DC Motor MCQ part-02
#1. The speed of a DC motor is controlled by varying the armature voltage. This method is primarily used for obtaining speeds:
Right answer
Armature voltage control maintains constant torque and is used for speeds below the base speed.
#2. Which part of the DC motor is responsible for providing the main magnetic field?
Right answer
The field poles, with their associated field windings, produce the main working magnetic flux.
#3. If a DC series motor’s load torque is quadrupled (4 times), the armature current will approximately become:
Right answer
Torque is proportional to I2a (T∝ I2a). If T is quadrupled (4T), then Ia must be doubled (4Ia = 2Ia)
#4. To reverse the direction of rotation of a DC motor, you must reverse the connections of:
Reversing the current flow in either the armature or the field (but not both) reverses the direction of the force and thus the torque.
#5. Which of the following speed control methods for a DC shunt motor gives speeds above the rated speed?
Since N∝1/Φ, decreasing the field flux (weakening the field) increases the speed above the base speed.
#6. What is the function of the yoke (frame) of a DC machine?
Right answer
The yoke is made of high-permeability cast iron or steel and completes the magnetic circuit, providing a path for the main flux.
#7. A DC long shunt compound motor has its series field connected in series with the:
A DC compound motor has two field windings: a series field and a shunt field. In a long shunt configuration, the series field is in series with the line, and the shunt field is parallel to the series combination of armature and series field.
#8. The efficiency of a DC motor is defined as:
Right answer
Since Output Power = Input Power − Losses, this is the correct definition for efficiency η = Output/Input
#9. If the terminal voltage of a DC shunt motor is halved, while the load torque is maintained, the speed will approximately:
The speed is roughly proportional to Eb. Halving V roughly halves Eb, leading to a halving of the speed, while the armature current slightly increases to maintain torque (T ∝ ΦIa).
#10. Interpoles (or commutating poles) in a DC motor are used to neutralize the effect of:
Interpoles are placed in the geometric neutral axis and neutralize the cross-magnetizing effect of armature reaction, thus improving commutation.