commutator, and extreme care must be used when cleaning  it to avoid marring or scratching the surface.  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.  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. 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  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.  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  Compound Wound DC Generators Parallel (Shunt) Wound DC Generators Series Wound DC Generators Types of DC Generators Figure 9-50.  Shunt wound generator.  Figure 9-49.  Diagram of a series wound generator.  A B N S + − Field rheostat Field winding Load Shunt circuit Main circuit Armature winding Armature Field coils N S + − Field rheostat Field winding Load 9-32