Electrical Machines
Semi closed slots or totally closed slots are used in induction motors, essentially to

 improve starting torque
 increase pull out torque
 reduce magnetising current and improve power factor
 decrease starting torque

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Electrical Machines
Slip speed is the

  sum of synchronous and rotor speeds.
  difference of actual rotor speed and synchronous speed.
  half of the sum of synchronous and rotor speeds.
  difference of synchronous speed and actual rotor speed.

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