Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  rotates at synchronous speed with respect to rotor.
  rotates at slip speed with respect to stator mmf.
  is at standstill with respect to stator mmf.
  rotates at the speed of rotor in the air gap.

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Any of the above.
 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in the field circuit.
 Connecting an additional resistance in stator circuit.

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