Measuring Devices

Prepared by:

Dr. Eng. Montaser Abd El Sattar

Department of Electrical Engineering

Faculty of Engineering, South Valley University, Qena

18-4-2021

Health Technical Institute

2

Moving Coil Instruments

There are two types of moving coil

instruments namely:

Permanent magnet moving coil (PMMC)

type which can only be used for direct

current, voltage measurements

Dynamometer type which can be used on

either direct or alternating current, voltage

measurements.

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Permanent Magnet Moving Coil (PMMC)

In PMMC meter or (D’Arsonval) meter or galvanometer all are the same instrument, a coil of fine wire is suspended in a magnetic field produced by permanent magnet.

The coil will rotate in the magnetic field when it carries an electric current by electromagnetic (EM) torque effect.

A pointer will deflect according to the amount of current to be measured which applied to the coil. The (EM) torque is counterbalance by the mechanical torque of control springs attached to the movable coil also.

4

PMMC Instruments

PMMC Instruments

The current flows through a coil in a

magnetic field.

The coil experiences a torque proportional to

current.

The movement of the coil is "opposed" by a

spring.

The deflection of the needle is proportional

to current.

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Permanent Magnet Moving Coil Instruments

Construction It consists of permanent magnet which is stationary.

Moving system consists of a spindle attached to a rectangular aluminum frame. A coil made up of thin copper wire is wound over the frame. The current to be measured is passed through this coil.

A soft iron core is placed in the in the space within the aluminum frame.

Two spiral springs are mounted on the spindle to produce control torque. Control spring also serves an additional purpose & acts as control lead.

Pointer is mounted on spindle. Mirror is provided below the scale to avoid parallax error. The spindle is supported by jeweled bearings.

Permanent Magnet Moving Coil Instruments

Construction 1. It consists of permanent magnet which is stationary.

2. Moving system consists of a spindle attached to a rectangular aluminum frame. A coil made up of thin copper wire is wound over the frame. The current to be measured is passed through this coil.

3. A soft iron core is placed in the in the space within the aluminum frame. This core is stationary and is provided to reduce the reluctance of the magnetic path between two poles of the permanent magnet.

4. Two spiral springs are mounted on the spindle to produce control torque. The control spring also serves an additional purpose and acts as control lead. Pointer is mounted on spindle. Mirror is provided below the scale to avoid parallax error. The spindle is supported by jeweled bearings.

Working 1. The current to be measured is passed through moving coil via control springs.

2. A current carrying moving coil is now in a magnetic field. According to Flemings left hand rule, torque is produced on the coil and coil moves, pointer deflects.

3. Damping torque is provided by eddy current damping method.

Torque equation- Deflection is proportional to current.

PMMC Instruments

When the torques are balanced the moving

coil will stopped and its angular deflection

represent the amount of electrical current to

be measured against a fixed reference,

called a scale.

If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear.

PMMC Instruments

PMMC Instruments

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Torque Equation and Scale

Assume there are (N) turns of wire and the coil is (L) in long by (W) in wide. The force F acting perpendicular to both the direction of the current flow and the direction of magnetic field is given by:

F=BIL Newtons

where N: turns of wire on the coil I: current in the movable coil , B: flux density in the air gap L: vertical length of the coil

Since the force acts on each side of the coil, the total force for a coil of N turns

F=2BILN Newtons

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Torque Equation and Scale (cont.)

The force on each sides acts at a radius r,

producing deflecting turque

TD=2BILNr [N.m]

=BILN(2r)

=BILND [N.m]

where D is the coil diameter

Fleming's left-hand rule

14

Fleming's left-hand rule is used for electric motors, while Fleming's

right-hand rule is used for electric generators.

Separate hands need to be used for motors and generators because of

the differences between cause and effect.

In an electric motor, the electric current and magnet field exist

(which are the causes), and they lead to the force that creates the

motion (which is the effect), and so the left hand rule is used.

In an electric generator, the motion and magnetic field exist

(causes), and they lead to the creation of the electric current (effect),

and so the right hand rule is used.

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PMMC

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Torque Equation and Scale (cont.)

The controlling torque is proportional to the angle or deflection of the pointer

Tc= Kθ , K is constant

Deflecting torque = Controlling torque

Kθ =BILND

θ = C I , C is constant

The angular deflection proportional linearly with applied current.

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Galvanometer

A galvanometer is a PMMC instrument designed to be

sensitive to extremely low current levels.

The galvanometer is the device used for detecting

the presence of small current and voltage or for

measuring their magnitude.

The simplest galvanometer is a very sensitive

instrument with the type of center-zero scale.

A typical galvanometer has a "full-scale-current"

(Ifs) of 10 A to 10 mA

The resistance of the coil is typically 10 to 1000 .

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Galvanometers were the first instruments

used to detect and measure electric currents.

Sensitive galvanometers were used to detect

signals from long submarine cables, and

were used to discover the electrical activity

of the heart and brain.

Galvanometer (cont.)

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– An ammeter is always connected in series.

– low internal resistance.

– maximum pointer deflection is produced by

a very small current.

– For a large currents, the instrument must be

modified by connecting a very low shunt

resister.

DC Ammeter

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Example 1: An ammeter as shown in Figure 3-9 has a PMMC instrument

with a coil resistance of Rm = 99 and FSD current of 0.1 mA. Shunt

resistance Rs = 1. Determine the total current passing through the

ammeter at (a) FSD, (b) 0.5 FSD, and 0.25 FSD

m

mmsh

msh

sh

mmsh

mmshsh

msh

II

RIR

III

I

RIR

RIRI

VV

A direct-current ammeter consists of a

PMMC instrument and a low-resistance

shunt.

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Solution

(a) At FSD

mA10

mA0.1mA9.9IIIcurrenttotal

mA9.9Ω1

mV9.9

R

VI

VRIand

Ω99mA0.1

RIVvoltagemeter

ms

s

ms

mss

mmm

(b) At 0.5 FSD

mA5

mA500.mA4.95IIIcurrenttotal

mA4.95Ω1

mV4.95

R

VI

mV4.95Ω99mA0.05RIV

mA0.05mA0.10.5I

ms

s

ms

mmm

m

(b) At 0.25 FSD

mA2.5

mA0.025mA2.475IIIcurrenttotal

mA2.475Ω1

mV2.475

R

VI

mV2.475Ω99mA0.025RIV

mA0.025mA0.10.25I

ms

s

ms

mmm

m

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Example 2: A PMMC instrument has FSD of 100 A and a coil

resistance of 1 k. Calculate the required shunt resistance

value to convert the instrument into an ammeter with (a) FSD

= 100 mA and (b) FSD = 1 A.

Solution

(a) FSD = 100 mA

Ω1.001mA99.9

mV100

I

VR

mA99.9Aμ100mA100III

III

mV100kΩ1Aμ100RIV

s

ms

ms

ms

mmm

(b) FSD = 1 A

Ω0.1001mA999.9

mV100

I

VR

mA999.9Aμ100A1III

mV100RIV

s

ms

ms

mmm

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Swamping Resistance

A PMMC instrument is wound with thin copper wire, and its resistance can

change significantly when its temperature changes.

The heating effect of the coil current may be enough to produce a resistance

change, which will introduce an error.

A swamping resistance made of manganin or constantan is connected in series

with the coil (resistance temperature coefficients very close to zero).

Manganin is a trademarked name for an alloy of typically 86% copper, 12%

manganese, and 2% nickel. It was first developed by Edward Weston in

1892.

Constantan is a copper-nickel alloy usually consisting of 55% copper and

45% nickel, 1887.

The ammeter shunt must also be made of manganin or constantan to avoid

shunt resistance variations with temperature.

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• Make-before-break switch

• The instrument is not left without a shunt in

parallel with it.

• During switching there are actually two shunts

in parallel with the instrument.

Multi-range Ammeters

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Ayrton Shunt

At B Total resistance R1+R2+R3

Meter resistance Rm

At C Total resistance R1+R2

Meter resistance Rm+R3

At D?

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Example 3: A PMMC instrument has a three-resistor Ayrton

shunt connected across it to make an ammeter as shown

in Figure 3-13. The resistance values are R1 = 0.05, R2

= 0.45 and R3 = 4.5. The meter has Rm = 1k and

FSD = 50A. Calculate the three ranges of the

ammeter.

Solution

Switch at contact B:

mA10.05

mA10μA50III

mA10Ω4.5Ω0.45Ω0.05

mV50

RRR

VI

mV50kΩ1μA50RIV

sm

321

ss

mms

27

Switch at contact C:

mA100.05

mA100μA50III

mA100Ω0.45Ω0.05

mV50

RR

VI

mV50Ω4.5kΩ1μA50RRIV

sm

21

ss

3mms

Switch at contact C:

1.00005A

1A50μ0III

1A0.05Ω

50mV

R

VI

50mV0.45Ω4.5Ω1kΩ50μ0RRRIV

sm

1

ss

23mms