Question

What is a back EMF in a DC Motor?

Answer 1

Hint: The conversion of energy in a motor takes place when a current carrying coil rotates due to the torque acting on the coil due to the magnetic field produced by the electromagnets. But as the coil rotates in the magnetic field, the magnetic flux associated with coil changes and an EMF is induced in the coil.

Complete Step-by-Step solution:
A motor is a device that converts electrical energy into mechanical energy. When a coil carrying a current I is present in a magnetic field B produced by a pair of electromagnets, it experience a torque given by the formula,
$Torque\left( \tau \right)=l(I\times B)$
Where, $l$ is the length of the loop.
$I$ is the current flowing through the coil.
B is the magnetic field produced by the electromagnets.
So this torque acting on the coil rotates the coil as a result the axle connected to the coil starts rotating which completes the transfer of electrical energy into mechanical energy.
But when the coil rotates the number of magnetic field lines passing through the coil varies as a result, the magnetic flux associated with the coil changes.
We know that according to Maxwell’s Induction law, an EMF is induced in a coil whenever there is a change in the magnetic flux associated with the coil. The EMF induced in this way will always try to counteract the source which produced the change in magnetic flux.
So in the case of the DC motor, an EMF is induced on the coil, which tries to counteract the potential that is causing the current to flow through the coil. So this so-called EMF which tries to reduce the applied voltage in the coil is called the counter EMF or the Back EMF.
So the potential difference across the coil is determined by the external voltage applied and the back EMF which is induced.
${{\text{V}}_{\text{coil}}}={{\text{V}}_{\text{ext}}}-{{\text{V}}_{\text{induced}}}$

Note: The back EMF even though it opposes the external voltage in the circuit is advantageous in many ways.
A) When a back EMF is induced it opposes the voltage applied across the coil. So, the external voltage which is responsible for the current in the coil should do work against the induced EMF in order to maintain the current flowing through the coil. So, the work done in this way is converted into mechanical energy which is used to rotate the coil.

B) The back EMF is responsible for making the DC motor a self-regulatory machine. Since the induced EMF depends on the current flowing through the coil, a sudden rise or drop in current is compensated by the induced EMF, which increases or decreases according to the need and hence provides a regulatory method for the motor.