Electrical Machines
A large series motor is never started without some mechanical on it because otherwise it will

  Develop excessive speed and damage itself.
 Produce sparking at brushes.
 Draw too much current.
 Open fuse or circuit breaker.

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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
 there was no residual magnetism in the field circuit
 field winding was wrongly connected

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