The correct answer is A. This will make armature to take heavy current, possibly burning it.
When the shunt field circuit of a DC shunt motor suddenly opens, the field current decreases to zero. This causes the magnetic field to collapse, which in turn causes the armature current to increase. The increased armature current causes the motor to speed up. If the armature current is allowed to increase too much, the armature windings may overheat and burn out.
To prevent this from happening, a current-limiting device is usually connected in series with the armature. This device will limit the armature current to a safe value, even if the field current is zero.
Here is a more detailed explanation of each option:
- Option A: This will make armature to take heavy current, possibly burning it.
When the shunt field circuit of a DC shunt motor suddenly opens, the field current decreases to zero. This causes the magnetic field to collapse, which in turn causes the armature current to increase. The increased armature current causes the motor to speed up. If the armature current is allowed to increase too much, the armature windings may overheat and burn out.
- Option B: This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses.
This is not correct. The armature current will increase, but the speed will not increase excessively. The current-limiting device will limit the armature current to a safe value, even if the field current is zero.
- Option C: Nothing will happen to motor.
This is not correct. The armature current will increase, and the motor will speed up. If the armature current is allowed to increase too much, the armature windings may overheat and burn out.
- Option D: Motor will come to stop.
This is not correct. The motor will not come to stop. The armature current will increase, and the motor will speed up. If the armature current is allowed to increase too much, the armature windings may overheat and burn out.