DC Motors
If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

This will make armature to take heavy current, possibly burning it
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
Nothing will happen to motor
Motor will come to stop

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DC Motors
As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

A resistance is connected parallel to the armature
A high value resistor is connected across the field winding
A resistance is connected in series with armature
Armature is temporarily open circuited

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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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