Back emf: When the armature of DC machine rotates under the influence of driving torque, the armature conductors move in the magnetic field and cut it and hence emf is induced in them. The induced emf acts in opposite direction to the applied voltage as per Lenz‟s law. Hence known as back or counter emf Eb.
Significance of back emf:
i) If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore the speed of armature is reduced. Hence back emf falls. This allows a larger current flow through armature means increased driving torque, thus the driving torque increases as motor slows down.
ii) If load on motor is decreased, the driving torque is momentarily in excess so armature is accelerated and armature speed increases, which increases back emf and causes armature current to decrease.
It follows therefore that back emf in DC motor regulates the flow of armature current i.e. it automatically changes the armature current to meet load requirements.