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%.

 72.5%
 82.5%
 78.5%
 67.5%

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 it reduces field structure iron losses
 commutator action is not possible otherwise
 dc machines are comparatively low rating machines.
 it gives more uniform air-gap flux distribution

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Electrical Machines
The armature mmf of a DC machine has

 stepped distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.
  triangular space distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.

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