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
In a 3 - Φ induction motor

  power factor at starting is low as compared to that while running.
  power factor at starting is high as compared to that while running.
  power factor at starting in the same as that while running.
  none of them.

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Electrical Machines
Consider the following statements about commutating poles which are fitted on most large dc Shunt motors: 1. The commutating poles are placed in the geometric neutral plane and their number is usually Equal to the number of main poles 2. The winding on the commutating poles is connected in series with the shunt-field winding on the main poles 3. The polarity of the commutating pole must be that of the next pole further ahead 4. The commutating poles neutralize the reactance voltage produced in the coil undergoing Commutation. Of these statements,

 1, 2 and 4 are correct
 1, 2 and 3 are correct
 1 and 4 are correct
 2, 3 and 4 are correct

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Electrical Machines
What will happen if the the supply terminals are reversed in case of a DC shunt motor?

 The motor will stop.
 The motor will run much faster in the same direction.
 The motor will run at its normal speed in the same direction as before.
 The motor will run in reverse direction.

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