Virtually all generators in power plants are synchronous machines, justifying a substantial amount of attention to this class of electric machine. Synchronous machines are named for the property that they rotate in synchronism with the electrical system to which they are connected. Synchronous generators come in a very wide range of ratings, up to, as of this writing, nearly two giga‐volt‐amperes (GVA).
Synchronous machines can have either wound field or permanent magnet excitation. Most machines used as generators and many synchronous motors have field windings to permit control of voltage and reactive power; this is the type of machine we deal with in this chapter. Permanent magnet synchronous machines, controlled by power electronic converters, are sometimes used as high‐performance motors. This class of machine will be dealt with in Chapter 16.
The basic operation of a synchronous machine is quite simple. The field winding (or permanent magnet array) produces magnetic flux that varies, roughly sinusoidally, around the periphery of the rotor. The number of cycles of variation of this flux density is referred to as the number of pole pairs of the rotor. The armature of the machine, usually mounted on the stator, consists of a number of phase windings (typically three), wound in such a way as to link the flux from the rotor winding. As the rotor turns, the variation of flux from the rotor induces a voltage in the armature windings.
There are two types ...