Back emf: When the armature of DC motor rotates under the fixed magnetic field and cuts it, by Faraday's law an 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.
The significance of back emf:
i) If the motor is at standstill or rest Eb is zero. This causes large current flow through armature, which produce high starting torque. ii) When motor takes speed, the back emf increases, causes armature current to decrease hence decrease in torque. iii)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.