Electrical Machines
Slip speed is the

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

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

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

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