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Electrical Machines

Electrical Machines
A commutator with a diameter of 50 cm rotating at 1000 rpm. For a brush width of 1.5 cm, find the time of commutation in ms.

 0.512.
  0.573.
 0.649.
 0.443.

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Electrical Machines
The armature of a DC motor is laminated to reduce

 friction and windage loss.
 hysteresis loss.
 copper loss.
 eddy current loss.

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Electrical Machines
In case of a dc motor the ratio Eb / E is indicative of

  efficiency.
  ratio of input to output voltage.
  ratio of starting to running torque.
  ratio of starting to running current.

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Electrical Machines
A smaller air gap in a polyphase induction motor helps to

  reduce the chance of crawling.
  reduce the magnetizing current.
  reduce the chance of cogging.
  increase the starting torque.

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Electrical Machines
The armature shaft must be able to withstand

 bending moment due to weight of the armature.
All of these
 twisting strains due to transmission of torque.
 any unbalanced magnetic pull on the armature core.

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Electrical Machines
In 3 - Φ induction motor some times copper bars are placed deep in the rotor to

  improve starting torque.
  improve power factor.
  improve efficiency.
  reduce copper loss.

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MORE MCQ ON Electrical Machines

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