DC Motors
The speed of a dc motor can be controlled by changing

Aplied voltage
Is flux
Armature circuit resistance
All of the listed here

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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
Motor will run at very slow speed
This will make armature to take heavy current, possibly burning it
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

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