DC Motors
In a dc motor if the back emf is absent

Motor will run at very slow speed
Motor will burn
Motor will run at very high speed
Motor will not run at all

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

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

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