Cooled

(TEBC)

A motor so enclosed as to prevent the free

exchange of air between the inside and

outside of the case, but not sufficiently

enclosed to be termed air-tight, and dust

does not enter in sufficient quantity to

interfere with satisfactory operation. Used

on inverter duty motors. Cooled with

external fan on a power supply independent

of the inverter output. Provides full cooling

even at lower motor speeds.

6. Explosion Proof

A totally enclosed motor whose enclosure is

designed and constructed to withstand an

explosion of a specified gas or vapor which

may occur within it and to prevent the

ignition of the specified gas or vapor

surrounding the motor by sparks, flashes, or

explosions of the gas or vapor which may

occur within the motor housing.

7. Explosion-Proof Non Ventilated

(EPNV)

A non-ventilated explosion proof motor.

See TENV and Explosion-Proof above for

more information.

.

3

8. Explosion-Proof Fan Cooled

(EPFC)

A fan cooled explosion-proof motor. See

TEFC and Explosion- Proof above for more

information. The motor ambient temperature

shall not exceed +40oC.

.

2

TOPIC SEVEN

ELECTRIC MOTOR CONTROLS

Once the proper motor is selected, understanding the many various control devices available and

their uses and limitations becomes an important part related to reliable operation and protection

of the motor and the personnel using the motor. There are four major motor control topics or

categories to consider. Each of these has several subcategories and sometimes the subcategories

overlap to some extent. Certain pieces of motor control equipment can accomplish multiple

functions from each of the topics or categories.

The four categories include:

1) Starting the Motor

Disconnecting Means

Across the Line Starting

Reduced Voltage Starting

2) Motor Protection

Overcurrent Protection

Overload Protection

Other Protection (voltage, phase, etc)

Environment

3) Stopping the Motor

Coasting

Electrical Braking

Mechanical Braking

4) Motor Operational Control

Speed Control

Reversing

Jogging

Sequence Control

An understanding of each of these areas is

necessary to effectively apply motor control

principles and equipment to effectively

operate and protect a motor.

Motor Starting All motors must have a control device to

start and stop the motor called a “motor

controller”.

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2

The above are some motor starters

a. Motor Controller

A motor controller is the actual device that

energizes and de-energizes the circuit to the

motor so that it can start and stop. Motor

controllers may include some or all of the

following motor control functions: S

starting, stopping, over-current protection,

overload protection, reversing, speed

changing, jogging, plugging, sequence

control, and pilot light indication.

S Controllers range from simple to complex

and can provide control for one motor,

groups of motors, or auxiliary equipment

such as brakes, clutches, solenoids, heaters,

or other signals.

b. Motor Starter

The starting mechanism that energizes the

circuit to an induction motor is called the

“starter” and must supply the motor with

sufficient current to provide adequate

starting torque under worst case line voltage

and load conditions when the motor is

energized. There are several different types

of equipment suitable for use as “motor

starters” but only two types of starting

methods for induction motors:

i) Across the Line Starting

ii). Reduced Voltage Starting

.

3

c. Across the Line Starting of Motors

Across the Line starting connects the motor

windings/terminals directly to the circuit

voltage “across the line” for a “full voltage

start”. This is the simplest method of starting

a motor. Motors connected across the line

are capable of drawing full in-rush current

and developing maximum starting torque to

accelerate the load to speed in the shortest

possible time.

All NEMA induction motors up to 200

horsepower, and many larger ones, can

withstand full voltage starts. (The electric

distribution system or processing operation

may not though, even if the motor will).

d. Across the Line Starters

i) Manual Starter

There are two different types of common

“across the line” starters including:

1. Manual Motor Starters

2. Magnetic Motor Starters

1. Manual Motor Starters

A manual motor starter is package

consisting of a horsepower rated switch with

one set of contacts for each phase and

corresponding thermal overload devices to

provide motor overload protection. The

main advantage of a manual motor starter is

lower cost than a magnetic motor starter

with equivalent motor protection but less

motor control capability.

Manual motor starters are often used for

smaller motors - typically fractional

horsepower motors but the National

Electrical Code allows their use up to 10

Horsepower. Since the switch contacts

remain closed if power is removed from the

circuit without operating the switch, the

motor restarts when power is reapplied

which can be a safety concern. They do not

allow the use of remote control or auxiliary

control equipment like a magnetic starter

does.

2. Magnetic Motor Starters

A magnetic motor starter is a package

consisting of a contactor capable of opening

and closing a set of contacts that energize

and de-energize the circuit to the motor

along with additional motor

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4

overload protection equipment. Magnetic

starters are used with larger motors (required

above 10 horsepower) or where greater

motor control is desired. The main element

of the magnetic motor starter is the

contactor, a set of contacts operated by an

electromagnetic coil. Energizing the coil

causes the contacts (A) to close allowing

large currents to be initiated and interrupted

by a smaller voltage control signal.

The control voltage need not be the same as

the motor supply voltage and is often low

voltage allowing start/stop controls to be

located remotely from the power circuit.

Closing the Start button contact energizes

the contactor coil. An auxiliary contact on

the contactor is wired to seal in the coil

circuit. The contactor de-energizes if the

control circuit is interrupted, the Stop button

is operated, or if power is lost. The overload

contacts are arranged so an overload trip on

any phase will cause the contactor to open

and de-energize all phases.

Reduced Voltage Starting of Motors

Reduced Voltage Starting connects the

motor windings/terminals at lower than

normal line voltage during the initial starting

period to reduce the inrush current when the

motor starts. Reduced voltage starting may

be required when: The current in-rush form

the motor starting adversely affects the

voltage drop on the electrical system;

needed to reduce the mechanical “starting

shock” on drive-lines and equipment when

the motor starts. Reducing the voltage

reduces the current in-rush to the motor and

also reduces the starting torque available

when the motor starts.

All NEMA induction motors can accept

reduced voltage starting however it may not

provide enough starting torque in some

situations to drive certain specific loads. If

the driven load or the power distribution

system cannot accept a full voltage start,

some type of reduced voltage or "soft"

starting scheme must be used. Typical

reduced voltage starter types include:

1. Solid State (Electronic) Starters

2. Primary Resistance Starters

3. Autotransformer Starters

4. Part Winding Starters

5. Wye-Delta Starters

Reduced voltage starters can only be used

where low starting torque is acceptable or a

means exists to remove the load from the

motor or application before it is stopped.

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5

Motor Protection Motor protection safeguards the motor, the

supply system and personnel from various

operating conditions of the driven load, the

supply system or the motor itself.

Motor protection categories include:

i. Overcurrent Protection

ii. Overload Protection

iii. Other Types of Protection.

The National Electrical Code requires that

motors and their conductors be protected

from both overcurrent and overload

conditions.

i. Overcurrent Protection

Overcurrent protection interrupts the

electrical circuit to the motor upon excessive

current demand on the supply system from

either short circuits or ground faults.

Overcurrent protection is required to protect

personnel, the motor branch circuit

conductors, control equipment, and motor

from these high currents. Overcurrent

protection is usually provided in the form of

fuses or circuit breakers. These devices

operate when a short circuit, ground fault or

an extremely heavy overload occurs. Most

overcurrent sources produce extremely large

currents very quickly.

ii. Overload Protection

Overload protection is installed in the motor

circuit and/or motor to protect the motor

from damage from mechanical overload

conditions when it is operating/running. The

effect of an overload is an excessive rise in

temperature in the motor windings due to

current higher than full load current.

Properly sized overload protection

disconnects the motor from the power

supply when the heat generated in the motor

circuit or windings approaches a damaging

level for any reason.

The larger the overload, the more quickly

the temperature will increase to a point that

is damaging to the insulation and lubrication

of the motor. Unlike common instantaneous

type fuses and breakers, overload devices

are designed to allow high currents to flow

briefly in the motor to allow for:

Typical motor starting currents

of 6 to 8 times normal running

current when starting.

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6

Short duration overloads such

as a slug of product going

through a system.

If the motor inlets and outlets

are covered by a blanket of lint

or if a bearing should begin to

lock, excessive heating of the

motor windings will “overload”

the motors insulation which

could damage the motor.

The overcurrent device will not react to this

low level overload. The motor overload

device prevents this type of problem from

severely damaging the motor and also

provides protection for the circuit

conductors since it is rated for the same or

less current as the conductors. Overload

protection trips when an overload exists for

more than a short time. The time it takes for

an overload to trip depends on the type of

overload device, length of time the overload

exists, and the ambient temperature in which

the overloads are located.

Other Motor Protection Devices i. Low Voltage Protection

Low Voltage Disconnects - Protection

device operates to disconnect the motor

when the supply

voltage drops below a preset value. The

motor must be manually restarted upon

resumption of normal supply voltage.

Low Voltage Release - Protection device

interrupts the circuit when the supply

voltage drops below a preset value and re-

establishes the circuit when the supply

voltage returns to normal.

ii. Phase Failure Protection

Interrupts the power in all phases of a three-

phase circuit upon failure of any one phase.

Normal fusing and overload protection may

not adequately protect a polyphase motor

from damaging single phase operation.

Without this protection, the motor will

continue to operate if one phase is lost.

Large currents can be developed in the

remaining stator circuits which eventually

burn out. Phase failure protection is the only

effective way to protect a motor properly

from single phasing.

iii. Phase Reversal Protection

Used where running a motor backwards

(opposite direction from normal) would

cause operational or safety problems. Most

three phase motors will run the opposite

direction by switching the connections of

any two of the three phases. The device

interrupts the power to the motor upon

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7

detection of a phase reversal in the three-

phase supply circuit. This type of protection

is used in applications like elevators where it

would be damaging or dangerous for the

motor to inadvertently run in reverse.

iv. Ground Fault Protection

Operates when one phase of a motor shorts

to ground preventing high currents from

damaging the stator windings and the iron

core.

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TOPIC EIGHT: CONTACTORS

CONTACTORS

A contactor is an electrically controlled switch used for switching a power circuit, similar to a

relay except with higher current ratings. A contactor is controlled by a circuit which has a much

lower power level than the switched circuit

.

Some contactors

Contactors come in many forms with

varying capacities and features. Unlike a

circuit breaker, a contactor is not intended to

interrupt a short circuit current. Contactors

range from those having a breaking current

of several amperes to thousands of amperes

and 24 V DC to many kilovolts. The

physical size of contactors ranges from a

device small enough to pick up with one

hand, to large devices approximately a meter

(yard) on a side. Contactors are used to

control electric motors, lighting, heating,

capacitor banks, thermal evaporators, and

other electrical loads.

A contactor has three components. The

contacts are the current carrying part of the

contactor. This includes power contacts,

auxiliary contacts, and contact springs. The

electromagnet (or "coil") provides the

driving force to close the contacts. The

enclosure is a frame housing the contact and

the electromagnet. Enclosures are made of

insulating materials like Bakelite, Nylon 6,

and thermosetting plastics to protect and

insulate the contacts and to provide some

measure of protection against personnel

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2

touching the contacts. Open-frame

contactors may have a further enclosure to

protect against dust, oil, explosion hazards

and weather.

Magnetic blowouts use blowout coils to

lengthen and move the electric arc. These

are especially useful in DC power circuits.

AC arcs have periods of low current, during

which the arc can be extinguished with

relative ease, but DC arcs have continuous

high current, so blowing them out requires

the arc to be stretched further than an AC

arc of the same current. The magnetic

blowouts in the pictured Albright contactor

(which is designed for DC currents) more

than double the current it can break,

increasing it from 600 A to 1,500 A.

Sometimes an economizer circuit is also

installed to reduce the power required to

keep a contactor closed; an auxiliary contact

reduces coil current after the contactor

closes. A somewhat greater amount of

power is required to initially close a

contactor than is required to keep it closed.

Such a circuit can save a substantial amount

of power and allow the energized coil to stay

cooler. Economizer circuits are nearly

always applied on direct-current contactor

coils and on large alternating current

contactor coils.

A basic contactor will have a coil input

(which may be driven by either an AC or

DC supply depending on the contactor

design). The coil may be energized at the

same voltage as a motor the contactor is

controlling, or may be separately controlled

with a lower coil voltage better suited to

control by programmable controllers and

lower-voltage pilot devices. Certain

contactors have series coils connected in the

motor circuit; these are used, for example,

for automatic acceleration control, where the

next stage of resistance is not cut out until

the motor current has dropped.

Applications of Contactors

1. Lighting control

Contactors are often used to provide central

control of large lighting installations, such

as an office building or retail building. To

reduce power consumption in the contactor

coils, latching contactors are used, which

have two operating coils. One coil,

momentarily energized, closes the power

circuit contacts, which are then

mechanically held closed; the second coil

opens the contacts.

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3

2. Magnetic starter

A magnetic starter is a device designed to

provide power to electric motors. It includes

a contactor as an essential component, while

also providing power-cutoff, under-voltage,

and overload protection.

3. Vacuum contactor

Vacuum contactors utilize vacuum bottle

encapsulated contacts to suppress the arc.

This arc suppression allows the contacts to

be much smaller and use less space than air

break contacts at higher currents. As the

contacts are encapsulated, vacuum

contactors are used fairly extensively in

dirty applications, such as mining. Vacuum

contactors are only applicable for use in AC

systems.

The AC arc generated upon opening of the

contacts will self-extinguish at the zero-

crossing of the current waveform, with the

vacuum preventing a re-strike of the arc

across the open contacts. Vacuum contactors

are therefore very efficient at disrupting the

energy of an electric arc and are used when

relatively fast switching is required, as the

maximum break time is determined by the

periodicity of the AC waveform.

How Contactor Controls an Electric Motor

Control of electric motor with contactor

When a relay is used to switch a large

amount of electrical power through its

contacts, it is designated by a special name:

contactor. Contactors typically have

multiple contacts, and those contacts are

usually (but not always) normally-open, so

that power to the load is shut off when the

coil is de-energized. Perhaps the most

common industrial use for contactors is the

control of electric motors.

The top three contacts switch the respective

phases of the incoming 3-phase AC power,

typically at least 480 Volts for motors 1

horsepower or greater. The lowest contact is

an “auxiliary” contact which has a current

rating much lower than that of the large

motor power contacts, but is actuated by the

same armature as the power contacts. The

auxiliary contact is often used in a relay

logic circuit, or for some other part of the

.

4

motor control scheme, typically switching

120 Volt AC power instead of the motor

voltage.

One contactor may have several auxiliary

contacts, either normally-open or normally-

closed, if required. The three “opposed-

question-mark” shaped devices in series

with each phase going to the motor are

called overload heaters. Each “heater”

element is a low-resistance strip of metal

intended to heat up as the motor draws

current. If the temperature of any of these

heater elements reaches a critical point

(equivalent to a moderate overloading of the

motor), a normally-closed switch contact

(not shown in the diagram) will spring open.

This normally-closed contact is usually

connected in series with the relay coil, so

that when it opens the relay will

automatically de-energize, thereby shutting

off power to the motor.

Overload heaters are intended to provide

overcurrent protection for large electric

motors, unlike circuit breakers and fuses

which serve the primary purpose of

providing overcurrent protection for power

conductors. Overload heater function is

often misunderstood. They are not fuses;

that is, it is not their function to burn open

and directly break the circuit as a fuse is

designed to do.

Rather, overload heaters are designed to

thermally mimic the heating characteristic of

the particular electric motor to be protected.

All motors have thermal characteristics,

including the amount of heat energy

generated by resistive dissipation (I2R), the

thermal transfer characteristics of heat

“conducted” to the cooling medium through

the metal frame of the motor, the physical

mass and specific heat of the materials

constituting the motor, etc.

These characteristics are mimicked by the

overload heater on a miniature scale: when

the motor heats up toward its critical

temperature, so will the heater toward its

critical temperature, ideally at the same rate

and approach curve. Thus, the overload

contact, in sensing heater temperature with a

thermo-mechanical mechanism, will sense

an analogue of the real motor. If the

overload contact trips due to excessive

heater temperature, it will be an indication

that the real motor has reached its critical

temperature (or, would have done so in a

short while). After tripping, the heaters are

supposed to cool down at the same rate and

approach curve as the real motor, so that

they indicate an accurate proportion of the

.

5

motor’s thermal condition, and will not

allow power to be re-applied until the motor

is truly ready for start-up again.

Shown here below is a contactor for a three-

phase electric motor, installed on a panel as

part of an electrical control system at a

municipal water treatment plant. Three-

phase, 480 volt AC power comes in to the

three normally-open contacts at the top of

the contactor via screw terminals labeled

“L1,” “L2,” and “L3″ (The “L2″ terminal is

hidden behind a square-shaped “snubber”

circuit connected across the contactor’s coil

terminals). Power to the motor exits the

overload heater assembly at the bottom of

this device via screw terminals labeled “T1,”

“T2,” and “T3.”

The overload heater units themselves are

black, square-shaped blocks with the label

“W34,” indicating a particular thermal

response for a certain horsepower and

temperature rating of electric motor. If an

electric motor of differing power and/or

temperature ratings were to be substituted

for the one presently in service, the overload

heater units would have to be replaced with

units having a thermal response suitable for

the new motor. The motor manufacturer can

provide information on the appropriate

heater units to use.

A white pushbutton located between the

“T1″ and “T2″ line heaters serves as a way

to manually re-set the normally-closed

switch contact back to its normal state after

having been tripped by excessive heater

temperature. Wire connections to the

“overload” switch contact may be seen at

the lower-right of the photograph, near a

label reading “NC” (normally-closed). On

this particular overload unit, a small

“window” with the label “Tripped” indicates

a tripped condition by means of a colored

flag. In this photograph, there is no

“tripped” condition, and the indicator

appears clear

Contactor for a three-phase electric motor

installed on a panel as part of an electrical

control system

.

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TOPIC FOUR

PREVENTIVE MAINTENANCE

WHY PREVENTIVE MAINTENANCE?

Preventive maintenance is predetermined work performed to a schedule with the aim of

preventing the wear and tear or sudden failure of equipment components.

Preventive maintenance helps to:

Protect assets and prolong the

useful life of production equipment

Improve system reliability

Decrease cost of replacement

Decreases system downtime

Reduce injury

Mechanical, process or control

equipment failure can have adverse

results in both human and economic

terms. In addition to down time and

the costs involved to repair and/or

replace equipment parts or

components, there is the risk of injury

to operators, and of acute exposures to

chemical and/ or physical agents.

Preventive maintenance, therefore, is a

very

important ongoing accident prevention

activity, which you should integrate

into your operations/ product

manufacturing process. To be

effective, your preventive maintenance

function should incorporate the

following elements:

1. Planned replacements

Planned replacements of components

designed around the following:

Reliability of components

(equipment failure is caused by its

least reliable component)

- check manufacturer’s information

- check accepted industry best

practices

Maintaining equipment service

records

Scheduling replacement of

components at the end of their

useful service life

Acquiring and maintaining

inventories of:

- east reliable components

- critical components

- components scheduled for

replacements

.

2

Replacing service-prone equipment

with more reliable performers

By introducing the element of planning

into your maintenance function, you

are likely to reduce your repair and

manpower requirements.

2. Exploratory maintenance

Exploratory maintenance is to

anticipate and prevent breakdowns.

Diagnostic measures to

analyze your plant requirements

include:

Operating and performing

specifications of equipment

Past experience with components:

- inspection records

- servicing records

- replacement frequency

- inspected component failures

Regularly scheduled lubrication

program:

- identify lubrication points on

equipment

- colour code in order to identify

lubrication frequency

- consult manufacturer and accepted

industry best practices to establish

schedule

Preventive Maintenance

Identifying Maintenance Hazards

The hazards associated with

maintenance activities can be

classified as follows:

Safety Hazards

Mechanical

equipment

tools

Electrical

live equipment

Pneumatic

Hydraulic

Thermal

Combustion

Falls

slippery floors

working at heights

Health Hazards

Chemical Agents

process chemicals

cleaning solvents

unexpected reaction

products

dusts

other chemical agents

Physical Agents

noise

vibration

other

Ergonomic Hazards

.

3

Biomechanical

lifting, pushing, pulling

(manual handling)

stretching, ending (to reach

hard to access areas)

Work/process design

poorly designed tools

hard to access work locations

ill-fitting personal protective

equipment

complex procedures

Many of these hazards are interrelated.

Examine your process, the layout of

your process area, and the process

equipment used, to determine the exact

nature of the hazards likely to be

encountered during your maintenance

activities. For example, maintenance

work carried out in confined spaces

carries a greater risk of critical injuries

and acute exposures to chemical and

physical agents. These risks are

associated with equipment and

materials in the space itself and from

nearby operations. Fatalities are quite

common.

Controlling Maintenance Hazards Ideally, the hazards likely to occur

during maintenance activities should

be addressed in the

planning stage.

Process Selection

Depending on the nature of the

process, special precautions may be

needed to protect workers when

disassembling and cleaning equipment.

Consider this factor when you make a

decision to select one process over

another. Also consider the following

factors which contribute to the level of

risk of your maintenance activities:

How easy temporary structures

are to erect

How easy they are to access

Support and reassembly of

components of large scale

equipment

Use of hoists and mobile

working platforms

Safe use of ladders especially

near live electrical equipment

How much disassembly is

required to access affected

equipment

Need for temporary hoisting

equipment

.

4

Need for personal protective

equipment

Housekeeping hazards created

at floor level by the presence of

dismantled components

Equipment Selection

The process you select will determine

the type of equipment you will be

using. However, consider the

following:

Reliability:

manufacturer’s data

in-plant operating experience

trade association data

Ease of access to serviceable parts

Ease of disassembly

Complexity of repair procedures

Ease of frequency of required

lubrication

Manufacturer/supplier follow-up:

availability of parts

availability of service time

Developing Procedures

When servicing equipment, hazards

not related to your process operation

are likely to be introduced. For this

reason, it is important to prepare

written servicing procedures that

include the following:

A clear, step-by-step procedure, in

checklist form, for controlling

hazardous energy:

1. Preparing for shutdown

2. Shutting down machine, process or

equipment

3. Isolating energy to the machine,

process or equipment

4. Applying lockout devices

5. Controlling stored energy

6. Verification of isolation

7. Release from lockout control

Hazards identification

Selection and specification of personal

protective equipment:

appropriate for the hazard

proper fit

Selection and specification of tools to

be used:

right tool for the job

in good condition

appropriate for the environment

(non-sparking tools in flammable

atmospheres)

ergonomic design

Step-by-step procedure for

disassembly

Step-by-step checklist for inspection of

components (to establish a baseline for

reliability)

.

5

Identification of hazards associated

with sub-procedures:

entering and working in confined

spaces

welding in open and confined

spaces

removing insulation

cleaning

handling and using solvents

erecting temporary structures

using portable equipment

using ladders

abrasive blasting

painting

Erection and disassembly of

scaffolding and other temporary

platforms

Disassembly of small-scale equipment

Reassembly of small-scale equipment

Support and disassembly of large scale

equipment

Examine each procedure thoroughly to

ensure that the least hazardous method

is selected, and that all precautions

necessary to complete the job safely

are taken. Keep records of all your

maintenance activities, indicating the

machine(s) involved, the part(s)

involved, type of maintenance and date

on which performed.

Training

Maintenance personnel are often

involved in a complex and changing

set of problems. Therefore, they need

more thorough training in accident

prevention than regular workers.

Serious consequences to maintenance

and other workers can result from not

following established maintenance

procedures (e.g., use of work permits,

lockout procedures, confined space

entry procedures). Ensure that your

maintenance personnel are well trained

in, and can demonstrate that they

understand, all relevant procedures.

Also provide training in:

Hazard identification

Selection, use, and care of

equipment, machine tools, personal

protective clothing/equipment, etc.,

required to be used

First-aid and life-saving techniques

The hazards of and control

methods for substances which may

be encountered in the workplace,

such as:

irritating, toxic or corrosive dusts

gases

vapours

fluids

.

6

How to inspect chains, blocks, fall

protection devices and ropes

How to secure loads

Understanding stresses

It is a good practice to call the

maintenance crew together at the start

of each job, in order to discuss the

hazards involved and the method of

doing it safely. In the course of their

daily work, members of the

maintenance crew travel throughout

the plant, becoming familiar with

every machine and process. If properly

selected and trained, they can do much

to identify and correct unsafe

conditions. In small companies, the

maintenance staff may also be

responsible for inspecting and

maintaining portable power tools,

extension cords, and the like. If so,

special procedures and training are

needed. Train equipment operators to

recognize the signs of impending

failure, such as abnormal noise,

excessive vibration, declining or

abnormal output, and to report these

immediately to their supervisor.

Legislation

The following Regulations made under

the Occupational Health and Safety

Act contain provisions that deal with

maintenance:

Industrial Establishments

(R.R.O. 851/90)

Control of Exposure to

Biological or Chemical Agents

(R.R.O. 833/90)

Workplace Hazardous

Materials Information System

(R.R.O. 860/90)

Designated Substances