Compound Excitation

excitation of electrical machines in which the magnetic flux is automatically regulated as a function of the current in the armature of the machine.

Compound excitation of DC machines is provided by two field windings: a series winding and a shunt (or separate) winding. The shunt winding provides a magnetic excitation flux in the machine that corresponds to the nominal voltage under no-load conditions. The series winding is intended for automatic voltage regulation of the machine voltage depending on the load. Machines of this type are called compound-excitation machines; in terms of their electrical characteristics they occupy an intermediate position between series-excited and shunt-excited machines.

Compound excitation of AC machines is used primarily in automatic voltage regulation systems for high-power turbo generators and hydraulic generators. A compound excitation circuit includes current transformers, a rectifier, and load resistances. Upon a change of current in the armature of the synchronous generator, the current in the field winding of the exciter machine also changes, causing changes in the exciter voltage and the current in the excitation winding of the synchronous generator. Since a compound excitation system alone is not capable of maintaining the voltage of the synchronous generator with the required accuracy, voltage regulation of the synchronous generator is used simultaneously with compound excitation. The voltage regulator consists of an instrument potential transformer, a magnetic amplifier loaded by a rectifier, and a device that converts AC voltage variations into DC signals in the control windings of the magnetic amplifier. If the voltage of the synchronous generator deviates from a predetermined value, the current in the control windings of the magnetic amplifier will change, resulting in a change in the output voltage of the rectifier (and hence of the current in the field winding of the exciter). In many cases compound excitation systems with supplementary voltage regulation are used in combination with forced excitation controlled by relays.