Back Electromotive Force (Back emf)

Definition - What does Back Electromotive Force (Back emf) mean?

Back Electromotive Force, also known as counter-electromotive force is the voltage that occurs in an electric motor when it spins. This voltage or electromotive force is actually supplied by an external power source and it pushes against the current which induces it. Back emf results in a voltage drop in the AC circuit of an electric motor caused by magnetic induction.

Petropedia explains Back Electromotive Force (Back emf)

A DC motor or any type of motor has motor windings that resist sudden changes in the alternating current because of magnetic induction. This happens because the magnetic field gets developed in the motor windings due to alternating current passing through these windings. This induced magnetic field takes time to build up or decrease in intensity. Thus, whenever the supply of an alternating current is stopped, the motor winding tries to maintain the current flowing through the winding and becomes a source of power that generates a voltage in order to maintain this current. In such scenario, the motor is converted into power source instead of power consumer and thus, the voltage is reversed for the same current which was earlier flowing in the same direction. This voltage that is generated by motor is called back emf.

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