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 electromagnet on the
stator of an AC
universal motor. 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 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 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: :*
Distribution transformer - A transformer in an
electric power grid which transforms the high voltage from the electric
power line to the lower voltage used by utility customers. :*
Autotransformer - a transformer with only one winding. Different portions of the winding, accessed with taps, act as primary and secondary windings of the transformer. :*
Toroidal transformer - the core is in the shape of a
toroid. This is a commonly used shape as it decreases the leakage flux, resulting in less electromagnetic interference. :*
Induction coil or
trembler coil - an early transformer which uses a vibrating interrupter mechanism to break the primary current so it can operate off of DC current. :**
Ignition coil - an induction coil used in
internal combustion engines to create a pulse of high voltage to fire the
spark plug which initiates the fuel burning. :*
Balun - a transformer which matches a
balanced transmission line to an unbalanced one. :*
Bifilar coil - a coil wound with two parallel, closely spaced strands. If AC currents are passed through it in the same direction, the
magnetic fluxes 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
Ground Fault Interrupters. They are also used in low inductance wirewound resistors for use at RF frequencies. :*
Audio transformer - A transformer used with
audio signals. They are used for
impedance matching. :**
Hybrid coil - a specialized audio transformer with 3 windings used in
telephony circuits to convert between
two-wire and
four-wire circuits
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 . 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 :*
Inductive sensor - a coil which senses when a magnet or iron object passes near it :*
Recording head - a coil which is used to create a magnetic field to write data to a
magnetic storage medium, such as
magnetic tape, or a
hard disk. Conversely it is also used to read the data in the form of changing magnetic fields in the medium. :*
Induction heating coil - an AC coil used to heat an object by inducing
eddy currents in it, a process called
induction heating. :*
Loop antenna - a coil which serves as a
radio antenna, to convert radio waves to electric currents. :*
Rogowski coil - a toroidal coil used as an AC measuring device :*
Musical instrument pickup - a coil used to produce the output
audio signal in an
electric guitar or
electric bass. :*
Flux gate - a sensor coil used in a
magnetometer :*
Magnetic phonograph cartridge - a sensor in a
record player that uses a coil to translate vibration of a needle to an audio signal in playing vinyl
phonograph records. There are also types of coil which don't fit into these categories. ==Winding technology==