The two main types of generator are ‘turbo’ or
cylindrical-rotor and salient-pole generators. Both these types are synchronous
machines in which the rotor turns in exact synchronism with the rotating
magnetic field in the stator.
The largest generators used in major power stations are
usually turbo-generators. They operate at high speeds and are usually directly
coupled to a steam or gas turbine.
The general construction of a turbo-generator is shown in
Fig. 5.1.
The rotor is made from solid steel for strength, and
embedded in slots within the rotor are the field or excitation windings. The
outer stator also contains windings which are located in slots, this is again
for mechanical strength and so that the teeth between the slots form a good
magnetic path.
Most of the constructional features are very specialized,
such as hydrogen cooling instead of air, and direct water cooling inside the
stator windings, so only passing reference is made to this class of machines in
the following descriptions.
The stator construction is similar in form to the
turbo-generator stator shown in Fig. 5.1. Less commonly used are induction
generators and inductor alternators.
Induction generators have a simple form of rotor
construction as shown in Fig. 5.3, in which aluminium bars are cast into a
stack of laminations. These aluminium bars require no insulation and the rotor
is therefore much cheaper to manufacture and much more reliable than the
generators shown in Figs 5.1 and 5.2.
The machine has characteristics which suit wind turbines
very well, and they also provide a low-cost alternative for small portable
generators.
Inductor alternators have laminated rotors with slots,
producing a flux pulsation in the stator as the rotor turns. These machines are
usually used for specialized applications requiring high frequency.
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