Electrical Machines
An ideal synchronous motor has no starting torque because

 rotor is made up of salient poles.
  relative velocity between stator and rotor mmf’s is not zero.
 rotor winding is highly reactive.
 relative velocity between the stator and rotor mmf’s is zero.

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Electrical Machines
The introduction of interpoles in between the main poles improves the performance of a dc machine, because

 the interpoles produce additional flux to augment the developed torque
 a counter-emf is induced in the coil undergoing commutation
 the inequality of air-gap flux on the top and bottom halves of the armature is removed
 the flux waveform is improved with reduction in harmonics

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Electrical Machines
Cogging of induction motor occurs at

 high voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are not equal.
 high voltage and when stator and rotor poles are not equal.

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