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

Motor will run at very slow speed
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 the motor

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DC Motors
The speed of a dc motor is

inversely proportional to applied voltage
inversely proportional to flux per pole
None of the listed here
Directly proportional to flux per pole

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