Commutator Action - Electrical Machines Questions and Answers

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Action of commutator”.

1. What is Commutator?
a) rotating segment to which brushes are attached
b) stationary segment to which rotating brushes are attached
c) rotating segment to which rotating brush segment is attached
d) any of the mentioned
View Answer

Answer: a
Explanation: Commutator is a rotating segment to which brushes are attached.

2. Commutator action is analogous to _________ in analog electronics.
a) full wave rectifier
b) half wave rectifier
c) comparator
d) amplifier
View Answer

Answer: a
Explanation: Commutator converts the alternating voltage induced at the armature to the unidirectional voltage at the slip rings.

3. What is Inductive kick?
a) reactance voltage in commutator segments
b) reactance voltage in shorted commutator segments
c) impulsive voltage in commutator segments
d) none of the mentioned
View Answer

Answer: a
Explanation: Inductive kick is the name given to the reactance voltage which occurs at the commutation taking place in the commutator.It occurs due to the high circulating current during the commutation.

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4. Inductive kick causes ________
a) sparking at the brushes of the machine
b) similar problems when neutral phase shift takes place in the armature
c) large current to flow in the equalizer rings
d) none of the mentioned
View Answer

5. Operation of a dc machine will ________
a) not be affected by commutating poles
b) be affected by commutating poles
c) be affected significantly upto 30% spread of commutating poles
d) none of the mentioned
View Answer

Answer: a
Explanation: Commutating poles are placed under the armature poles and it compensates only poles under these regions.

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6. A 200 V dc machine working at rated speed. Voltage across the commutating terminals should ideally be ________
a) zero
b) 2 V
c) 1 V
d) 200 V
View Answer

Answer: a
Explanation: The commutating poles undergoing commutation should have zero voltage across it so that there is no reactive voltage induced.
So it will not cause large circulating current in the winding.

7. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. Then number of commutator segment is?
a) 16
b) 32
c) 4
d) 8
View Answer

Answer: a
Explanation: Commmutator segment is equal to number of coil slots.

8. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils. The commutator pitch for the machine should be ________
a) 1
b) -1
c) +1 and 1
d) none of the mentioned
View Answer

Answer: a
Explanation: Commutator pitch for the wave connected progressive winding should be 1.

9. The brushes in dc machine are normally placed electrically in interpolar region to ________
a) make an angle of 90 deg. electrical
b) make an angle of 90 deg. mechanical
c) make an angle of 0 deg. electrical
d) none of the mentioned
View Answer

Answer: a
Explanation: To obtain proper commutation without any sparks, the magnetic neutral axis and geometrical neutral axis should be aligned to each other,such that coils undergoing commutation experience zero emf across them.

10. The armature rotation in the dc machine and the magnetic field are independent of each other and obey laws of conservation of energy.
a) True
b) False
View Answer

Answer: a
Explanation: The armature rotation does not affect the stored magnetic energy at the steady state of operation.

11. Under commutation results ________
a) sparking at the trailing edge of brush
b) sparking at the leading edge of brush
c) sparking at the middle of brush
d) any of the mentioned
View Answer

Answer: a
Explanation: Under commutation takes place when the commutation speed is slow and it takes more time than critical defined time for commutation. Because of this the trailing edge of the brush still remains under the commutation.

12. While the operation of dc motor, there are sparkings at the commutation observed. These can be happening due to ________
a) under commutation
b) over commutation
c) too rapid reversal of the current
d) any of the mentioned
View Answer

Answer: d
Explanation: The over commutation, over commutation or rapid reversal of the current, any of these reasons can lead to sparking at the brushes.

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