Page 416 - Power Electronics Handbook

Electrical machine control 405
14.3.5 A.C. generators
The principal objective in generator control is regulation of its output
voltage. Referring to the elementary machine of Figure 14.11, the voltage
induced in coil a-b is directly proportional to the rate with which it cuts the
stator flux, Le. to the relative speed between rotor and stator field and to
stator field strength. In d.c. and synchronous generators, changing the
rotor to stator field speed involves control of the mechanical drive, which is
inconvenient. In a synchronous machine a change in speed also produces a
change in output frequency, therefore field control is normally used, the
power electronic drive system operating in d.c. line control or controlled
rectification modes, depending on whether the supply to the controller is
d.c. or a.c.
Since the excitation field in an induction generator is rotating, the speed
of this field can be controlled by the use of thyristor frequency changers, so
as to vary the generated output. It must be remembered that, due to the
complex nature of the transformer action involved, only part of the
induced rotor current is supplied to the stator output, the rest being
converted into rotor heat. There is an optimum relative speed at which the
output is a maximum. Excitation control may be used to control the
magnitude of the output, although it is normally kept proportional to
frequency, to maintain constant flux.
Synchronous generators, called alternators, are popularly used for
power generation and these are treated here in further detail. Figure 14.56

3@+zp 0
Regula tor Rectifier

- Rectifier