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

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

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Electrical Machines
When the supply terminal of a DC shunt motor are interchanged

 The motor will run much faster in the same direction.
 The direction of rotation will reverse.
  The motor will run at its normal speed in the same direction as before.
 The motor will stop.

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Electrical Machines
Consider the following statements regarding the speed control of dc motors : 1. ward-leonard method is suitable forconstant-torque drives 2. Ward-leonard method is suitable for constant-power drives 3. field-control method facilitates speed control below base speed 4. Armature-resistance control method is more efficient than ward-leanard method 5. Field-control method is suitable for constant-torque drives 6. Armature-resistance control method is suitable for constant-torque drives From these, the correct answer is

 1, 2, 6
 1, 4, 6
 1, 2, 5
 1, 3, 5

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