# Test: Effect of Brush Shift & Compensation Windings

## 15 Questions MCQ Test Electrical Machines | Test: Effect of Brush Shift & Compensation Windings

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Attempt Test: Effect of Brush Shift & Compensation Windings | 15 questions in 10 minutes | Mock test for Electrical Engineering (EE) preparation | Free important questions MCQ to study Electrical Machines for Electrical Engineering (EE) Exam | Download free PDF with solutions
QUESTION: 1

### The brushes are placed

Solution:

The brushes are placed along magnetic neutral axi so that in ideal conditions commutations take place without sparking.

QUESTION: 2

### The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.

Solution:

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.

QUESTION: 3

### With no commutating poles used the brushes are given backward lead in the dc motor.

Solution:

The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.

QUESTION: 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

Solution:

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

QUESTION: 5

The coil under going commutation lies along _____ while the coil getting short circuited lies along ____ for a practical dc machine.

Solution:

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.

QUESTION: 6

The principle contributors to magneto motive force is/are

Solution:

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

QUESTION: 7

The absence of compensating winding lead to

Solution:

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

QUESTION: 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)

Solution:

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

QUESTION: 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

Solution:

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

QUESTION: 10

Each of the following is valid for interpoles except

Solution:

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

QUESTION: 11

Mark the most incorrect. In dc generator, commutation can be improved by

Solution:

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

QUESTION: 12

The commutation process involves basically reversal of current in armature coil as it crosses MNA.

Solution:

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

QUESTION: 13

The armature reaction is produced mainly by

Solution:

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

QUESTION: 14

The compensating winding in dc generator is provided to

Solution:

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

QUESTION: 15

If the magnetic circuit of the dc machine is in the saturation region, the armature reaction

Solution:

In the saturation region the flux will be constant.

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