Back E.M.F.

What is Back e.m.f. ?

When the armature of a dc motor rotates under the influence of the driving torque, the armature conductor move through the mangetic field and hence e.m.f. is induced in them as in generator. The induced e.m.f. acts in opposite direction to the applied voltage V (by Lenz's Law) and is known as back e.m.f. or counter e.m.f. E_b. The value of back emf depends upon the speed of rotation of armature conductors.

Significance of Back e.m.f.

The presence of back e.m.f. makes the d.c. motor a self regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. The back e.m.f. is always less than applied voltage.

Let,

V = Applied voltage

E_b = Back e.m.f.

R_a = Resistance of armature conductor

Then the current in the armature conductor I_a at any instant is given by,

I_a = ^{Net voltage}/_Resistance

=^V-E_b/_Ra

OR V = E_b + I_aR_a 

When the motor is running on no load small torque is required to overcome the friction and windage losses. Therefore, the armature current is I_a small and the back e.m.f. is nearly equal to the applied voltage. If the motor is suddenly loaded the first effect is to cause the armature to slow down. Therefore the speed at which the armature conductors move through the field is reduced and hence the back e.m.f. E_b falls. The decreased back e.m.f. E_b allows larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.

If the load on the motor is suddenly decreased, the driving toque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increase and causes the armature current I_a to decrease as back e.m.f. increases too. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.

It follows, therefore, that back e.m.f. in d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.