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

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

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Electrical Machines
Armature reaction in a DC machine

 is in direct opposition to the main poles.
 makes an angle with the main pole axis which is load dependent.
 is in the same direction as the main poles.
  makes an angle of 90° with the main pole axis.

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