# Electrical Machines Questions and Answers – Constructional Features of Rotating Electrical Machines-1

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Constructional Features of Rotating Electrical Machines-1”.

1. Which of the following two windings produces/induces the working flux and working EMF respectively?
a) field winding and armature winding
b) armature winding and field winding
c) both are produced in field winding
d) both are produced in armature winding

Explanation: Field winding produces the working flux and armature winding has the working EMF, which is induced by the working flux.

2. Armature winding carries which of the following current?
a) magnetizing current only
c) both magnetizing current and load current
d) none of the mentioned

Explanation: The winding on the machine that carries only load current is called armature winding.

3. Field winding carries which of the following current?
a) exciting current only
c) both exciting current and load current
d) none of the mentioned

Explanation: The winding that handles only exciting current is called field winding.
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4. Which of the following statements are correct regarding exciting current?

```(i) exciting current varies with the load
(ii) exciting current doesn't vary with load
(iii) exciting current produces only a working magnetic flux
(iv) exciting current is not responsible for the production of working  magnetic flux
```

a) (i) & (iv)
b) (ii) & (iii)
c) (i) & (iii)
d) (ii) & (iv)

Explanation: The current that produces only a working magnetic flux, and does not vary with the load on the machine is called exciting current, magnetizing current or field current.

5. Current in the field winding is ___________
a) AC always
b) DC always
c) Both AC and DC
d) Either AC or DC

Explanation: Field winding is always given DC supply and hence current is also DC.

6. The rating of the armature winding is ___________
a) 1/2 to 2% of the rated power of the machine
b) less than the power rating of the machine
c) greater than the power rating of the machine
d) equal to the power rating of the machine

Explanation: The armature winding handles all the power that is being converted or transformed.

7. The power rating of the field winding is ___________
a) equal to the power rating of the machine
b) 1/2 to 2% of the rated power of the machine
c) greater than the power rating of the machine
d) less than the power rating of the machine

Explanation: The power input to the DC field winding is dissipated as i2R loss in the field winding (once the required field current is established).

8. What is the reason behind armature structure for both DC and AC machines being laminated?
a) reduce i2R losses
b) reduce the leakage flux
c) reduce the eddy current losses
d) for better operating power factor

Explanation: The armature windings of both the DC and AC machines have to deal with alternating current only; this is the reason why the armature structure of all rotating machines are laminated in order to reduce the eddy current losses.

9. Armature winding is one in which working ___________
a) flux is produced by field current
b) flux is produced by the working emf
c) emf is produced by the working flux
d) emf is produced by the leakage flux

Explanation: In an electrical machine, armature winding is one in which the working emf is induced by the working flux.

10. Why is the air gap between stator and rotor should be kept as small as mechanically possible in induction motor?
a) to reduce the leakage flux between stator and rotor
b) as it leads to better operating power factor of the induction motor
c) reduce the eddy current losses
d) both, to reduce leakage flux between stator and rotor, and also for better operating power factor

Explanation: The leakage flux and power factor depend on the air gap and it should be as small as possible.

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