# Counter-electromotive force

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Counter-electromotive force

The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF)[1] 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.[2] Counter EMF is a voltage developed in an inductor network by a pulsating current or an alternating current [1]. The voltage's polarity is at every moment the reverse of the input voltage.[1][3]

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.[4]

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 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.

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.[5]

## Efficiency

Because back EMF increases the apparent resistance of the circuit by providing an opposing voltage, an electric motor that is already spinning consumes less power than one that is starting or changing direction. In order to maximize this effect, the motors coils should utilize copper, silver, or any material (usually metallic) having a low electrical resistivity, and the nearby stationary parts of the motor should be relatively free of iron, most types of steel, or any material which has a high magnetic hysteresis.

## References

1. ^ a b c Graf, "counterelectromotive force", Dictionary of Electronics
2. ^ "Back-EMF". Acroname.com. 2006-08-07. Retrieved 2010-02-07.
3. ^ Naval Electrical Engineering Training Series, Module 02 - Introduction to Alternating Current and transformers", Inductance, self-inductance
4. ^ "Nuclear Power Fundamentals Training Manuals". DC Generators, Counter-Electromotive Force (CEMF), DC Equipment Terminology, Electrical Science Volume 2.
5. ^ "Back EMF Motion Feedback" Back EMF Motion Feedback

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### Look at other dictionaries:

• counter electromotive force — noun Etymology: counter (VI) : the electromotive force that develops in some circuits from chemical or magnetic effects of the current and that opposes the impressed electromotive force producing the current called also back electromotive force * …   Useful english dictionary

• counter electromotive force — Elect. an electromotive force that is created by a chemical or magnetic effect upon a circuit and that acts in opposition to the applied electromotive force of the circuit. Also called back electromotive force. * * * …   Universalium

• back electromotive force — noun : counter electromotive force * * * Elect. See counter electromotive force. [1890 95] …   Useful english dictionary

• back electromotive force — Elect. See counter electromotive force. [1890 95] * * * …   Universalium

• counter voltage — noun Etymology: counter (VI) : counter electromotive force …   Useful english dictionary

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• противо-электродвижущая сила — противо эдс — [Л.Г.Суменко. Англо русский словарь по информационным технологиям. М.: ГП ЦНИИС, 2003.] Тематики информационные технологии в целом Синонимы противо эдс EN counter electromotive forceopposing electromotive force …   Справочник технического переводчика

• siemens's law — ˈsēmənz(ə̇z) , ˈzē noun Usage: usually capitalized S Etymology: after Werner von Siemens died 1892 German electrical engineer and inventor, its formulator : a statement in electricity: the greater the ratio of counter electromotive force to… …   Useful english dictionary