Field coil

Most DC field coils generate a constant, static field. Most three-phase AC field coils are used to generate a rotating field as part of an electric motor. Single-phase AC motors may follow either of these patterns: • Small motors are usually universal motors, like the brushed DC motor with a commutator, but run from AC. • Larger AC motors are generally induction motors, whether these are three-phase or single-phase. == Stators and rotors ==

Many 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. == Bipolar and multipolar fields ==

In the early years of generator development, the stator field went through an evolutionary improvement from a single bipolar 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. 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 DC 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. == Winding materials ==

Coils are typically wound with enamelled copper wire, sometimes termed magnet wire. The winding material must have a low resistance, to reduce the power consumed by the field coil, but more importantly to reduce the waste heat produced by resistive heating. Excess heat in the windings is a common cause of failure. Owing to the increasing cost of copper, aluminium windings are increasingly used. 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 calutrons 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. ==See also==