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.
To experience the effect of counter-electromotive force one can perform this simple exercise. With a window closed, lift the switch of an electric window in a car that is running at idle, and hold it momentarily and notice the idle RPM drop. The electric motor in the door is stationary and therefore the inrush current will be very high. The alternator will try to provide for the large current which subsequently drags down the engine. As soon as the power window motor overcomes its inertia and starts spinning, back EMF will be produced, exerting less load on the alternator. Hence, the engine speed will return to normal.