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

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Electrical Machines
When the supply terminal of a DC shunt motor are interchanged

 The motor will stop.
  The motor will run at its normal speed in the same direction as before.
 The direction of rotation will reverse.
 The motor will run much faster in the same direction.

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