Electrical Machines Questions and Answers – Effect of Brush Shift and Compensation Windings

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This set of Electrical Machines MCQs focuses on “Effect of Brush Shift and Compensation Windings”.

1. The brushes are placed __________
a) along geometrical neutral axis
b) perpendicular to geometrical neutral axis
c) along magnetic neutral axis
d) perpendicular to magnetic neutral axis

Explanation: The brushes are placed along geometrical neutral axis so that in ideal conditions commutations take place without sparking. Also placing brushes along the magnetic neutral axis requires consistent variations as load varies which is not desirable.

2. The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.
a) forward
b) reverse to rotation
c) all of the mentioned
d) none of the mentioned

Explanation: For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.

3. With no commutating poles used the brushes are given backward lead in the dc motor.
a) True
b) False

Explanation: The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.
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4. If the critical commutation time is 2 ms but it was observed that the practical commutation took 2.5 ms. This is the case of __________
a) under commutation
b) over commutation
c) critical commutation
d) none of the mentioned

Explanation: Since the time taken is more than the prescribed time limit for the commutation, it is under commutation.

5. The coil under going commutation lies along _____ while the coil getting short circuited lies along ____ for a practical dc machine.
a) gna, mna
b) mna, gna
c) gna, gna
d) mna, mna

Explanation: In a practical dc machine, the coil undergoing commutation lies along GNA so as the brushes and the coils which actually get short circuited lies along mna. This lead to sparking as a voltage difference is created at the coils.

6. The principle contributors to magneto motive force is/are __________
a) field current and armature current
b) field excitation
c) electromagnetic torque
d) all of the mentioned

Explanation: The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf.

7. The absence of compensating winding lead to __________
a) statically induced emf in armature
b) fluctuations in supply
c) reduction in the flux
d) any of the mentioned

Explanation: The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.

8. The number of compensating conductors/pole faces is _______ (where Z is the number of active armature conductors/pole; A is number of parallel paths; Ia is the armature current).
a) Z/A*Ia
b) 2*A*Ia/Z
c) Z*Ia/A
d) Z/2A*Ia

Explanation: Compensating conductors per pole face is Z/A*Ia.

9. A 400-V, 1000-A, lap wound dc machine has 10 poles, 860 armature conductors. The number of conductors in the pole face to give full compensation if pole face covers 70% of pole span is?
a) 3010
b) 4300
c) 2400
d) 2800

Explanation: AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.

10. Each of the following is valid for interpoles except __________
a) they are connected in parallel with the armature so that they carry part of armature current
b) they are small yoke fixed poles spaced in between main poles
c) their polarity, in case of generator is same as that of main poles ahead
d) they automatically neutralize not only reactance voltage but cross-magnetization also

Explanation: All the options are valid for the interpoles except a because they are connected in series not in parallel.

11. Mark the most incorrect. In dc generator, commutation can be improved by __________
a) using interpoles
b) using carbon brushes
c) shifting brush axis in direction of armature rotation
d) none of the mentioned

Explanation: All the given measures are used to eliminate the effect of armature reaction and hence improves commutation also.

12. The commutation process involves basically reversal of current in armature coil as it crosses MNA.
a) True
b) False

Explanation: By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.

13. The armature reaction is produced mainly by __________
c) all of the mentioned
d) none of the mentioned

Explanation: The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.

14. The compensating winding in dc generator is provided to __________
a) neutralize cross-magnetizing flux
b) neutralize demagnetizing flux
c) neutralize armature mmf
d) maintain magnetizing mmf

Explanation: The compensating winding functions such that to nullify the cross magnetization effects in the armature.

15. If the magnetic circuit of the dc machine is in the saturation region, the armature reaction __________
a) does not affect flux/pole
b) increases the flux/pole
c) decreases flux/pole
d) affects the flux/pole only when armature current is small

Explanation: In the saturation region the flux will be constant.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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