# Electrical Machines Questions and Answers – EMF Generated in the Armature

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This set of Electrical Machines Interview Questions and Answers focuses on “EMF Generated in the Armature”.

1. If the speed of rotation of a separately excited dc machine is halved and the filed strength is also reduced to half of the original. Then the emf induced in the armature _____________
a) reduces to 1/4th of original
b) reduces to half of original
c) remains same
d) doubles

Explanation: Emf is proportional to speed of rotation and flux.

2. If the speed of rotation for a four pole, 220V dc machine is 1 m/s, flux density as 2T having length of bore as 0.1m and radius of 0.5m. The emf induced under pole faces will be ________
a) 0.2 V
b) 0.1 V
c) 0.2*pi V
d) 220 V

Explanation: E = B*l*v
= 2*0.1*1
= 0.2 Volts.

3. The induced emf in the armature of a 4-pole dc machine is ________
a) directly proportional to speed and field strength applied to it
b) directly proportional to speed
c) directly proportional to field strength applied to it
d) inversely proportional to speed and inversely proportional to field strength applied to it

Explanation: Emf is proportional to speed of rotation and flux.
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4. If the armature current is by mistake increased to 50% for the dc machine then ________
a) the induced emf remains unchanged
b) the induced emf doubles
c) the induced emf halves
d) none of the mentioned

Explanation: Emf is proportional to speed of rotation and flux.

5. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. The same machine is then wave wound, the emf induced in the lap wound dc machine to that of wave wound will be ________
a) 2
b) 0.5
c) 1
d) 4

Explanation: E = 1/(P/2)
= 1/(4/2) = 1/2 V.

6. A duplex lap wound armature is used in six pole dc machine with six brush set. The number of current paths in this machine is?
a) 12
b) 6
c) 3
d) 2

Explanation: The number of parallel paths for a lap wound machine is number of parallel paths. As it is a duplex winding, it will have 6*2 = 12 parallel paths.

7. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils has simplex winding. The number of current paths is?
a) 2
b) 6
c) 3
d) 4

Explanation: The number of parallel path for a wave wound machine is equal to two.

8. For a four pole, 200 V dc shunt generator having armature resistance of 1 ohms. The current flowing in the armature is 10 A. What is the back emf generated if the terminal voltage of 220 V supply is fed?
a) 230 Volts
b) 210 Volts
c) 190 Volts
d) 200 Volts

Explanation: E = V+I*ra
= 220+10*1
= 230 Volts.

9. For a 4 pole 200 V, 20 A dc generator running at the rated rpm with no load will be ________
b) 4000 W no load losses
d) none of the mentioned

Explanation: As there is no load and assuming no armature resistance, the no load losses should ideally be zero.

10. Which of the following cannot reduce the terminal voltage of a dc shunt generator?
a) Commutation
b) Armature reaction
c) Armature ohmic losses
d) Any of the mentioned

Explanation: Commutation ideally does not reduce the terminal voltage of a dc machine. If it takes place properly at desired timings, the losses can be avoided. Armature reaction, ohmic losses contribute to the losses in the terminal voltage.

Explanation: Emf is proportional to speed of rotation.

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