Electrical Machines
The armature mmf of a DC machine has

  triangular space distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.
  triangular space distribution and is stationary in space.
 stepped distribution and rotates at the speed of armature.

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

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