Inductor

An inductor is a passive electrical component designed to provide inductance in a circuit.

Inductors store energy in a magnetic field created when an electric current flows through them. They are usually implemented by some sort of coiled conductive winding which may surround a magnetic core. Large inductors used at low frequencies may have thousands of turns of wire around an iron core; however even a straight piece of wire (i.e., with turns and core reduced to zero) has significant inductance.

An "ideal inductor" has inductance, but no resistance or capacitance, and does not dissipate energy. A real inductor is equivalent to a combination of a significant ideal inductance, some resistance, and capacitance, usually small. The resistance, a necessary property of a wire except at superconducting temperatures, may contribute significantly to the impedance, and may dissipate significant power. At some frequency, usually much higher than the working frequency, a real inductor behaves as a resonant circuit (due to its self capacitance).

Physics

Overview

Inductance ("L") (measured in henrys) is an effect, resulting from the magnetic field that forms around a current-carrying conductor that tends to resist changes in the current. Electric current through the conductor creates a magnetic flux proportional to the current. A change in this current creates a change in magnetic flux that, in turn, by Faraday's law generates an electromotive force (EMF) that acts to oppose this change in current. Inductance is a measure of the amount of EMF generated for a unit change in current. For example, an inductor with an inductance of 1 henry produces an EMF of 1 volt when the current through the inductor changes at the rate of 1 ampere per second. The number of loops, the size of each loop, and the material it is wrapped around all affect the inductance. For example, the magnetic flux linking these turns can be increased by coiling the conductor around a material with a high permeability such as iron. This can increase the inductance by 2000 times, although less so at high frequencies.

Hydraulic model

Electric current can be modeled by the hydraulic analogy. An inductor can be modeled by the flywheel effect of a heavy turbine rotated by the flow. When water first starts to flow (current), the stationary turbine will cause an obstruction in the flow and high pressure (voltage) opposing the flow until it gets turning. Once it is turning, if there is a sudden interruption of water flow the turbine will continue to turn by inertia, generating a high pressure to keep the flow moving. Magnetic interactions such as in transformers are not modeled hydraulically.

Applications

Inductors are used extensively in analog circuits and signal processing. Inductors in conjunction with capacitors and other components form tuned circuits which can emphasize or filter out specific signal frequencies. This can range from the use of large inductors as chokes in power supplies, which in conjunction with filter capacitors remove residual hum or other fluctuations from the direct current output, to such small inductances as generated by a ferrite bead or torus around a cable to prevent radio frequency interference from being transmitted down the wire. Smaller inductor/capacitor combinations provide tuned circuits used in radio reception and broadcasting, for instance.

Two (or more) inductors which have coupled magnetic flux form a transformer, which is a fundamental component of every electric utility power grid. The efficiency of a transformer may decrease as the frequency increases due to eddy currents in the core material and skin effect on the windings. Size of the core can be decreased at higher frequencies and, for this reason, aircraft use 400 hertz alternating current rather than the usual 50 or 60 hertz, allowing a great saving in weight from the use of smaller transformers.

An inductor is used as the energy storage device in some switched-mode power supplies. The inductor is energized for a specific fraction of the regulator's switching frequency, and de-energized for the remainder of the cycle. This energy transfer ratio determines the input-voltage to output-voltage ratio. This "X"_L is used in complement with an active semiconductor device to maintain very accurate voltage control.

Inductors are also employed in electrical transmission systems, where they are used to depress voltages from lightning strikes and to limit switching currents and fault current. In this field, they are more commonly referred to as reactors.

As inductors tend to be larger and heavier than other components, their use has been reduced in modern equipment; solid state switching power supplies eliminate large transformers, for instance, and circuits are designed to use only small inductors, if any; larger values are simulated by use of gyrator circuits.

Inductor construction

An inductor is usually constructed as a coil of conducting material, typically copper wire, wrapped around a core either of air or of ferromagnetic material. Core materials with a higher permeability than air increase the magnetic field and confine it closely to the inductor, thereby increasing the inductance. Low frequency inductors are constructed like transformers, with cores of electrical steel laminated to prevent eddy currents. 'Soft ferrites' are widely used for cores above audio frequencies, since they don't cause the large energy losses at high frequencies that ordinary iron alloys do. This is because of their narrow hysteresis curves, and their high resistivity prevents eddy currents. Inductors come in many shapes. Most are constructed as enamel coated wire wrapped around a ferrite bobbin with wire exposed on the outside, while some enclose the wire completely in ferrite and are called "shielded". Some inductors have an adjustable core, which enables changing of the inductance. Inductors used to block very high frequencies are sometimes made by stringing a ferrite cylinder or bead on a wire.

Small inductors can be etched directly onto a printed circuit board by laying out the trace in a spiral pattern. Some such planar inductors use a planar core.

Small value inductors can also be built on integrated circuits using the same processes that are used to make transistors. In these cases, aluminium interconnect is typically used as the conducting material. However, practical constraints make it far more common to use a circuit called a "gyrator" which uses a capacitor and active components to behave similarly to an inductor.

In electric circuits

An inductor opposes changes in current. An ideal inductor would offer no resistance to a constant direct current; however, only superconducting inductors have truly zero electrical resistance.

In general, the relationship between the time-varying voltage "v"("t") across an inductor with inductance "L" and the time-varying current "i"("t") passing through it is described by the differential equation:

:$v(t)\; =\; L\; frac\{di(t)\}\{dt\}$

When there is a sinusoidal alternating current (AC) through an inductor, a sinusoidal voltage is induced. The amplitude of the voltage is proportional to the product of the amplitude ($I\_P$) of the current and the frequency (" f ") of the current.

:$i(t)\; =\; I\_P\; sin(2\; pi\; f\; t),$

:$frac\{di(t)\}\{dt\}\; =\; 2\; pi\; f\; I\_P\; cos(2\; pi\; f\; t)$

:$v(t)\; =\; 2\; pi\; f\; L\; I\_P\; cos(2\; pi\; f\; t),$

In this situation, the phase of the current lags that of the voltage by 90 degrees. #

If an inductor is connected to a DC current source, with value "I" via a resistance, "R", and then the current source short circuited, the differential relationship above shows that the current through the inductor will discharge with an exponential decay:

:$i(t)\; =\; I\; (e^\{frac\{-tR\}\{L)$

Laplace circuit analysis (s-domain)

When using the Laplace transform in circuit analysis, the transfer impedance of an ideal inductor with no initial current is represented in the "s" domain by:

:$Z(s)\; =\; Ls,$::: where:::: "L" is the inductance, and:::: "s" is the complex frequency

If the inductor does have initial current, it can be represented by:
* adding a voltage source in series with the inductor, having the value::$L\; I\_0\; ,$("Note that the source should have a polarity that opposes the initial current")
* or by adding a current source in parallel with the inductor, having the value::$frac\{I\_0\}\{s\}$::: where:::: "L" is the inductance, and:::: "$I\_0$ is the initial current in the inductor.

Inductor networks

Inductors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent inductance ("L"_eq):

:

:$frac\{1\}\{L\_mathrm\{eq\; =\; frac\{1\}\{L\_1\}\; +\; frac\{1\}\{L\_2\}\; +\; cdots\; +\; frac\{1\}\{L\_n\}$

The current through inductors in series stays the same, but the voltage across each inductor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total inductance:

:

:$L\_mathrm\{eq\}\; =\; L\_1\; +\; L\_2\; +\; cdots\; +\; L\_n\; ,!$

These simple relationships hold true only when there is no mutual coupling of magnetic fields between individual inductors.

Stored energy

The energy (measured in joules, in SI) stored by an inductor is equal to the amount of work required to establish the current through the inductor, and therefore the magnetic field. This is given by:

:$E\_mathrm\{stored\}\; =\; \{1\; over\; 2\}\; L\; I^2$

where "L" is inductance and "I" is the current through the inductor.

"Q" factor

An ideal inductor will be lossless irrespective of the amount of current through the winding. However, typically inductors have winding resistance from the metal wire forming the coils. Since the winding resistance appears as a resistance in series with the inductor, it is often called the "series resistance". The inductor's series resistance converts electrical current through the coils into heat, thus causing a loss of inductive quality. The quality factor (or "Q") of an inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency. The higher the Q factor of the inductor, the closer it approaches the behavior of an ideal, lossless, inductor.

The Q factor of an inductor can be found through the following formula, where "R" is its internal electrical resistance and $omega\{\}L$ is capacitive or inductive reactance at resonance:

:$Q\; =\; frac\{omega\{\}L\}\{R\}$

By using a ferromagnetic core, the inductance is greatly increased for the same amount of copper, multiplying up the Q. Cores however also introduce losses that increase with frequency. A grade of core material is chosen for best results for the frequency band. At VHF or higher frequencies an air core is likely to be used.

Inductors wound around a ferromagnetic core may saturate at high currents, causing a dramatic decrease in inductance (and Q). This phenomenon can be avoided by using a (physically larger) air core inductor. A well designed air core inductor may have a Q of several hundred.

An almost ideal inductor (Q approaching infinity) can be created by immersing a coil made from a superconducting alloy in liquid helium or liquid nitrogen. This supercools the wire, causing its winding resistance to disappear. Because a superconducting inductor is virtually lossless, it can store a large amount of electrical energy within the surrounding magnetic field (see superconducting magnetic energy storage).

Inductance formulae

The table below lists some common formulae for calculating the theoretical inductance of several inductor constructions.

See also

* Electronic component
* Capacitor
* Resistor
* Memristor
* Electricity
* Electronics
* Gyrator
* Inductance (including "mutual inductance")
* Induction coil
* Induction cooking
* Induction loop
* Magnetic core
* Saturable reactor
* Transformer
* Reactance (electronics)
* RL circuit
* RLC circuit

Synonyms

* coil
* Choke (electronics)
* reactor

Notes

External links

; General
* [http://electronics.howstuffworks.com/inductor1.htm How stuff works] The initial concept, made very simple
* [http://www.lightandmatter.com/html_books/4em/ch07/ch07.html Capacitance and Inductance] - A chapter from an online textbook
* [http://www.mpdigest.com/issue/Articles/2005/aug2005/agilent/Default.asp Spiral inductor models] . Good article on inductor characteristics and modeling.
* [http://www.66pacific.com/calculators/coil_calc.aspx Online coil inductance calculator] . Online calculator calculates the inductance of conventional and toroidal coils using formulas 3, 4, 5, and 6, above.
* [http://www.phys.unsw.edu.au/~jw/AC.html AC circuits]
* [http://www.mikroe.com/en/books/keu/03.htm] - Understanding coils and transforms
* [http://library.thinkquest.org/10784/circuit_symbols.html] - Circuit Schematic Symbols