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. For a machine using field coils, as is the case in most large generators, the field must be established by a current in order for the generator to produce electricity. Although some of the generator's own output can be used to maintain the field once it starts up, an external source of current is needed for starting the generator. In any case, it is important to be able to control the field since this will maintain the system voltage.
Amplifier principle Except for permanent magnet generators, a generator produces output voltage proportional to the magnetic flux, which is the sum of flux from the magnetization of the structure and the flux proportional to the field produced by the excitation current. If there is no excitation current the flux is tiny and the armature voltage is almost nil. The
field current controls the generated voltage allowing a power system’s voltage to be regulated to remove the effect of increasing armature current causing increased
voltage drop in the armature winding conductors. In a system with multiple generators and a constant system voltage the current and power delivered by an individual generator is regulated by the field current. A generator is a current to voltage, or
transimpedance amplifier. To avoid damage from progressively larger over-corrections, the field current must be adjusted more slowly than the effect of the adjustment propagates through the power system.
Separate excitation For large, or older, generators, it is usual for a separate
exciter dynamo to be powered in parallel with the main
power generator. This is a small permanent-magnet or battery-excited dynamo that produces the field current for the larger generator.
Self excitation Modern generators with field coils are usually
self-excited; i.e., some of the power output from the rotor is used to power the field coils. The rotor iron retains a degree of residual
magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field induces a weak current in the rotor coils, which in turn creates an initial field current, increasing the field strength, thus increasing the induced current in the rotor, and so on in a feedback process until the machine "builds up" to full voltage.
Starting Self-excited generators must be started without any external load attached. Any external load will sink the electrical power from the generator before the capacity to generate electrical power can increase.
Variants Multiple versions of self-exitation exist: • a shunt, the simplest design, uses the main winding for the excitation power; • an
excitation boost system (EBS) is a shunt design with a separate small generator added to temporarily provide an energy boost when the main coil voltage drops (for example, due to a fault). The boost generator is not rated for permanent operation; • an auxiliary winding is not connected to the main one and thus is not subject to voltage changes caused by the change of the load.
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 field coil 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.
Brushless excitation Brushless excitation creates the magnetic flux on the rotor of electrical machines without the need of carbon brushes. It is typically used for reducing the regular maintenance costs and to reduce the risk of brush-fire. It was developed in the 1950s, as a result of the advances in high-power
semiconductor devices. The concept was using a rotating diode rectifier on the shaft of the synchronous machine to harvest induced alternating voltages and rectify them to feed the generator
field winding. Brushless excitation has been historically lacking the fast flux de-regulation, which has been a major drawback. However, new solutions have emerged. Modern rotating circuitry incorporates active de-excitation components on the shaft, extending the passive
diode bridge. Moreover, their recent developments in high-performance wireless communication have realized fully controlled topologies on the shaft, such as the thyristor rectifiers and chopper interfaces. ==References==