If the field of a dc shunt motor gets opened while the motor is running, then the
A. motor will become slow.
B. motor will attain dangerously high speed
C. armature current will drop
D. armature will oscillate about original speed as the mean speed
Answer: B
Share your understanding of this question with the correct explanation.
If the field of a DC shunt motor gets opened while the motor is running, the correct answer is B. The motor will attain dangerously high speed.
When the field circuit of a DC shunt motor is open, the field current becomes zero, resulting in the loss of the magnetic field that is necessary for motor operation. Without the magnetic field, there is no force acting on the armature conductors, and the motor loses its ability to generate torque.
As a result, the motor speeds up rapidly due to the absence of the counteracting electromagnetic torque. The speed can increase to a dangerous level, which can lead to mechanical failure or other hazardous conditions.
It is important to note that when the field circuit is open, the armature current may also decrease (as mentioned in option C), but the primary concern is the rapid increase in speed (as mentioned in option B) rather than the change in armature current.