Synchronous machines - Excitement

Synchronous electrical machines that convert mechanical energy into electrical energy (alternators) or vice versa (synchronous motors) require a DC power supply for their inductor or excitation system. In addition, the existence of this device makes it possible to regulate the voltage or reactive power of the synchronous machine. The performance obtained must be in line with the machine's operating conditions, in particular the protection provided by the synchronous machine itself and, where applicable, by the electrical network to which it is connected.

Behavior in the face of small disturbances depends mainly on the presence or absence of a variable-flux intermediate amplifier, DC exciter or alternator-exciter, and on the design of the associated controller.

Behavior with respect to large disturbances depends mainly on the power source of the excitation system. A source that is truly independent of the disturbances to which the synchronous machine is subjected can be obtained from a permanent magnet alternator mounted on the shaft line, but this requires the use of an intermediate amplifying machine and is limited to lower-power machines. A pure bypass self-supply is the easiest and most economical to implement, but its performance depends directly on the disturbances experienced by the synchronous machine. Finally, a number of compound self-feeding combinations enable the synchronous machine to perform well under disturbed conditions, at the cost of a more complex and more expensive design.

The sizing of excitation systems must also take into account the voltage and current constraints corresponding to their operating conditions.