Question

When speed of a dc motor increases, the armature current:
A. Increases
B. Decreases
C. Does not change
D. Increases and decreases continuously

Answer 1

Hint: Armature current is the component in an electric machine which carries alternating current. So, to find the relation between armature current and current, firstly use the relation between torque and speed. Then, consider the relation between torque and armature current. Equate both the relations and then find the relation between speed and armature current of a dc motor.

Complete answer:
A DC motor is a device which converts electrical energy such as voltage and current into mechanical energy like speed.
Armature conducts alternating current. Even on DC motors, armature conducts alternating current.
Greater is the load on the motor, higher the torque is required to move it. So, when load increases, torque has to be increased too. To increase the torque, we have to increase the armature current. Thus, it can be inferred torque is directly proportional to current. It can be expressed as,
$\tau \propto I_a$ ...(1)
But, we know, torque is inversely proportional to speed which can be expressed as,
$\tau \propto {\displaystyle \frac{1}{v}}$ ...(2)
From the equation. (1) and equation. (2) we get,
$v\propto {\displaystyle \frac{1}{I_a}}$ ...(3)
From the equation. (3) we can say, speed is inversely proportional to armature current. Thus, when the speed of the dc motor is increased, the armature current will decrease.

So, the correct answer is “Option B”.

Note:
In DC motors, speed can be controlled by changing the armature current. While in the case of AC motor, speed can be controlled by changing the frequency. DC motors are widely used in small domestic appliances while AC motors are used in large industries. DC motors require more maintenance as compared to AC motors. In DC motor, armature rotates while in AC motor armature is stationary.