When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator The direction of this induced e.m.f. acts in opposite to the applied voltage V, (according to the Lenz’s law) and it is referred as back or counter e.m.f. Eb. The back e.m.f. Eb(= P φ ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.
The rotating armature generating the back emf Eb is like a battery of e.m.f. Eb put across a supply mains of V volts. Obviously, V has to drive Ia against the opposition of Eb . The power required to overcome this opposition is EbIa.
The current flowing through the armature is given by the relation:
When the mechanical load applied on the motor decreases, its speed decreases which reduces the value of Eb. As a result the value (V – Eb) increases which consequently increases Ia. Hence, motor draws extra current from the supply mains.
So, we find that Eb acts like a governor i.e. it makes the motor self-regulating so that it draws as much current as is just necessary.