Electrical Machines
In 3 - Φ induction motor some times copper bars are placed deep in the rotor to

  reduce copper loss.
  improve starting torque.
  improve efficiency.
  improve power factor.

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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 have maximum efficiency and maximum starting torque.
  they are capable of operation under range of leading and lagging pf.

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