Electrical Machines
The low voltage winding of a 400/230 V single phase 50 Hz transformer is to be connected to keep the magnetization current at the same level in both the cases the voltage at 25 Hz should be

 65 V
 460 V
 230 V
 115 V

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

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

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