Counter-electromotive force: Difference between revisions
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The '''counter-electromotive force''' also known as '''back electromotive force''' (abbreviated '''counter emf''', or '''CEMF''' ) <ref name="Graf">Graf, "counterelectromotive force", Dictionary of Electronics </ref> is the [[voltage]], or [[electromotive force]], that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is represented by [[Lenz's Law]] of [[electromagnetism]]. '''Back electromotive force''' is a voltage that occurs in [[electric motor]]s where there is relative motion between the [[armature (electrical engineering)|armature]] of the motor and the external [[magnetic field]]. One practical application is to use this phenomenon to indirectly measure motor speed and position.<ref>{{cite web|url=http://www.acroname.com/robotics/info/articles/back-emf/back-emf.html |title=Back-EMF |publisher=Acroname.com |date=2006-08-07 |accessdate=2010-02-07}}</ref> ''Counter emf'' is a voltage developed in an [[inductor]] network by a [[pulsating direct current|pulsating current]] or an [[alternating current]] <ref name="Graf"/>. The voltage's [[Polarity (physics)|polarity]] is at every moment the reverse of the input voltage.<ref name="Graf"/><ref>[[Naval Electrical Engineering Training Series]], Module 02 - Introduction to Alternating Current and transformers", ''Inductance, [http://www.tpub.com/neets/book2/2a.htm self-inductance]''</ref> |
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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.<ref>"Nuclear Power Fundamentals Training Manuals". DC Generators, [http://www.tpub.com/content/doe/h1011v2/css/h1011v2_79.htm Counter-Electromotive Force] (CEMF), DC Equipment Terminology, Electrical Science Volume 2. |
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.<ref>"Nuclear Power Fundamentals Training Manuals". DC Generators, [http://www.tpub.com/content/doe/h1011v2/css/h1011v2_79.htm Counter-Electromotive Force] (CEMF), DC Equipment Terminology, Electrical Science Volume 2. |
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Revision as of 07:46, 16 August 2010
lmAO
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.[1]
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, gold, 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
- ^ "Nuclear Power Fundamentals Training Manuals". DC Generators, Counter-Electromotive Force (CEMF), DC Equipment Terminology, Electrical Science Volume 2.