DC Motors
In variable speed motor

None of these
Same commutating field is needed at low speed than at high speed
A weaker commutating field is needed at low speed than at high speed
A stronger commutating field is needed at low speed than at high speed

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

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