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commutator, and extreme care must be used when cleaning it to avoid marring or scratching the surface. 

## Types of DC Generators 

There are three types of DC generators: series wound, parallel (shunt) wound, and series-parallel (or compound wound). The appropriate generator is determined by the connections to the armature and field circuits with respect to the external circuit. The external circuit is the electrical load powered by the generator. In general, the external circuit is used for charging the aircraft battery and supplying power to all electrical equipment being used by the aircraft. As their names imply, windings in series have characteristics different from windings in parallel. 

## Series Wound DC Generators 

The series generator contains a field winding connected in series with the external circuit. [Figure 9-49]  Series generators have very poor voltage regulation under changing load, since the greater the current is through the field coils to the external circuit, the greater the induced EMFs and the greater the output voltage is. When the aircraft electrical load is increased, the voltage increases; when the load is decreased, the voltage decreases. 

Since the series wound generator has such poor voltage and current regulation, it is never employed as an airplane generator. Generators in airplanes have field windings, that are connected either in shunt or in compound formats. 

## Parallel (Shunt) Wound DC Generators 

A generator having a field winding connected in parallel with the external circuit is called a shunt generator. [Figure 9-50] It should be noted that, in electrical terms, shunt means 

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parallel. Therefore, this type of generator could be called either a shunt generator or a parallel generator. 

In a shunt generator, any increase in load causes a decrease in the output voltage, and any decrease in load causes an increase output voltage. This occurs since the field winding is connected in parallel to the load and armature, and all the current flowing in the external circuit passes only through the armature winding (not the field). 

As shown in Figure 9-50A , the output voltage of a shunt generator can be controlled by means of a rheostat inserted in series with the field windings. As the resistance of the field circuit is increased, the field current is reduced; consequently, the generated voltage is also reduced. As the field resistance is decreased, the field current increases and the generator output increases. In the actual aircraft, the field rheostat would be replaced with an automatic control device, such as a voltage regulator. 

## Compound Wound DC Generators 

A compound wound generator employs two field windings one in series and another in parallel with the load. [Figure 9-51] This arrangement takes advantage of both the series and 

