Back EMF and Its Significance in DC Motor



What is Back EMF?

When the armature of a DC motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence emf is induced in them as in a generator.

The induced emf acts in opposite direction to the applied voltage  V (Lenz’s law) and is known as Back EMF or Counter EMF (Eb). 

The equation to find out back emf in a DC motor is given below,


Back Emf
Direction of Back Emf

Back Emf opposes the applied voltage and this can be proved by Lenz’s law, which states Direction of the induced emf is such that it is opposes the cause producing it. and in this case, induced emf (i.e, back emf) is produced due to applied voltage. That’s why back emf opposes applied voltage.

Value of the Back Emf is given by :

Eb = (P*Z*N)/(60*A) …….eq.1

where, P = Number of Poles in DC Motor.

Z = Total Number of Armature Conductors.

 =   Flux  per pole in webers.

N = Speed of Armature in rpm.

A =  Number of Parallel Paths for Armature Current.

For Wave Winding, A = 2
For Lap Winding, A = P (No. of poles)

Note: Value of Back Emf is always less than the applied voltage.

Net Voltage across the armature circuit = V – Eb

If R is the armature resistance, then

Armature Current =  Net Voltage across the armature circuit / Armature resistance.

I = (V – Eb) / R ……..eq2


A very important advantage of back emf is that it makes the DC Motor to draw only as much as armature current which is just sufficient to develop the torque required by the load.
Suppose if the motor is running at no load, it means that speed of the motor will be more. As a result. back emf will also increase (as from eq.1 ) and will become nearly equal (but lesser) to applied voltage. So as we can see from eq.2 , when the back emf increases then armature current will also get decreased.

If the load on motor is increased, then its speed of armature will decrease. Therefore, the speed at which armature conductors move through the field is reduced and back emf falls. This decreased back emf will allow a larger armature current to flow and driving torque will also increased.

Conclusively, back emf in DC motor regulate the flow of armature current to meet the load requirements.

READ HERE  What is a DC Machine : Construction & Its Working


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