Power passes from the engine through the coupling to the generator which will take one of two forms:

• A single-bearing generator with the frame spigot mounted directly to the engine crankcase and the driven end of the generator shaft supported by the engine crankshaft via a coupling

• For larger sizes, a two-bearing generator may be used. The engine and generator are solidly mounted on a base frame and the generator is driven through a flexible coupling.

Diesel engine driven generators will run at engine speed (1000 or 1500 rpm for 50 Hz) and will have salient pole rotors. Gas turbine driven generators will usually run at 3000 rpm (for 50 Hz) and will probably have cylindrical rotors.

The distribution voltages in common use within the United Kingdom are 400 and 11,000 volts and generators will usually use one of these voltages. It is not generally economical to manufacture a high voltage machine for ratings below about 1 MVA, so below this size generation may be expected to be at low voltage. The economics depend on the material content of the stator windings, a low voltage machine with a high rating would include an excessive amount of copper and a small amount of insulation, whereas for a high voltage machine with a low rating the reverse situation would apply.

However, a rating of 1 MVA at 400 volts results in a line current of 1443 amperes, and cables for such a current are quite large and may be...