DC Motors
In case the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be

1
Anywhere between 75% and 90%
Around 90%
Less than 50%

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

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
Nothing will happen to the motor

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