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

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

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Electrical Machines
If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced by 50%, the maximum flux density will (assuming that the magnetic circuit remains unsaturated)

 changes to 0.5 times the original value
 changes to 1.5 times the original value
 remain the same as the original
 changes to three times the original value

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Electrical Machines
Commutation in a DC machine may be explained as a process

 how alternate flux affects the main field flux.
  in which magnetic field is set up.
 by virtue of which emf is induced in the armature conductors.
  by which electric current in a coil is reversed.

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 dc machines are comparatively low rating machines.
 it gives more uniform air-gap flux distribution
 commutator action is not possible otherwise
 it reduces field structure iron losses

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