Counter-electromotive force
Encyclopedia
The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF) is the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

, or electromotive force
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is the effect of Lenz's Law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...

 of electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

. Back electromotive force is a voltage that occurs in electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

s where there is relative motion between the armature
Armature (electrical engineering)
In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine–generally in a motor or generator, but it may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....

 of the motor and the external magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. One practical application is to use this phenomenon to indirectly measure motor speed and position. Counter EMF is a voltage developed in an inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

 network by a pulsating current
Pulsating direct current
A 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 current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 . 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
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

, 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
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...

.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force". (by Lenz's law
Lenz's law
Lenz's law is a common way of understanding how electromagnetic circuits must always obey Newton's third law and The Law of Conservation of Energy...

.) 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 and hold it momentarily and notice the idle RPM drop. The electric motor in the door is stationary and therefore the inrush current
Inrush current
Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...

 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.

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
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, or any material (usually metallic) having a low electrical resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...

, and the nearby stationary parts of the motor should be relatively free of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, most types of steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, or any material which has a high magnetic hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

.

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