Electrical Engineering XYZ MCQ
A DC cumulatively compounded motor delivers rated load torque at rated speed. If the series field is short-circuited, then the armature current and speed will respectively:
- Both will decrease
- Decrease and increase respectively
- Both will increase
- Increase and decrease respectively
Correct answer: 3. Both will increase
Explanation:
In a cumulatively compounded DC motor, both shunt and series field windings are connected in such a way that their magnetic effects add up. This type of motor is designed to provide a high starting torque and good speed regulation.
When the series field is short-circuited in a cumulatively compounded motor, the total field strength decreases because the series field contributes to the overall magnetic field. With the series field short-circuited, the total field strength is reduced, leading to a decrease in back electromotive force (EMF) in the armature.
As a result:
- Armature Current: The decrease in back EMF causes an increase in armature current, as the motor tries to maintain the same mechanical power output.
- Speed: Since speed is inversely proportional to back EMF, a decrease in back EMF leads to an increase in speed.
Therefore, both the armature current and speed will increase when the series field is short-circuited in a cumulatively compounded DC motor.