Discussion
In a dc motor, if the field coils get opened, the speed of the motor will :
Answer: C
If the field opens, the speed of a DC motor will increase, dangerously.
The speed of a DC motor is
\[ N \propto \frac{E_{b}}{\phi} = \frac{V - I_{a}R_{a}}{\phi} \]
When the field circuit of a shunt motor opens, the flux falls to the small residual value. The back emf collapses, the armature current rises sharply, and to restore the back emf the motor races to a very high speed. Unloaded, it can run away and damage itself, which is why field-failure protection is fitted.
The speed does not decrease, stop or become zero. Those would follow only if no torque were produced at all, but the residual flux and the very large armature current still produce enough torque to accelerate the motor.
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