Electrical Machines
The air gap between the yoke and armature is dc machine is kept very small

All of these
 to achieve a stronger magnetic field.
 to avoid over heating.
 to avoid locking of the armature.

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Electrical Machines
Cogging of induction motor occurs at

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

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