Back EMF in DC Motor

As the armature of the DC motor starts rotating, the magnetic flux inthe air gap, which is responsible for their rotation, is cut by armature conductors and consequently an e.m.f. is induced in them in accordance with Faraday's law of electromagnetic induction.

The direction of this induced emf is such that it always opposes the cause of its production. The emf is induced because of cutting of the magnetic flux, the flux is cut because of rotating motion, the rotating motion is because of force on the rotor or armature, the force is because of armature current and armature current is because of applied voltage. Hence the main cause of emf is the applied voltage. Therefore, the induced emf opposes the cause of its production i.e applied voltage. Due to its opposing nature, it is termed as back emf E₁, or counter e.m.f.

Significance Of Back Emf In D.C. Motor:

Armature current, la =  (V – Eb)/Ra

i) If the motor is suddenly loaded, the first effect is to cause the armatudown.slow down. Therefore the speed of armature is reduced. Hence back emf falls. This allows a larger current to flow through armature and produces 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.

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.

iv) It also decides the output mechanical power.