Question

When the polarity of the voltage applied to a dc motor is reversed, the direction of motion does not reverse. Why not? How could the direction of motion be reversed?

Short answer

In order to change the direction, the magnetic moment of the rotor or magnetic field should also be reversed

Step-by-step solution

Magnetic moment of the rotor

For a permanent magnet on a DC motor, reverse the polarity of the voltage applies reverses the direction of the current flow.

Hence, the direction of the rotation does not change,$\tau =\left(-u\right)\left(-B\right)=uB$

Now, in order to change the direction of the rotation, the magnetic moment of the rotor or the magnetic field should also be reversed.

Magnetic moment of the motor

For a permanent magnet DC motor, the reversing of the polarity of the applied voltage does reverse the direction of the rotation. But the electromagnetic DC motor, series motors shunt motors, reversing the polarity applied voltage reverse the current that is flowing in the circuit. Hence in order to change the direction of the rotation, the magnetic moment of the rotor should be reversed.