An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.
Excitation in generators
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| A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c. 1917. |
A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations.
A separately excited DC generator with bipolar field magnets. Separately excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator.
Separate excitation
For older and very large power generating equipment, it has been traditionally necessary for a small separate exciter generator to be operated in conjunction with the main power generator. This is a small permanent-magnet or battery-excited generator which produces the initial current flow necessary for the larger generator field to function.
Self excitation
Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.
Starting
Self-excited generators must be started without any external load attached. An external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage.
Field flashing
If the machine does not have enough residual magnetism to build up to full voltage, usually provision is made to inject current into the rotor from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart.
(source:wikipedia)
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