Electrical Machines
Satisfactory commutation of DC machine requires

 brushes should smoothly run in the holders.
 brushes should be of proper grade and size.
All of these
 smooth, concentric commutator properly undercut.

ANSWER DOWNLOAD EXAMIANS APP

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

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

ANSWER DOWNLOAD EXAMIANS APP