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{{Short description|Electrical component}}
{{Other uses}}
{{Redirect|Winding}} {{Redirect|Winding}}
]
]
An '''electromagnetic coil''' (or simply a "coil") is formed when a conductor (usually an insulated solid copper ]) is wound around a core or form to create an ] or ]. When electricity is passed through a coil, it generates a magnetic field. One loop of wire is usually referred to as a ''turn'' or a ''winding'', and a coil consists of one or more turns. For use in an ], ] terminals called taps are often connected to a coil. Coils are often coated with varnish or wrapped with insulating tape to provide additional ] and secure them in place. A completed coil assembly with one or more set of coils and taps is often called the '''windings'''.


An '''electromagnetic coil''' is an electrical ] such as a ] in the shape of a ] (] or ]).<ref name="Stauffer">{{cite book
Windings are used in ]s, ]s, ]s, ]s, ]s, and many other applications.
| last = Stauffer
| first = H. Brooke
| title = NFPA's Pocket Dictionary of Electrical Terms
| publisher = Jones and Hymel Tucker
| year = 2002
| page = 36
| url = https://books.google.com/books?id=9sW_knz5P0wC&pg=PA36
| isbn = 978-0877655992}}</ref><ref name="Laplante">{{cite book
| last = Laplante
| first = Phillip A.
| title = Comprehensive Dictionary of Electrical Engineering
| publisher = Springer
| year = 1999
| pages = 114–115
| url = https://books.google.com/books?id=soSsLATmZnkC&dq=coil&pg=PA114
| isbn = 978-3540648352}}</ref> Electromagnetic coils are used in ], in applications where ]s interact with ]s, in devices such as ]s, ]s, ]s, ]s, ]s, sensor coils such as in medical ] imaging machines. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely, an external ''time-varying'' magnetic field through the interior of the coil generates an ] (]) in the conductor.


A current through any conductor creates a circular magnetic field around the conductor due to ].<ref name="Arun">{{cite book
==General principles==
| last = Arun
]
| first = P.
| title = Electronics
| publisher = Alpha Sciences International Ltd.
| year = 2006
| pages = 73–77
| url = https://books.google.com/books?id=HsavX0cnxTcC&pg=PA73
| isbn = 978-1842652176}}</ref> The advantage of using the coil shape is that it increases the strength of the magnetic field produced by a given current. The magnetic fields generated by the separate turns of wire all pass through the center of the coil and add (]) to produce a strong field there.<ref name="Arun" /> The greater the ] of wire, the stronger the field produced. Conversely, a ''changing'' external magnetic flux induces a voltage in a conductor such as a wire, due to ].<ref name="Arun" /><ref>{{cite book| url = https://books.google.com/books?id=lROa-MpIrucC&dq=Faradays+law&pg=PA129| title = Newnes 2002, p. 129| isbn = 9780080524054| last1 = Amos| first1 = S. W.| last2 = Amos| first2 = Roger| date = 4 March 2002| publisher = Elsevier}}</ref> The induced voltage can be increased by winding the wire into a coil because the field lines intersect the circuit multiple times.<ref name="Arun" />


The direction of the magnetic field produced by a coil can be determined by the ]. If the fingers of the right hand are wrapped around the magnetic core of a coil in the direction of ] through the wire, the thumb will point in the direction the ]s pass through the coil. The end of a magnetic core from which the field lines emerge is defined to be the North pole.
The ] used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn.<ref name="Dixon (1997)">{{cite journal|last=Dixon|first=L.H., Jr.|url=http://focus.ti.com/lit/ml/slup197/slup197.pdf| title=Eddy Current Losses in Transformer Windings|year=1997|publisher=Texas Instrument|pages=R2-1-to-R2-10}}</ref> For small power and signal applications, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from ], such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of ].<ref name="cegb_1982">{{cite book
| author=Central Electricity Generating Board
| title=Modern Power Station Practice
| year = 1982
|publisher = Pergamon Press}}</ref>


There are many different types of coils used in electric and electronic equipment.
[[Image:transformer min stray field geometry.svg|thumb|left|250px|
Cut view through transformer windings.
White: insulator.
Green spiral: ].
Black: Primary winding made of ].
Red: Secondary winding.
Top left: Toroidal transformer.
Right: C-core, but E-core would be similar. The black windings are made of film.
Top: Equally low capacitance between all ends of both windings. Since most cores are at least moderately conductive they also need insulation.
Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers.
Bottom left: Reduction of ] would lead to increase of capacitance.
]]


]
===Effects of frequency===
High-frequency coils operating in the tens to hundreds of kilohertz often have windings made of braided ] to minimize the skin-effect and ] losses.<ref name="dixon"/> Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings.<ref name="cegb_1982"/> Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the low winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture.<ref name="cegb_1982"/>


==Windings and taps==
The windings of signal coils minimize leakage inductance and stray capacitance to improve high-frequency response. Coils are split into sections, and those sections interleaved between the sections of the other winding.
]
The wire or conductor which constitutes the coil is called the '''winding'''.<ref name="google2">{{cite book|title=NFPA's Pocket Dictionary of Electrical Terms|author=Stauffer, H.B.|date=2005|publisher=Jones & Bartlett Learning, LLC|isbn=9780877655992|url=https://books.google.com/books?id=9sW_knz5P0wC&pg=PA273|page=273|access-date=2017-01-07}}</ref> The hole in the center of the coil is called the '''core''' area or ''magnetic axis''.<ref name="Amos">{{cite book
| last = Amos
| first = S W
|author2=Roger Amos
| title = Newnes Dictionary of Electronics
| publisher = Newnes
| year = 2002
| page = 191
| url = https://books.google.com/books?id=lROa-MpIrucC&dq=magnetic+core&pg=PA191
| isbn = 978-0080524054}}</ref> Each loop of wire is called a '''turn'''.<ref name="Laplante" /> In windings in which the turns touch, the wire must be insulated with a coating of nonconductive ] such as plastic or enamel to prevent the current from passing between the wire turns. The winding is often wrapped around a ''coil form'' made of plastic or other material to hold it in place.<ref name="Laplante" /> The ends of the wire are brought out and attached to an external circuit. Windings may have additional electrical connections along their length; these are called '''taps'''.<ref name="google3">{{cite book|title=Comprehensive Dictionary of Electrical Engineering|author=Laplante, P.A.|date=1999|publisher=Springer Berlin Heidelberg|isbn=9783540648352|url=https://books.google.com/books?id=soSsLATmZnkC&pg=PA633|page=633|access-date=2017-01-07}}</ref> A winding that has a single tap in the center of its length is called '''center-tapped'''.<ref name="google4">{{cite book|title=NFPA's Pocket Dictionary of Electrical Terms|author=Stauffer, H.B.|date=2005|publisher=Jones & Bartlett Learning, LLC|isbn=9780877655992|url=https://books.google.com/books?id=9sW_knz5P0wC&pg=PA29|page=29|access-date=2017-01-07}}</ref>


Coils can have more than one winding, insulated electrically from each other. When there are two or more windings around a common magnetic axis, the windings are said to be '''inductively coupled''' or '''magnetically coupled'''.<ref name="google5">{{cite book|title=Newnes Dictionary of Electronics|author1=Amos, S.W.|author2=Amos, R.|date=2002|publisher=Elsevier Science|isbn=9780080524054|url=https://books.google.com/books?id=lROa-MpIrucC&pg=PA167|page=167|access-date=2017-01-07}}</ref> A time-varying current through one winding will create a time-varying magnetic field that passes through the other winding, which will induce a time-varying voltage in the other windings. This is called a ].<ref name="google6">{{cite book|title=Newnes Dictionary of Electronics|author1=Amos, S.W.|author2=Amos, R.|date=2002|publisher=Elsevier Science|isbn=9780080524054|url=https://books.google.com/books?id=lROa-MpIrucC&pg=PA326|page=326|access-date=2017-01-07}}</ref> The winding to which current is applied, which creates the magnetic field, is called the '']''. The other windings are called '']s''.
===Taps===
Coils may have ] at intermediate points on the winding for various uses.


==Magnetic core==
In transformers, taps may be manually reconnected, or a manual or automatic switch may be provided for changing taps. Automatic on-load ]s are used in electric power transmission or distribution, on equipment such as ] transformers, or for automatic voltage regulators for sensitive loads. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A ] is often used in the output stage of an audio power ] in a ]. Modulation transformers in ] transmitters are very similar.
Many electromagnetic coils have a ], a piece of ] material like ] in the center to increase the magnetic field.<ref name="Laplante1">{{cite book
| last = Laplante
| first = Phillip A.
| title = Comprehensive Dictionary of Electrical Engineering
| publisher = Springer
| year = 1998
| page = 143
| url = https://books.google.com/books?id=soSsLATmZnkC&dq=core&pg=PA143
| isbn = 978-3540648352}}</ref> The current through the coil magnetizes the iron, and the field of the magnetized material adds to the field produced by the wire. This is called a '''ferromagnetic-core''' or '''iron-core coil'''.<ref name="google7">{{cite book|title=Comprehensive Dictionary of Electrical Engineering|author=Laplante, P.A.|date=1999|publisher=Springer Berlin Heidelberg|isbn=9783540648352|url=https://books.google.com/books?id=soSsLATmZnkC&pg=PA346|page=346|access-date=2017-01-07}}</ref> A ferromagnetic core can increase the magnetic field and ] of a coil by hundreds or thousands of times over what it would be without the core. A '''ferrite core coil''' is a variety of coil with a core made of ], a ] ceramic compound.<ref name="google8">{{cite book|title=Comprehensive Dictionary of Electrical Engineering|author=Laplante, P.A.|date=1999|publisher=Springer Berlin Heidelberg|isbn=9783540648352|url=https://books.google.com/books?id=soSsLATmZnkC&pg=PA243|page=243|access-date=2017-01-07}}</ref> Ferrite coils have lower ]es at high frequencies.
*A coil with a core which forms a closed loop, possibly with some narrow air gaps, is called a '''closed-core''' coil. By providing a closed path for the magnetic field lines, this geometry minimizes the ] and produces the strongest magnetic field. It is often used in transformers.
**A common form for closed-core coils is a '''toroidal core''' coil, in which the core has the shape of a ] or doughnut, with either a circular or rectangular cross section. This geometry has minimum ] and radiates minimum ] (EMI).
*A coil with a core which is a straight bar or other non-loop shape is called an '''open-core''' coil. This has lower magnetic field and inductance than a closed core, but is often used to prevent ] of the core.
A coil without a ferromagnetic core is called an '''air-core coil'''.<ref name="google9">{{cite book|title=Comprehensive Dictionary of Electrical Engineering|author=Laplante, P.A.|date=1999|publisher=Springer Berlin Heidelberg|isbn=9783540648352|url=https://books.google.com/books?id=soSsLATmZnkC&pg=PA19|page=19|access-date=2017-01-07}}</ref> This includes coils wound on plastic or other nonmagnetic forms, as well as coils which actually have empty air space inside their windings.


===Potting=== ==Types of coils==
Coils can be classified by the ] of the current they are designed to operate with:
Coils are often ] to prevent the wires from moving in use. Traditionally a ] material was soaked into the windings after assembly and oven dried.
:*''Direct current'' or ''DC'' coils or electromagnets operate with a steady ] in their windings
:*''Audio-frequency'' or ''AF'' coils, inductors or transformers operate with ]s in the ] range, less than 20 kHz
:*''Radio-frequency'' or ''RF'' coils, inductors or transformers operate with alternating currents in the ] range, above 20 kHz


Coils can be classified by their function:
Dry-type coil winding insulation systems can be either of standard open-wound 'dip-and-bake' construction or of higher quality designs that include ] (VPI), ] (VPE), and ] processes.<ref name="Lane (2007)">{{cite web|last=Lane|first=Keith (2007)|title=The Basics of Large Dry-Type Transformers|url=http://ecmweb.com/content/basics-large-dry-type-transformers|publisher=EC&M|accessdate=29 January 2013}}</ref> In the VPI process, a combination of heat, vacuum and pressure is used to thoroughly seal, bind, and eliminate entrained air voids in the winding polyester resin insulation coat layer, thus increasing resistance to corona. VPE windings are similar to VPI windings but provide more protection against environmental effects, such as from water, dirt or corrosive ambients, by multiple dips including typically in terms of final epoxy coat.<ref>Heathcote, pp. 720–723</ref>


===Electromagnets===
Construction of mdoil-filled transformers requires that the insulation covering the windings be thoroughly dried before the oil is introduced. Drying is carried out at the factory, and may be required as a field service. Drying may be done by circulating hot air around the core, or by vapour-phase drying (VPD) where evaporated solvent transfers heat by condensation on the coil and core. For small transformers resistance heating by injection of current into the windings is used. The heating can be controlled very well and it is energy efficient. The method is called low-frequency heating (LFH) since the current is injected at a much lower frequency than the nominal of the grid, which is normally 50 or 60&nbsp;Hz. A lower frequency reduces the effect of the inductance in the transformer, so the voltage needed to induce the current can be reduced.<ref>{{cite book|last=Fink|first= Donald G.|coauthors= Beatty, H. Wayne (Eds.)|year=1978|title=Standard Handbook for Electrical Engineers|edition= 11th|publisher= McGraw Hill|isbn=978-0-07-020974-9 |pages=10-38 through 10-40}}</ref> The LFH drying method is also used for service of older transformers.<ref name=Figueroa>{{cite web|last=Figueroa|first=Elisa et al|title=Low Frequency Heating Field Dry-Out of a 750 MVA 500 kV Auto Transformer|url=http://www.electricity-today.com/et/issue0109/transformer_field_dry-out.pdf|publisher=Electricity Today|date=Jan/Feb 2009|accessdate=Feb 28, 2012}}</ref>
{{main|Electromagnet}}
] electromagnet on the ] of an AC ]. ]]
]
Electromagnets are coils that generate a ] for some external use, often to exert a mechanical force on something.<ref name="google10">{{cite book|title=Newnes Dictionary of Electronics|author1=Amos, S.W.|author2=Amos, R.|date=2002|publisher=Elsevier Science|isbn=9780080524054|url=https://books.google.com/books?id=lROa-MpIrucC&pg=PA113|page=113|access-date=2017-01-07}}</ref> or remove existing background fields.<ref>{{cite journal | last1 = Hobson | first1 = P. J. |display-authors=et al | year = 2022 | title = Bespoke magnetic field design for a magnetically shielded cold atom interferometer | journal = Sci. Rep. | volume = 12 | issue = 1 | pages = 10520 | doi= 10.1038/s41598-022-13979-4 | pmid = 35732872 | pmc = 9217970 | arxiv = 2110.04498 | bibcode = 2022NatSR..1210520H | s2cid = 238583775 }}</ref> A few specific types:
:*] - an electromagnet in the form of a straight hollow helix of wire
:*Motor and generator windings - iron core electromagnets on the ] or ] of electric motors and generators which act on each other to either turn the shaft (motor) or generate an electric current (generator)
:**] - an iron-core coil which generates a steady magnetic field to act on the armature winding.
:**] winding - an iron-core coil which is acted on by the magnetic field of the field winding to either create torque (motor) or induce a voltage to produce power (generator)
:*], ] - air-core coils which serve to cancel an external magnetic field
:*] coil - a coil used to demagnetize parts
:*] - a coil used in a moving-coil ], suspended between the poles of a magnet. When the ] is passed through the coil, it vibrates, moving the attached speaker cone to create sound waves. The reverse is used in a dynamic ], where sound vibrations intercepted by something like a ] physically transfer to a voice coil immersed in a magnetic field, and the coil's terminal ends then provide an electric analog of those vibrations.


===Capacitance=== ===Inductors===
{{main|Inductor}}
Coils, particularly multilayer coils can have interlayer ].
]s or reactors are coils which generate a magnetic field which interacts with the coil itself, to induce a back EMF which opposes changes in current through the coil. Inductors are used as ]s in electrical circuits, to temporarily store energy or resist changes in current. A few types:


:*Tank coil - an inductor used in a ]
When this capacitance is problematic multiple layers are often replaced with a single layer, as in a pancake coil, or a solenoid with a single layer is used.
:*] - an inductor used to block high frequency AC while allowing through low frequency AC or DC.
]'s flat spiral coil.]]
:*] - an inductor used to add inductance to an antenna, to make it resonant, or to a cable to prevent distortion of signals.
:*] - an adjustable inductor consisting of two coils in series, an outer stationary coil and a second one inside it which can be rotated so their magnetic axes are in the same direction or opposed.
:*] - Although called a transformer, this is actually an inductor which serves to store energy in ] and horizontal deflection circuits for ] televisions and monitors
:*] - an iron-core inductor used to control AC power by varying the saturation of the core using a DC control voltage in an auxiliary winding.
:*] - an inductor used in ] circuits, such as ]s, to limit the current through the lamp.
{{Clear}}


==Specific types== ===Transformers===
]
{{main|Transformer}}


A transformer is a device with two or more magnetically coupled windings (or sections of a single winding). A time varying current in one coil (called the ]) generates a magnetic field which induces a voltage in the other coil (called the ]). A few types:
===Solenoid===
:*] - A transformer in an ] which transforms the high voltage from the electric ] to the lower voltage used by utility customers.
{{main|Solenoid}}
:*] - a transformer with only one winding. Different portions of the winding, accessed with taps, act as primary and secondary windings of the transformer.
]
:*] - the core is in the shape of a ]. This is a commonly used shape as it decreases the leakage flux, resulting in less electromagnetic interference.
:*] or ] - an early transformer which uses a vibrating interrupter mechanism to break the primary current so it can operate off of DC current.
:**] - an induction coil used in ]s to create a pulse of high voltage to fire the ] which initiates the fuel burning.
:*] - a transformer which matches a ] to an unbalanced one.
:*] - a coil wound with two parallel, closely spaced strands. If AC currents are passed through it in the same direction, the ]es will add, but if equal currents in opposite directions pass through the windings the opposite fluxes will cancel, resulting in zero flux in the core. So no voltage will be induced in a third winding on the core. These are used in instruments and in devices like ]s. They are also used in low inductance wirewound resistors for use at RF frequencies.
:*] - A transformer used with ]s. They are used for ].
:**] - a specialized audio transformer with 3 windings used in ] circuits to convert between ] and ]s
{{Clear}}


===Electric machines===
A solenoid (from the ] ''solénoïde'', derived in turn from the ] ''solen'' "pipe, channel" + combining form of Greek ''eidos'' "form, shape"<ref>{{Cite web|url=http://www.etymonline.com/index.php?term=solenoid|title=Solenoid|work=]}}</ref>) is a coil wound into a tightly packed ]. The term was invented by ] to designate a helical coil.<ref>{{cite web |url=http://www.ampere.cnrs.fr/parcourspedagogique/zoom/courant/electrodynamique/index.php|title=Parcours pédagogique&nbsp;— Ampère et l'histoire de l'électricité | publisher=] | accessdate=July 31, 2012}}</ref>
]s such as ] and ] have one or more windings which interact with moving magnetic fields to convert electrical energy to mechanical energy. Often a machine will have one winding through which passes most of the power of the machine (the ]), and a second winding which provides the magnetic field of the rotating element ( the "field winding") which may be connected by brushes or slip rings to an external source of electric current. In an ], the "field" winding of the rotor is energized by the slow relative motion between the rotating winding and the rotating magnetic field produced by the stator winding, which induces the necessary exciting current in the rotor.


===Transducer coils===
In ], the term refers specifically to a long, thin loop of wire, often wrapped around a ]lic core, which produces a uniform ] in a volume of space (where some experiment might be carried out) when an ] is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as ]s.
].]]
These are coils used to translate time-varying magnetic fields to electric signals, and vice versa. A few types:
:*Sensor or pickup coils - these are used to detect external time-varying magnetic fields
:*] - a coil which senses when a magnet or iron object passes near it
:*] - a coil which is used to create a magnetic field to write data to a ] medium, such as ], or a ]. Conversely it is also used to read the data in the form of changing magnetic fields in the medium.
:*] - an AC coil used to heat an object by inducing ]s in it, a process called ].
:*] - a coil which serves as a ], to convert radio waves to electric currents.
:*] - a toroidal coil used as an AC measuring device
:*] - a coil used to produce the output ] in an ] or ].
:*] - a sensor coil used in a ]
:*] - a sensor in a ] that uses a coil to translate vibration of a needle to an audio signal in playing vinyl ]s.


There are also types of coil which don't fit into these categories.
In ], the term may also refer to a variety of ] devices that convert ] into linear motion. The term is also often used to refer to a ], which is an integrated device containing an electromechanical solenoid which actuates either a ] or ] valve, or a solenoid switch, which is a specific type of ] that internally uses an electromechanical solenoid to operate an electrical switch; for example, an ], or a linear solenoid, which is an electromechanical solenoid.


==Winding technology==
===Transformers===
{{main|transformer}} {{main|Coil winding technology}}
A ] is an electromagnetic device that has a ''primary winding'' and a ''secondary winding'' that transfers energy from one electrical circuit to another by ] without moving parts. The term ''tickler coil'' usually refers to a feedback coil, which is often the third coil placed in relation to a primary coil and secondary coil. A ] is a wiring feature found on some electrical transformers, inductors and coil pickups, all of which are sets of wire coils. The coil tap(s) are points in a wire coil where a conductive patch has been exposed (usually on a loop of wire that extends out of the main coil body).

===Inductor coil===
An inductor coil is typically has a relatively simple ] winding over a magnetic core.


===Electric motor=== ==See also==
*]
Electric motor windings can be quite complex. They are often 3 phase design, with 3 sets of windings. Many different layouts, interleavings and windings are in use.


==References== ==References==
{{Reflist}}
{{reflist|refs=
<ref name="dixon">{{Cite conference | first = Lloyd
| last = Dixon
| booktitle = Magnetics Design Handbook
| title = Power Transformer Design
| url = http://focus.ti.com/lit/ml/slup126/slup126.pdf
| year=2001
| publisher = Texas Instruments}}</ref> Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time.
}}


==Further reading== ==Further reading==
Line 88: Line 148:
* Weymouth, F. Marten, "''Drum armatures and commutators (theory and practice) : a complete treatise on the theory and construction of drum winding, and of commutators for closed-coil armatures, together with a full résumé of some of the principal points involved in their design; and an exposition of armature reactions and sparking''". London, "The Electrician" Printing and Publishing Co., 1893. * Weymouth, F. Marten, "''Drum armatures and commutators (theory and practice) : a complete treatise on the theory and construction of drum winding, and of commutators for closed-coil armatures, together with a full résumé of some of the principal points involved in their design; and an exposition of armature reactions and sparking''". London, "The Electrician" Printing and Publishing Co., 1893.
* "''Coil winding proceedings''". International Coil Winding Association. * "''Coil winding proceedings''". International Coil Winding Association.
* Chandler, R. H., "''Coil coating review, 1970-76''". Braintree, R. H. Chandler Ltd, 1977. * Chandler, R. H., "''Coil coating review, 1970–76''". Braintree, R. H. Chandler Ltd, 1977.


== External links == ==External links==
{{Wiktionary|coil|winding}} {{Wiktionary|coil|winding}}
{{Commonscat|Electromagnetic coils}}
* R. Clarke, "''''". Surrey.ac.uk, 2005 October 9
* Online calculator for determining the inductance of single-layer and multilayer coils

* R. Clarke, "'' {{Webarchive|url=https://web.archive.org/web/20070707141302/http://www.ee.surrey.ac.uk/Workshop/advice/coils/ |date=2007-07-07 }}''". Surrey.ac.uk, 2005 October 9
<!-- In other languages, alphabetically by language (not code) name -->
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Latest revision as of 07:52, 27 September 2024

Electrical component "Winding" redirects here. For other uses, see Winding (disambiguation).
The magnetic field lines (green) of a current-carrying loop of wire pass through the center of the loop, concentrating the field there

An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in devices such as electric motors, generators, inductors, electromagnets, transformers, sensor coils such as in medical MRI imaging machines. Either an electric current is passed through the wire of the coil to generate a magnetic field, or conversely, an external time-varying magnetic field through the interior of the coil generates an EMF (voltage) in the conductor.

A current through any conductor creates a circular magnetic field around the conductor due to Ampere's law. The advantage of using the coil shape is that it increases the strength of the magnetic field produced by a given current. The magnetic fields generated by the separate turns of wire all pass through the center of the coil and add (superpose) to produce a strong field there. The greater the number of turns of wire, the stronger the field produced. Conversely, a changing external magnetic flux induces a voltage in a conductor such as a wire, due to Faraday's law of induction. The induced voltage can be increased by winding the wire into a coil because the field lines intersect the circuit multiple times.

The direction of the magnetic field produced by a coil can be determined by the right hand grip rule. If the fingers of the right hand are wrapped around the magnetic core of a coil in the direction of conventional current through the wire, the thumb will point in the direction the magnetic field lines pass through the coil. The end of a magnetic core from which the field lines emerge is defined to be the North pole.

There are many different types of coils used in electric and electronic equipment.

In a coil of multiple turns of wire the magnetic field of the turns adds in the center of the coil, creating a strong field. This drawing shows a cross section through the center of the coil. The crosses are wires in which current is moving into the page; the dots are wires in which current is emerging from the page.

Windings and taps

Diagram of typical transformer configurations

The wire or conductor which constitutes the coil is called the winding. The hole in the center of the coil is called the core area or magnetic axis. Each loop of wire is called a turn. In windings in which the turns touch, the wire must be insulated with a coating of nonconductive insulation such as plastic or enamel to prevent the current from passing between the wire turns. The winding is often wrapped around a coil form made of plastic or other material to hold it in place. The ends of the wire are brought out and attached to an external circuit. Windings may have additional electrical connections along their length; these are called taps. A winding that has a single tap in the center of its length is called center-tapped.

Coils can have more than one winding, insulated electrically from each other. When there are two or more windings around a common magnetic axis, the windings are said to be inductively coupled or magnetically coupled. A time-varying current through one winding will create a time-varying magnetic field that passes through the other winding, which will induce a time-varying voltage in the other windings. This is called a transformer. The winding to which current is applied, which creates the magnetic field, is called the primary winding. The other windings are called secondary windings.

Magnetic core

Many electromagnetic coils have a magnetic core, a piece of ferromagnetic material like iron in the center to increase the magnetic field. The current through the coil magnetizes the iron, and the field of the magnetized material adds to the field produced by the wire. This is called a ferromagnetic-core or iron-core coil. A ferromagnetic core can increase the magnetic field and inductance of a coil by hundreds or thousands of times over what it would be without the core. A ferrite core coil is a variety of coil with a core made of ferrite, a ferrimagnetic ceramic compound. Ferrite coils have lower core losses at high frequencies.

  • A coil with a core which forms a closed loop, possibly with some narrow air gaps, is called a closed-core coil. By providing a closed path for the magnetic field lines, this geometry minimizes the magnetic reluctance and produces the strongest magnetic field. It is often used in transformers.
    • A common form for closed-core coils is a toroidal core coil, in which the core has the shape of a torus or doughnut, with either a circular or rectangular cross section. This geometry has minimum leakage flux and radiates minimum electromagnetic interference (EMI).
  • A coil with a core which is a straight bar or other non-loop shape is called an open-core coil. This has lower magnetic field and inductance than a closed core, but is often used to prevent magnetic saturation of the core.

A coil without a ferromagnetic core is called an air-core coil. This includes coils wound on plastic or other nonmagnetic forms, as well as coils which actually have empty air space inside their windings.

Types of coils

Coils can be classified by the frequency of the current they are designed to operate with:

  • Direct current or DC coils or electromagnets operate with a steady direct current in their windings
  • Audio-frequency or AF coils, inductors or transformers operate with alternating currents in the audio frequency range, less than 20 kHz
  • Radio-frequency or RF coils, inductors or transformers operate with alternating currents in the radio frequency range, above 20 kHz

Coils can be classified by their function:

Electromagnets

Main article: Electromagnet
Field coil electromagnet on the stator of an AC universal motor.
Canon AF-10 Date old camera shutter driver coil.

Electromagnets are coils that generate a magnetic field for some external use, often to exert a mechanical force on something. or remove existing background fields. A few specific types:

  • Solenoid - an electromagnet in the form of a straight hollow helix of wire
  • Motor and generator windings - iron core electromagnets on the rotor or stator of electric motors and generators which act on each other to either turn the shaft (motor) or generate an electric current (generator)
    • Field winding - an iron-core coil which generates a steady magnetic field to act on the armature winding.
    • Armature winding - an iron-core coil which is acted on by the magnetic field of the field winding to either create torque (motor) or induce a voltage to produce power (generator)
  • Helmholtz coil, Maxwell coil - air-core coils which serve to cancel an external magnetic field
  • Degaussing coil - a coil used to demagnetize parts
  • Voice coil - a coil used in a moving-coil loudspeaker, suspended between the poles of a magnet. When the audio signal is passed through the coil, it vibrates, moving the attached speaker cone to create sound waves. The reverse is used in a dynamic microphone, where sound vibrations intercepted by something like a diaphragm physically transfer to a voice coil immersed in a magnetic field, and the coil's terminal ends then provide an electric analog of those vibrations.

Inductors

Main article: Inductor

Inductors or reactors are coils which generate a magnetic field which interacts with the coil itself, to induce a back EMF which opposes changes in current through the coil. Inductors are used as circuit elements in electrical circuits, to temporarily store energy or resist changes in current. A few types:

  • Tank coil - an inductor used in a tuned circuit
  • Choke - an inductor used to block high frequency AC while allowing through low frequency AC or DC.
  • Loading coil - an inductor used to add inductance to an antenna, to make it resonant, or to a cable to prevent distortion of signals.
  • Variometer - an adjustable inductor consisting of two coils in series, an outer stationary coil and a second one inside it which can be rotated so their magnetic axes are in the same direction or opposed.
  • Flyback transformer - Although called a transformer, this is actually an inductor which serves to store energy in switching power supplies and horizontal deflection circuits for CRT televisions and monitors
  • Saturable reactor - an iron-core inductor used to control AC power by varying the saturation of the core using a DC control voltage in an auxiliary winding.
  • Inductive ballast - an inductor used in gas-discharge lamp circuits, such as fluorescent lamps, to limit the current through the lamp.

Transformers

Transformer
Main article: Transformer

A transformer is a device with two or more magnetically coupled windings (or sections of a single winding). A time varying current in one coil (called the primary winding) generates a magnetic field which induces a voltage in the other coil (called the secondary winding). A few types:

Electric machines

Electric machines such as motors and generators have one or more windings which interact with moving magnetic fields to convert electrical energy to mechanical energy. Often a machine will have one winding through which passes most of the power of the machine (the "armature"), and a second winding which provides the magnetic field of the rotating element ( the "field winding") which may be connected by brushes or slip rings to an external source of electric current. In an induction motor, the "field" winding of the rotor is energized by the slow relative motion between the rotating winding and the rotating magnetic field produced by the stator winding, which induces the necessary exciting current in the rotor.

Transducer coils

The sensor coil of a metal detector.

These are coils used to translate time-varying magnetic fields to electric signals, and vice versa. A few types:

There are also types of coil which don't fit into these categories.

Winding technology

Main article: Coil winding technology

See also

References

  1. Stauffer, H. Brooke (2002). NFPA's Pocket Dictionary of Electrical Terms. Jones and Hymel Tucker. p. 36. ISBN 978-0877655992.
  2. ^ Laplante, Phillip A. (1999). Comprehensive Dictionary of Electrical Engineering. Springer. pp. 114–115. ISBN 978-3540648352.
  3. ^ Arun, P. (2006). Electronics. Alpha Sciences International Ltd. pp. 73–77. ISBN 978-1842652176.
  4. Amos, S. W.; Amos, Roger (4 March 2002). Newnes 2002, p. 129. Elsevier. ISBN 9780080524054.
  5. Stauffer, H.B. (2005). NFPA's Pocket Dictionary of Electrical Terms. Jones & Bartlett Learning, LLC. p. 273. ISBN 9780877655992. Retrieved 2017-01-07.
  6. Amos, S W; Roger Amos (2002). Newnes Dictionary of Electronics. Newnes. p. 191. ISBN 978-0080524054.
  7. Laplante, P.A. (1999). Comprehensive Dictionary of Electrical Engineering. Springer Berlin Heidelberg. p. 633. ISBN 9783540648352. Retrieved 2017-01-07.
  8. Stauffer, H.B. (2005). NFPA's Pocket Dictionary of Electrical Terms. Jones & Bartlett Learning, LLC. p. 29. ISBN 9780877655992. Retrieved 2017-01-07.
  9. Amos, S.W.; Amos, R. (2002). Newnes Dictionary of Electronics. Elsevier Science. p. 167. ISBN 9780080524054. Retrieved 2017-01-07.
  10. Amos, S.W.; Amos, R. (2002). Newnes Dictionary of Electronics. Elsevier Science. p. 326. ISBN 9780080524054. Retrieved 2017-01-07.
  11. Laplante, Phillip A. (1998). Comprehensive Dictionary of Electrical Engineering. Springer. p. 143. ISBN 978-3540648352.
  12. Laplante, P.A. (1999). Comprehensive Dictionary of Electrical Engineering. Springer Berlin Heidelberg. p. 346. ISBN 9783540648352. Retrieved 2017-01-07.
  13. Laplante, P.A. (1999). Comprehensive Dictionary of Electrical Engineering. Springer Berlin Heidelberg. p. 243. ISBN 9783540648352. Retrieved 2017-01-07.
  14. Laplante, P.A. (1999). Comprehensive Dictionary of Electrical Engineering. Springer Berlin Heidelberg. p. 19. ISBN 9783540648352. Retrieved 2017-01-07.
  15. Amos, S.W.; Amos, R. (2002). Newnes Dictionary of Electronics. Elsevier Science. p. 113. ISBN 9780080524054. Retrieved 2017-01-07.
  16. Hobson, P. J.; et al. (2022). "Bespoke magnetic field design for a magnetically shielded cold atom interferometer". Sci. Rep. 12 (1): 10520. arXiv:2110.04498. Bibcode:2022NatSR..1210520H. doi:10.1038/s41598-022-13979-4. PMC 9217970. PMID 35732872. S2CID 238583775.

Further reading

  • Querfurth, William, "Coil winding; a description of coil winding procedures, winding machines and associated equipment for the electronic industry" (2d ed.). Chicago, G. Stevens Mfg. Co., 1958.
  • Weymouth, F. Marten, "Drum armatures and commutators (theory and practice) : a complete treatise on the theory and construction of drum winding, and of commutators for closed-coil armatures, together with a full résumé of some of the principal points involved in their design; and an exposition of armature reactions and sparking". London, "The Electrician" Printing and Publishing Co., 1893.
  • "Coil winding proceedings". International Coil Winding Association.
  • Chandler, R. H., "Coil coating review, 1970–76". Braintree, R. H. Chandler Ltd, 1977.

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