A machine in which mechanical energy is converted to electrical energy. Generators are made in a wide range of sizes, from very small machines with a few watts of power output to very large central-station generators providing 1000 MW or more. All electrical generators utilize a magnetic field to produce an output voltage which drives the current to the load. The electric current and magnetic field also interact to produce a mechanical torque opposing the motion supplied by the prime mover. The mechanical power input is equal to the electric power output plus the electrical and mechanical losses.
Generators can be divided into two groups, alternating current (ac) and direct current (dc). Each group can be subdivided into machines that use permanent magnets to produce the magnetic field (PM machines) and those using field windings. A further subdivision relates to the type of prime mover and the generator speed. Large generators are often driven by steam or hydraulic turbines, by diesel engines, and sometimes by electric motors. Generator speeds vary from several thousand rotations per minute for steam turbines to very low speeds for hydraulic or wind turbines. See Diesel engine, Hydraulic turbine, Motor, Prime mover, Steam turbine, Wind power
The field structure of a generator establishes the magnetic flux needed for energy conversion. In small generators, permanent magnets can be used to provide the required magnetic field. In large machines, dc field windings are more economical and permit changes in the magnetic flux and output voltage. This allows control of the generated voltage, which is important in many applications. In dc generators the field structure must be stationary to permit a rotating mounting for the commutator and armature windings. However, since the field windings require low voltage and power and have only two lead wires, it is convenient to place the field on the rotating member in ac generators. See Electric power generation, Electric rotating machinery, Windings in electric machinery