Industry Information >Back Electro-Magnetic field (EMF) in DC Motors

# Back Electro-Magnetic field (EMF) in DC Motors

Back EMF is produced due to rotating coils inside the magnetic field of the stator. It has the same unit as Voltage i.e. volt. Let's briefly discuss the characteristics of back EMF in DC motors.

How EMF is Produced in DC motor

The DC motor has a magnetic field due to a permanent magnet or an electromagnet. When voltage is supplied to the armature, the voltage is produced across the armature winding which opposes the flow of armature current. Due to the interacting magnetic fields, the armature is forced to rotate. The voltage produced across the armature is termed as back EMF or counter EMF in DC motor. Back EMF is always directed opposite to supply voltage V. To keep the armature of DC motor rotating the supply voltage has to force current against the back EMF through the armature windings. Back EMF is given by Mathematical Expression as

Eb = ΦNZ/60 *P/A

Where, Flux / Pole = Φ, Armature Speed = N, Total number of armature conductor = Z, Number of parallel paths in the Armature winding = A. Figure below shows a DC motor with all parts labeled for understanding how the back emf is produced inside the DC motor. Thus energy conversion in a DC motor is only because of the production of this BACK EMF. When DC motor is running at no-load, the voltage supplied is almost equal to back EMF produced in a DC motor. This is because a very small torque (Ta=KIa) is required by the DC motor to overcome the friction and winding resistance.

What can we conclude from the above discussion is that back EMF plays a dynamic role in running the DC motor. There is one another thing which back EMF does i.e. it makes the motor self-regulating. Well, how does it make a DC motor self-regulating, let's examine by the following cases?

BACK EMF make DC Motor self-regulating

Let’s suppose the motor is operating at no-load condition. At this stage, the DC motor requires only small torque for regulating the friction and windings losses. So, DC motor extracts less current. Both back EMF and current in DC motors depend on one another, so as current decreases back EMF value also decreases. The magnitude of the back EMF at this stage is approximately comparable to the supplied voltage.

The DC motor slows down when a sudden load is applied at the shaft of the DC motor. As the speed of DC motor decreases, the magnitude of the BACK EMF also been decreased. The small BACK EMF extracts heavy current from the power source. The armature current induces the greater torque in the armature, which is what DC motor requires. Thus, the DC motor functions continuously at the new speed.

Let the load on the DC motor is decreased suddenly, in this case, the driving torque on the DC motor will be greater than the load torque. The driving torque increases the speed of DC motor which also increases the BACK EMF.  Armature current is decreased by this high value of BACK EMF. So, the small value of armature current produces driving torque whose value is equal to the load torque. As a result, the DC motor will rotate uniformly at this new speed.

Hence, back EMF generated runs the DC motor by regulating the armature current to meet the load requirements. We will see some of the advantages of back EMF now.

Advantages of Back EMF in DC Motor

Armature rotated when the voltage supplied at the terminals of coils induces current in DC motor. The electrical work required for causing the current against the BACK EMF in DC motor is converted into mechanical energy. And that energy is induced in the armature of the motor. Thus, we can say that energy conversion in the DC motor is possible only because of the BACK EMF.

The back emf makes the DC motor self-regulating, i.e., it run the DC motor runs at a constant speed from no-load condition to maximum load condition. DC motor does so by developing the armature current according to the load applied to the motor. The armature current of the motor is calculated in the below equation. The figure below also shows armature current in DC motor.

I=(V-Eb)Ra Summary

Back EMF is important for running DC motors without fluctuating and responding to the output load efficiently.