DC Motors
If a dc motor is comnected across the AC supply it will

Run at lower speed
Not run
Burn due to heat produced in the field winding by eddy currents
Run at normal speed

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

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

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