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 .
Except for permanent magnet generators, a generator produces
output voltage proportional to the magnetic field, which is proportional to the
excitation current; if there is no excitation current there is zero voltage. A
small amount of (electric) power may control a large amount of power. This
principle is very useful for voltage control: if the system voltage is low,
excitation can be increased; if the system voltage is high, excitation can be
decreased. A synchronous condenser operates on the same principle, but there is
no "prime mover" power input; however, the "flywheel effect"
means that it can send or receive power over short periods of time.
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 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 a 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.
The critical field resistance is the maximum field circuit
resistance for a given speed with which the shunt generator would excite. The
shunt generator will build up voltage only if field circuit resistance is less
than critical field resistance. It is a tangent to the open circuit
characteristics of the generator at a given speed.