Back EMF of DC Motor | Importance and Formula

Before you understand the concept of Back EMF in a DC motor, you must know the simple construction and working principle of a DC Motor. So let's know the working and construction of the DC motor.

Generally, the DC motor works on the principle of electromagnetic attraction and repulsion. A DC motor has mainly two parts - a Stator and a Rotor. Stator and Rotor both have magnetic coils that are capable of creating the electromagnetic effect when an electric current flows through them.

So when the power supply is applied to the both stator and rotor(armature), electromagnetic flux is produced in the both stator and rotor coils. Due to the attraction and repulsion between the stator and rotor flux, a torque will be produced on the rotor or armature and tends to rotate. This is the basic concept of the DC motor working.

What is Back EMF in DC Motor?

The electromotive force or EMF produced in the armature winding of a DC motor in the opposite direction to the applied voltage is called Back EMF. In fact, as the direction of Back EMF is opposite to the applied voltage and gives back the energy towards the energy source, that's why it is called back EMF. It is also known as Counter EMF.

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How Back EMF is produced in DC Motor?

When the rotor of the motor is rotating then the coil of the rotor or armature cuts or links with the fluxes produced by the stator field. We know that when magnetic flux links with a movable conductor an EMF will induce in that conductor. When this happens in a DC motor, it is known as Back EMF.

Remember that when the rotor is rotating, that time only EMF will be induced. Suppose both the stator and rotor are connected to the power supply and they produce magnetic flux also. In this case, due to attraction and repulsion between the stator and rotor flux, a torque will be produced on the rotor that will try to force the rotor to rotate. But until the rotor is constant no back emf will be induced. So, once the rotor starts rotating, back emf is also induced.

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Characteristics and Properties of Back EMF

1. Back EMF always has the opposite direction to the Supplied Voltage.

2. Back EMF is always lower than the supply voltage.

3. Back EMF is always induced when the motor is running, there is no back EMF induced when the motor is not running.

4. Back EMF is dependent upon the speed of the rotor also, more speed more back EMF.

5. Back emf also depends upon the strength of the magnetic flux produced by the stator field.

6. Back emf varies with the variation of load on the motor.

7. One thing is to be noted the back emf of a DC motor determines or controls the current drawn by the motor. If the back emf is more, the current consumption will be less and if the back emf is low the current consumption will be more.

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Equation and Formula of Back EMF

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Importance and Significance of Back EMF

The main importance or significance of Back EMF is, that it makes the DC motor a Self Regulating Machine. Now the question is what is self-regulating? Self-regulating means, the dc machine can create and control automatically the torque, speed, and current consumption with the variation of load on it.

Let's understand it

When a DC motor runs with a constant load, it runs at a constant speed, so the back emf also constant, and the current-carrying by the motor is also constant.

If the load on the motor suddenly increases, the speed and torque will fall down. In this case, the back emf also below. So the motor will draw a higher current than before. This higher current will help to produce more torque on the motor to drive the new load with the same speed as before.

Now, if the load is suddenly decreased, the speed of the motor will increase. So the back emf also be increased. As we know when the back emf increases, the current consumption by the motor will decrease. So when current consumption decreases, the torque also be decreased, and therefore the speed of the motor also be decreased to before normal speed.

 

So the conclusion is that the back emf helps the DC motor run at a constant speed at any variation of load.

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