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 the voltage drop in an alternating current (AC) circuit caused by magnetic induction (see Faraday's law of induction, electromagnetic induction, Lenz's Law). For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil, and is equal to the current divided by the impedance of the inductor  . The voltage's polarity is at every moment the reverse of the input voltage . Back electromotive force also refers to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the external magnetic field. From Faraday's law, the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor. One practical application is to use this phenomenon to indirectly measure motor speed and position.
In a motor using a rotating armature 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, reducing the current load on the alternator. Hence, the engine speed will return to the normal operation.
In motor control and robotics, the term "Back-EMF" often refers to using the voltage generated by a spinning motor to infer the speed of the motor's rotation.
- Graf, "counterelectromotive force", Dictionary of Electronics
- Griffiths, David Jeffery. (2012). Introduction to Electrodynamics (3rd ed.).
- Naval Electrical Engineering Training Series, Module 02 - Introduction to Alternating Current and transformers", Inductance, self-inductance
- "Back-EMF". Acroname.com. 2006-08-07. Retrieved 2010-02-07.
- "Nuclear Power Fundamentals Training Manuals". DC Generators, Counter-Electromotive Force (CEMF), DC Equipment Terminology, Electrical Science Volume 2.
- "Back EMF Motion Feedback" Back EMF Motion Feedback