Electrical Machines
Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

 1, 2, and 3
 1 and 3
 2, 3 and 4
 1, 2 and 4

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Electrical Machines
Cogging and crawling are phenomena associated with

 the former at a fraction of its rate speed and the latter during starting.
 cage induction machines and they are essentially the same.
 any of the above.
 the former during starting and the latter at a fraction of its rated speed.

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Electrical Machines
The damper windings also called the squirrel cage windings or damper grids

  consists of short-circuited copper bars embedded in the field pole faces.
  are provided in a synchronous motor to make it self starting.
  both (A) and (B).
  are provided on the stator for improving power factor.

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