Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Back EMF

The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF) is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is the effect of Lenz's Law of electromagnetism. Back electromotive force is a voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. One practical application is to use this phenomenon to indirectly measure motor speed and position. Counter EMF is a voltage developed in an inductor network by a pulsating current or an alternating current . The voltage's polarity is at every moment the reverse of the input voltage.

In a motor using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil; the motor is acting like a generator. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force" (by Lenz's law). With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.

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