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Electrical Machines

Electrical Machines
It is an advisable to use auto transformer if the transformation ratio is

 near to 1.
 0.5.
 0.25.
 Greater than 1.

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Electrical Machines
Silicon content in steel lamination is kept within 5% as it

 Increases cost.
All of these
 makes the material brittle.
 increases hysteresis loss.

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Electrical Machines
1 KVA, 230 V, 50 Hz, single phase transformer has an eddy current loss of 30 watts. The eddy current loss when the transformer is excited by a dc source of same voltage will be

 zero watt
 less than 30 W
 30 W
 more than 30 W

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Electrical Machines
The total core loss of a 400 V, 50 Hz transformer is 2400W, when it is supplied by 200 V, 25 Hz loss is 800W. Calculate eddy current loss and hysteresis loss?

 1000W , 1400W
 1400W, 1000W
 1600W, 800W
 800W, 1600W

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Electrical Machines
In synchronous motor, the damper winding is generally used to

  reduce the eddy currents.
  provide starting torque only.
  reduce noise level only.
  prevent hunting and provide the starting torque.

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Electrical Machines
A DC shunt machine generates 250 V on open circuit at 1000 RPM. Armature resistance is 0.5 Ω, field resistance 250 Ω. When running as a motor takes 4 A at 250 V. Calculate the efficiency when running as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4%.

 82.5%
 72.5%
 78.5%
 67.5%

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