According to the faradays laws of electromagnetic induction, E.M.F. is induced in the armature winding. And as per Lenz’s law, this induced E.M.F. is always act in opposite direction to the supply voltage. Therefore this E.M.F. is called as Back EMF (Eb).
As the supply applied to the dc motor, armature winding of dc machine start rotating. Armature conductors move in the magnetic field which is responsible for rotation of armature.
This armature conductor cut these flux and consequently an EMF is induced in it accordance with Faraday’s law of electromagnetic induction.
This induced EMF always acts in opposition with the applied voltage (V) as per Lenz’s law and this E.M.F. is known as back E.M.F. or counter E.M.F. of DC motor. It is denoted by Eb.
Since the Eb opposes the applied voltage across the armature.
Mathematical representation of Eb is same as that of EMF induced in DC generator.
Significance / Important
Question: Explain significance of Back EMF in DC motor.
In the running condition, (Eb) is nearly equal to supply voltage (V).
As the internal resistance of the armature winding of a DC motor being very low. The back e.m.f. which is mainly limits the armature current in the running condition of the motor.
If load on the motor is suddenly increases, the first effect is to cause the armature speed is reduced. Hence back emf is also falls down.
This allows a larger current flow through armature winding, it means increased driving torque thus the driving torque increases as motor slows down.
If load on motor is decreased, the driving torque is momentarily increases armature speed, which increases Eb and causes armature current to decrease.
The Eb is proportional to the speed (N). It is zero at standstill. That means back emf opposes the supply voltage and limits the armature current to a safe value.
If Eb is not present then the armature winding is burn out and machine get damaged.
That’s why Back EMF is very important in dc machine.