DC Motors
If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking

None of these
Plugging
Regenerative
Dynamic braking

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

Nothing will happen to the motor
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

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