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What is a Field Coil?-Definition, Difference, And Windings

August 30, 2022
written by Adeel Abbas

A field coil is an electromagnet that is used to generate a magnetic field in an electrical machine. There is a coil of wire through which a current flows.

What is a field Coil?

The field coils in a rotating machine are wound around an iron magnetic core, which guides the magnetic field lines. The stator, which is stationary, and the rotor, which rotates within it, make up the magnetic core. The magnetic field lines 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 can be explained by thinking about poles as being like locations on a map. Just as there are North and South Poles on Earth, there are also north and south poles with magnets. These poles are at equal angles around the rotor, and they act like bridges that allow the magnetic field lines to pass from the stator to the rotor or vice versa.

The stator (and rotor) are classified by how many poles they have. Most arrangements use one field coil per pole, but some older or simpler arrangements might use just one field coil with a pole at each end.

Difference Between stator and rotor

Many rotary electrical machines need current in order to move a rotor using sliding contacts. This is done by using a commutator or slip rings. These contacts are usually the most complicated and unreliable part of the machine, and may also limit the maximum current that the machine can handle. For this reason, when machines use two sets of windings, the windings that carry less current are usually placed on the rotor, and those with more current are placed on the stator.

The field coils can be mounted on the rotor or stator, depending on which construction is more cost-effective.

In a brushed DC motor, the field is static and the armature current must be commutated to rotate continually. The armature windings on the rotor are supplied by a commutator, a rotating slip ring with switches. AC induction motors also use field coils on the stator, with the current on the rotor supplied by induction in a squirrel cage.

Difference Between Bipolar and multipolar fields

In the early years of generator development, there was a shift from single bipolar field magnets to multiple designs. This change was largely due to the advantages that multipolar field magnets had over bipolar generators, such as being able to generate electricity in larger 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 of a DC generator, it must be spun faster, but beyond a certain speed, this becomes impractical for very large power transmission generators.


The windings on a coil are typically made with enameled copper wire. This type of wire is sometimes also called magnet wire. It’s important for the winding material to have low resistance for a couple of reasons.

First, this reduces the power consumed by the field coil. Second, and more importantly, it reduces the waste heat produced by ohmic heating. Too much heat in the windings is a common cause of failure. Due to the rising cost of copper, aluminum windings are becoming more popular.

A material that would be even better than copper, except for its high cost, is silver. This is because silver has an even lower resistivity than copper. Although silver has only been used in rare cases, it was used during World War II in the Manhattan project to build the first atomic bomb. This is because silver is needed in order to build highly efficient low-resistance field coils for their magnets.