Electrical Machines
Under commutation results in

  sparking at the middle of the brush.
  sparking the trailing edge of the brush.
  no sparking.
  sparking at the leading edge of the brush.

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Electrical Machines
A dc shunt generator, when driven without any excitation, showed an open circuit voltage of 12 V. When the field winding was excited, the voltage dropped to zero. It happened because

 field resistance was higher than critical resistance
 there was a break in the armature circuit
 field winding was wrongly connected
 there was no residual magnetism in the field circuit

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