Electrical Machines
DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of

 Heavy sparking at the first stud of starting resistance steps.
 Both of above.
 Heavy sparking at brushes.
 None of these.

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

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