In a DC motor when the armature conductors rotates between the poles of magnet, cut the magnetic flux, opposes the current flowing through the conductors due to which an emf get induced in conductors and that emf is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.
Significance of back emf- As at starting, the back emf is zero so current is very high. It is controlled by adding resistor in series with the armature circuit. Once the motor starts running, resistance is cut gradually and are made zero when the motor attains the full speed.
As we know induced emf is directly proportional to speed of motor. So at full speed the induced emf controls the armature current as
Ia= (V−E)/Ra
Hence copper loss decreases across the armature.