Electrical Machines
The crawling in the induction motor is caused by

 low voltage supply
 harmonics developed in the motor
 high loads
 improper design of stator laminations

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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

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

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Electrical Machines
The 3 - Φ induction motors are, by a very considerable margin, the most widely used AC motors as

  they have maximum efficiency and maximum starting torque.
  they have very good speed regulation.
  they are capable of operation under range of leading and lagging pf.
  these are cheaper in cost, simple and rugged in construction.

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