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

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Electrical Machines
Armature reaction in a DC machine

  makes an angle of 90° with the main pole axis.
 is in direct opposition to the main poles.
 is in the same direction as the main poles.
 makes an angle with the main pole axis which is load dependent.

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