The
counter-electromotive force (abbreviated
counter emf, or
CEMF ) is the
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
, or
electromotive forceIn physics, electromotive force, or most commonly emf , or electromotance is "that which tends to cause current to flow."...
, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is represented by
Lenz's LawLenz's law is an extension of the law of conservation of energy to the non-conservative forces in electromagnetic induction. It can be used to give the direction of the induced electromotive force and current resulting from electromagnetic induction...
of
electromagnetismElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
.
Back electromotive force is a voltage that occurs in
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
s where there is relative motion between the
armatureIn electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine– a motor or generator, but may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay. The...
of the motor and the external
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
. One practical application is to use this phenomenon to indirectly measure motor speed and position .
The
counter-electromotive force (abbreviated
counter emf, or
CEMF ) is the
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
, or
electromotive forceIn physics, electromotive force, or most commonly emf , or electromotance is "that which tends to cause current to flow."...
, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is represented by
Lenz's LawLenz's law is an extension of the law of conservation of energy to the non-conservative forces in electromagnetic induction. It can be used to give the direction of the induced electromotive force and current resulting from electromagnetic induction...
of
electromagnetismElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
.
Back electromotive force is a voltage that occurs in
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
s where there is relative motion between the
armatureIn electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine– a motor or generator, but may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay. The...
of the motor and the external
magnetic fieldMagnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges...
. One practical application is to use this phenomenon to indirectly measure motor speed and position .
Counter emf is a voltage developed in an
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
network by a
pulsating currentA pulsating direct current is a direct current that changes in value at regular or irregular intervals.A pulsating direct current may change in value, i.e., be always present but at different levels, or it may be a current that is interrupted completely at regular or irregular intervals, but when...
or an
alternating currentIn alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again...
. The voltage's
polarityIn physics, polarity is a description of an attribute, typically a binary attribute , or a vector . For example:* An electric charge has a polarity of either positive or negative....
is at every moment the reverse of the input voltage .
In a generator using a rotating armature and, in the presence of a
magnetic fluxMagnetic flux, represented by the Greek letter Φ , is a measure of quantity of magnetism, taking into account the strength and the extent of a magnetic field...
, 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 inductionFaraday's law of induction is a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:...
.) This voltage opposes the original applied voltage; therefore, it is called "
counter-electromotive force". (by
Lenz's lawLenz's law is an extension of the law of conservation of energy to the non-conservative forces in electromagnetic induction. It can be used to give the direction of the induced electromotive force and current resulting from electromagnetic induction...
.) With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.
Efficiency
If it is assumed that a motor is 100% efficient with no friction or
windageWindage is a force created on an object by friction when there is relative movement between air and the object.There are two possible causes of windage:# the object is moving and being slowed by resistance from the air...
losses, the speed of the armature will increase until the back electromotive force is equal to the applied electromotive force, i.e. there will be no net electromotive force, no current flow and hence, no net force. The armature will spin at a constant rate, of its own accord.