Types and Use of DC Generators in Aircraft Electrical System
Aircraft need electrical energy to power things like avionics, instruments, and lights on the exterior and interior. This energy is provided by the aircraft’s generators, which work in direct current (DC). DC generators transform mechanical energy into electrical energy by generating voltage with a rotating armature surrounded by magnets, and then transferring this voltage to the aircraft’s stationary loads via a set of slip rings and brushes. However, the voltage created by this arrangement is AC, so a modified slip ring arrangement, known as a commutator, is used to change the AC produced in the generator loop into a DC voltage.
There are three primary types of DC generators: series wound, parallel (shunt) wound, and series-parallel (or compound wound). They are determined by the connections to the armature and field circuits with respect to the external circuit, which is the electrical load powered by the generator.
Series wound DC generators contain a field winding connected in series with the external circuit. Series generators have poor voltage regulation under changing load, since the greater the current running through the field coils to the external current, the greater the induced electromagnetic field and the greater the output voltage is. Since series wound generators have such poor voltage and current regulation, they are never used on aircraft. Instead, generators in aircraft have field windings that are connected in either shunt or compound formats.
Parallel (shunt) wound DC generators have a field winding that is connected in parallel with the external circuit. In a shunt generator, any increase in load causes a decrease in the output voltage, and any decrease in load causes an increase in output voltage.
Compound wound DC generators employ two field windings, one in series and one in parallel with the load. This arrangement takes advantage of both the series and parallel characteristics described earlier. The output of a compound wound generator is relatively constant, even with changes in the load.
DC generators are typically rated for their voltage and power output. Each generator is designed to operate at a specific voltage, either 14 or 28 volts. All electrical systems are designed to operate at one of these two voltages, which depends on if the battery operates at either 12 or 14 volts when fully charged (generators must have a voltage output slightly higher than the battery voltage).
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