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A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a electric motor or generator. It consists of a coil of wire through which the field current flows.
In a rotating machine, the field coils are wound on an iron magnetic core which guides the magnetic field lines. The magnetic core is in two parts; a stator which is stationary, and a rotor, which rotates within it. The magnetic pass in a continuous loop or magnetic 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 by poles, 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 simple through to complex lab instruments such as mass spectrometers and NMR machines. Field coils were once widely used in loudspeakers before the general availability of lightweight permanent magnets.
The field coils can be mounted on either the rotor or the stator, depending on whichever method is the most cost-effective for the device design.
In a brushed 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 a commutator, 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 a squirrel cage.
For generators, the field current is smaller than the output current.Strictly it is the output electrical power that is greater than the field power, although in practice this usually implies that the current is greater too. 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 in electroplating, this could require particularly large and complex brushgear.
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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. Hawkins Electrical Guide, Volume 1, Copyright 1917, Theo. Audel & Co., Chapter 14, Classes of Dynamo, page 182
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, a Direct current 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 the Gramme 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, the armature 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 the magnetic flux is shorter than in a two-pole design.
An even better material than copper, except for its high cost, would be silver as this has even lower resistivity. Silver has been used in rare cases. During World War II the Manhattan Project to build the first atomic bomb used electromagnetic devices known as to enrich uranium. Thousands of tons of silver were borrowed from the U.S. Treasury reserves to build highly efficient low-resistance field coils for their magnets.
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