It consists of a coil wire through which a current flows.
In a rotating machine, the field coils are wrapped around an iron magnetic core that guides the magnetic field lines. The magnetic coil has two parts, a stator and a rotor. The stator is stationary and the rotor rotates within it. The magnetic field lines pass in a continuous loop, called a magnetic circuit, from the stator to the rotor and back to the stator again.
Field coils, which energize, may be attached to the stator or the rotor. A rotating machine’s magnetic path is characterized by poles, or locations at equal angles around the rotor at which the magnetic field lines pass from stator to rotor or vise versa. The stator and rotor are classified by the number of poles they have. Most configurations use just one coil per pole, but some older or simpler arrangements use a single fuel coil with a pole at each end.
The wire used to wind a field coil must have a low resistance in order to reduce the amount of power it consumes and to reduce the production of waste heat by ohmic heating. This is important because excess heat in the wires is a common cause of failure. Typically, coils are wound with enamelled copper wire, which is sometimes referred to as magnet wire. However, due to increasing cost of copper, field coils are frequently being wound with aluminum.
Some time ago, before the general availability of lightweight permanent magnets, field coils were routinely used in loudspeakers. Today, field coils provide catalyst services for many usages, such as dynamos, alternators, generators, electric motors, starter motors, magnet coils and electric brakes. Non-rotating machines that use field coils include simple electromagnets as well as complex lab instruments such as mass spectrometers and NMR machines.
To meet the complex needs of the market, field coils are available in many different types, including encapsulated, glass taped, tape wrapped, injection molded, transfer molded and loose wound, bobbin wound or self-supported.