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
Turbo-generator with inner water-cooled stator and rotor under construction.
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.
More commonly used in smaller and medium power ranges is the salient-pole generator. Here, the rotor windings are wound around the poles which project from the centre of the rotor. The stator construction is similar in form to the turbo-generator stator shown
Stator Salient Pole Generator
Rotor Salient Pole Generator 12 MW 16 Poles
Less commonly used are induction generators and inductor alternators. Induction generators have a simple form of rotor construction as shown
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.
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.