Electrical Machines
Dummy coils are used for

 mechanical balancing.
 increasing efficiency.
 reducing armature reaction.
All of these

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Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at synchronous speed with respect to rotor.
  rotates at the speed of rotor in the air gap.
  rotates at slip speed with respect to stator mmf.
  is at standstill with respect to stator mmf.

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