Electrical Machines
A small air gap in poly phase induction motor helps to

 reduce chance of cogging.
  reduce magnetizing current.
 reduce chance of crawling.
 increase starting torque.

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Electrical Machines
Flashing the field of DC generator means

  making the magnetic losses forces parallel.
  neutralising residual magnetism.
 increasing flux density by adding extra turns of winding.
  creating residual magnetism by a DC source.

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

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

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