Excitation (magnetic)

In electromagnetism, excitation is the process of generating a magnetic field by means of an electric current.

Field coils yield the most flexible form of magnetic flux regulation and de-regulation, but at the expense of a flow of electric current.

Multiple versions of self-exitation exist:[1] If the machine does not have enough residual magnetism to build up to full voltage, usually a provision is made to inject current into the field coil from another source.

Brushless excitation creates the magnetic flux on the rotor of electrical machines without the need of carbon brushes.

[3][4][5] Brushless excitation has been historically lacking the fast flux de-regulation, which has been a major drawback.

[6] Modern rotating circuitry incorporates active de-excitation components on the shaft, extending the passive diode bridge.

[7][8][9] Moreover, their recent developments in high-performance wireless communication[10][11] have realized fully controlled topologies on the shaft, such as the thyristor rectifiers and chopper interfaces.

A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c. 1917
A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations.
A separately-excited DC generator with bipolar field magnets. Separately-excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator.
A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator).
Alternator of 1930s diesel generating set, with excitation dynamo above