Electrical Machines
The sparking at the the brushes, in a DC machines is due to

 high resistance of carbon brushes.
 armature reaction.
 presence of commutator.
  reactance voltage.

ANSWER DOWNLOAD EXAMIANS APP

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

  rotates at the speed of rotor in the air gap.
  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.

ANSWER DOWNLOAD EXAMIANS APP