Question

The back emf in a DC motor is maximum when
A.)The motor has picked the maximum speed
B.)The motor has just started moving
C.)The speed of the motor is still on the increase
D.)The motor has just been switched off

Answer 1

Hint: Define the working of the DC motor. Define what is back emf and how it is produced. Obtain the properties of the DC motor. The back emf of a dc motor is dependent on the speed of the motor i.e. on the angular speed of the motor Then we can find the answer to this question.

Complete step by step answer:
When we place a current carrying conductor in a magnetic field, torque is induced in the conductor. Due to this torque the conductor rotates in the magnetic field and hence the magnetic flux through the conductor changes. This process produces an emf or electromotive force on the conductor. This induced emf is in the opposite direction to the applied voltage and hence it is called the back emf.

The back emf produced in a DC motor is directly proportional to the angular speed of the motor.

So, we can mathematically write that, $emf\propto \omega$, where $\omega$ is the angular speed of the motor.

So, with the increase of the speed of the motor the back emf produced will also increase and with the decrease of the speed of the motor the back emf will also decrease. So, we can say that the back emf in a DC motor will be maximum when the motor has picked the maximum speed.
The correct option is (A).

Note: When the motor starts at first the back emf is zero. Even if the motor gets full voltage and current flow, if the motor is not turning the back emf will be zero. With the increase in the speed of the motor the back emf will also increase and when the motor picks up the maximum speed the back emf will be maximum.