DC Motors
For a D.C. shunt motor if the excitation is changed

Torque, power and speed, all will change
Torque will remain constant
Torque will change but power will remain constant
Torque and power both will change

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

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
Motor will come to stop

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