A cumulatively compounded dc motor delivers rated load power at rated speed.
If the series field is short-circuited, then
A. both armature current Ia and speed ωm increase
B. Ia remains constant but ωm increase
C. Ia increases and ωm decreases
D. both decrease
Answer: B
Share your understanding of this question with the correct explanation.
If the series field of a cumulatively compounded DC motor is short-circuited, it means that the series field current becomes zero. In this case, the motor essentially becomes a shunt motor.
In a shunt motor, the speed is inversely proportional to the armature current. When the series field is short-circuited, the armature current remains constant, but the speed of the motor increases. This is because the absence of the series field current reduces the magnetic field strength, resulting in a higher speed of rotation.
Therefore, the correct answer is B. The armature current (Ia) remains constant, but the speed (ωm) increases when the series field is short-circuited in a cumulatively compounded DC motor.