Afield coil is anelectromagnet used to generate amagnetic field in an electro-magnetic machine, typically a rotatingelectrical machine such as amotor orgenerator. It consists of a coil of wire through which thefield current flows.
In a rotating machine, the field coils are wound on an ironmagnetic core which guides the magnetic field lines. The magnetic core is in two parts; astator which is stationary, and arotor, which rotates within it. The magneticfield lines pass in a continuous loop ormagnetic circuit from the stator through the rotor and back through the stator again. The field coils may be on the stator or on the rotor.
The magnetic path is characterized bypoles, locations at equal angles around the rotor at which the magnetic field lines pass from stator to rotor or vice versa. The stator (and rotor) are classified by the number of poles they have. Most arrangements use one field coil per pole. Some older or simpler arrangements use a single field coil with a pole at each end.
Although field coils are most commonly found in rotating machines, they are also used, although not always with the same terminology, in many other electromagnetic machines. These include simpleelectromagnets through to complex lab instruments such asmass spectrometers andNMR machines. Field coils were once widely used inloudspeakers before the general availability of lightweight permanent magnets.
Most[note 1]DC field coils generate a constant, static field. Mostthree-phase AC field coils are used to generate a rotating field as part of anelectric motor. Single-phaseAC motors may follow either of these patterns:
Many[note 1] rotary electrical machines require current to be conveyed to (or extracted from) a moving rotor, usually by means of sliding contacts: acommutator orslip rings. These contacts are often the most complex and least reliable part of such a machine, and may also limit the maximum current the machine can handle. For this reason, when machines must use two sets of windings, the windings carrying the least current are usually placed on the rotor and those with the highest current on the stator.
The field coils can be mounted on either therotor or thestator, depending on whichever method is the most cost-effective for the device design.
In abrushed DC motor the field is static but the armature current must be commutated, so as to continually rotate. This is done by supplying the armature windings on the rotor through acommutator, a combination of rotating slip ring and switches. AC induction motors also use field coils on the stator, the current on the rotor being supplied by induction in asquirrel cage.
For generators, the field current is smaller than the output current.[note 2] Accordingly, the field is mounted on the rotor and supplied through slip rings. The output current is taken from the stator, avoiding the need for high-current sliprings. In DC generators, which are now generally obsolete in favour of AC generators with rectifiers, the need for commutation meant that brushgear and commutators could still be required. For the high-current, low-voltage generators used inelectroplating, this could require particularly large and complex brushgear.
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In the early years of generator development, the stator field went through an evolutionary improvement from a singlebipolar field to a later multipole design.
Bipolar generators were universal prior to 1890 but in the years following it was replaced by the multipolar field magnets. Bipolar generators were then only made in very small sizes.[1]
The stepping stone between these two major types was the consequent-pole bipolar generator, with two field coils arranged in a ring around the stator.
This change was needed because higher voltages transmit power more efficiently over small wires. To increase the output voltage, aDC generator must be spun faster, but beyond a certain speed this is impractical for very large power transmission generators.
By increasing the number of pole faces surrounding theGramme ring, the ring can be made to cut across more magnetic lines of force in one revolution than a basic two-pole generator. Consequently, a four-pole generator could output twice the voltage of a two-pole generator, a six-pole generator could output three times the voltage of a two-pole, and so forth. This allows output voltage to increase without also increasing the rotational rate.
In a multipolar generator, thearmature and field magnets are surrounded by a circular frame or "ring yoke" to which the field magnets are attached. This has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of themagnetic flux is shorter than in a two-pole design.[1]
Coils are typically wound withenamelled copper wire, sometimes termedmagnet wire. The winding material must have a low resistance, to reduce the power consumed by the field coil, but more importantly to reduce thewaste heat produced byresistive heating. Excess heat in the windings is a common cause of failure. Owing to the increasing cost of copper, aluminium windings are increasingly used.[citation needed]
An even better material than copper, except for its high cost, would be silver as this has even lowerresistivity. Silver has been used in rare cases. DuringWorld War II theManhattan Project to build the firstatomic bomb used electromagnetic devices known ascalutrons toenrich uranium. Thousands of tons of silver were borrowed from theU.S. Treasury reserves to build highly efficient low-resistance field coils for their magnets.[2][3]