Electrical Machines
A 240 V dc series motor takes 40 A when giving its rated output at 1500 rpm. Its resistance is 0.3 Ω. The value of resistance which must be added to obtain rated torque at 1000 rpm is

 2.2 Ω
 6 Ω
 5.7 Ω
 1.9 Ω

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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

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

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