DC Motors
In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

50% of the previous value
10% of the previous value
25% of the previous value
100% of the previous value

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DC Motors
If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
Motor will run at very slow speed
This will make armature to take heavy current, possibly burning it
Nothing will happen to the motor

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