BRAKING DC MOTOR

PREPARED BY:CHRISTIANE JOSEPH M. AGUILARREX DANCEL U. NIEBLA

TYPES DC MOTORS

1. PERMANENT MAGNET DC MOTOR

2. SERIES WOUND DC MOTOR

3. SHUNT WOUND DC MOTOR

1) PERMANENT MAGNET(PM) DC MOTOR

• A PM motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque.

• Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control.

• PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding.

• Most larger DC motors are of the "dynamo" type, which have stator windings.

2) SHUNT WOUND DC MOTOR• The shunt wound dc motor falls under the category of

self excited dc motors, where the field windings are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive of.

Itotal = Ia + Ish

Where: Ia = It – Ish = E / Ra

E = Eb + IaRa = Ish Rsh

PHYSICAL CONSTRUCTION OF SHUNT WOUND DC MOTOR

3) SERIES WOUND DC MOTORA series DC motor connects the armature and field

windings in series with a common D.C. power source. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.

I total = I se = Ia

Where:

E = Eb + Itotal ( Rsh + Ra)

Z4 SERIES DC MOTOR

• A series wound motors has linear relationship existing between the field current and the amount of torque produced. i.e. torque is directly proportional to current over the entire range of the graph.

• Series motors are generally operated for a very small duration, about only a few seconds, just for the purpose of starting. Because if its run for too long, the high series current might burn out the series field coils thus leaving the motor useless.

Speed Regulation of Series Wound DC Motor

Unlike in the case of a DC shunt motor, the dc series motor has very poor speed regulation. i.e. the series motor is unable to maintain its speed on addition of external load to the shaft

1&2) Compound connection• A compound DC motor connects the armature and fields

windings in a shunt and a series combination to give it characteristics of both a shunt and a series DC motor. 

• This motor is used when both a high starting torque and good speed regulation is needed.

• The motor can be connected in two arrangements: cumulatively or differentially.

• Cumulative compound motors connect the series field to aid the shunt field, which provides higher starting torque but less speed regulation. Differential compound DC motors have good speed regulation and are typically operated at constant speed.

BRAKING

DCMOTORS

• The use of the electric traction motors of a vehicle as generators when slowing. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors, and regenerative if the power is returned to the supply line.

• Dynamic braking lowers the wear of friction-based braking components, and additionally regeneration reduces energy consumption

• During dynamic braking, the traction motors, which are now acting as generators, are connected to braking grids of large resistors which limit the current flow and dissipate the converted energy as heat in the resistors instead of the motor.

• Dynamic braking can also be achieved by shorting the motor terminals, thus bringing the motor to a fast abrupt stop. This method causes an enormous current surge through the motor itself, dissipating all the energy as heat, and can only be used in low-power intermittent applications due to cooling limitations. It is not suitable for traction applications.

• Permanent magnet motors do not require an excitation field, this field is provided by the permanent magnets.

TYPES OF DC MOTOR BRAKING

1) ELECTRIC BRAKING forShunt wound DC MOTORS

2)ELECTRIC BRAKING for Series Wound DC MOTORS

1) Electric Braking for D.C. Shunt Motor

   (a) Rheostatic or Dynamic Braking :      In this method, the armature is simply disconnected from the supply and connected to a resistance, while the field remains connected to the supply as shown in Fig. 1(a) and (b).

(b) Plugging :       This involves the reversing of the armature connections of the motor. We have seen that if the direction of current through armature or field is reversed. In case of plugging, motor is subjected to the reversely directed torque. To limit high inrush of current an additional resistance R is connected. In this method, motor tries to accelerate in the other direction, after coming to rest. So some auxiliary device is required to cut-off the supply as soon as motor comes to rest. In case of failure of supply this method is ineffective. This method gives more braking torque that the rheostatic braking.

The method is generally used in braking elevators, rolling mills, machine tools etc.

(c) Regenerative braking :        In this method, instead of being disconnected from the supply, it remains connected and returns the braking energy to the line.

The method is used while lowering the cage of a hoist or the downgrade motion of an electric train.

2) ELECTRIC BRAKING FOR SERIES DC MOTORS       (a) Rheostatic or dynamic braking :

The basic principle of disconnecting the armature from the supply remains the same. The direction of the current through the armature reverse while braking. The motor runs as a generator. In case of series motor care should be taken to see that the direction of current through field does not change

         (b) Plugging :     The basic principle of reversing the armature connection remains same. Similar to the shunt motor, the resistance R added can be controlled to control the magnitude of the braking torque.

(c) Regenerative braking        In case of a d.c. series motor, increase in excitation results decrease in speed. As such it is not possible to get e.m.f. more than voltage. It is not possible to make field current more than the armature current. Hence regeneration braking with series motor is not possible. But can be used with traction motors with some special arrangements.Note : The flow of disturbance current through the series field causes the electrical instability for the series motors when used for regenerative braking.       Hence while using series motors for regenerative braking the fields must be excited separately and use stabilizing circuits.

The armature of all the motors are either connected in series or parallel while the fields are connected in series. The resistance R1 and R2  are the protective resistances.

Fig. 6  Series motor used for regenerative braking

END