Electrical Machines
Variation in speed by ward-Leonard control of DC motors occurs due to variation in

  Field excitaion.
  Armature current.
  Supply voltage.
  Armature voltage.

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Electrical Machines
Cogging in induction motor occurs when

 number of stator teeth - number of rotor teeth = 0.
 number of stator teeth - number of rotor teeth = negative number.
 number of stator teeth - number of rotor teeth = odd number.
 number of stator teeth - number of rotor teeth = even number.

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Electrical Machines
With the increase in speed of a DC motor

 back emf falls and line current increases.
 back emf increases but line current falls.
 both back emf as well as line current increase.
 both back emf as well as line current fall.

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