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the field coils. This current is changed to DC by the commutator and carried out of the generator by two of four carbon brushes, depending on the size of the generator. Part of this current is sent to the field to create the magnetic field. The amount of current is controlled by the regulator mentioned above. It senses the voltage output and adjusts We are indebted to Frank Cianfagthe field current to an appropriate level lione, formerly of the EAA Ottawa Canato maintain the output voltage at 14.5 dian Chapter for the following article. volts. The AC alternator works on the same An alternator is an alternating current (AC) generator, and a generator is nor- principle as the DC generator except mally thought of as a direct current (DC) that the field rotates. The current generator. Both of these devices con- supplied to the field comes from an external source such as the aircraft batvert mechanical energy into electrical energy. For many years, all light aircraft tery, unlike the DC generator which is self starting. Therefore, if your battery used DC generators. In recent years, however, alternators have been used in is dead, and you start your engine by light aircraft without exception. How- swinging the prop, your alternator may not generate any current because it ever, amateur aircraft builders buy older model engines mostly equipped with needs current first before it will give you any. How does this happen to an alterDC and AC generators. nator but not to a generator? When a Direct current (DC) flows in one direction. Alternating current (AC) flows in generator is stopped, there is some left one direction for a moment and then over magnetism from the last time it opflows in the opposite direction, that is, erated. Not much, but some. This is called residual magnetism. If for some it alternates. The current in your home is AC and most automobiles today genreason, such as a mechanical blow or not using the generator for a long time, erate AC. Both the generator and alternator the voltage refuses to build up after the produce electric current, but do so in generator is turning, the reason may be slightly different ways. Both produce AC that the residual magnetism has disapinside the unit. The generator uses a peared. The method of restoring the recommutator and brushes to convert AC sidual magnetism is to flash the field. to DC. The alternator uses diodes to This is accomplished by connecting a make the conversion. wire from the battery terminal of the regThe generator has two major parts, ulator to the field terminal, while the the armature or rotating part, and the generator is turning, and then only for field frame assembly or stator (as it is an instant. The generator should spring called in an alternator) is the stationary to life. This residual magnetism will part. Both generator and alternator cause a voltage to develop the instant have voltage regulators. This is necesthe generator is turned and it is this insary to maintain the voltage at a certain itial voltage which causes a current to prescribed level (14.5 volts). The DC flow through the field coil and build up generator uses a two or three unit regrapidly to full voltage. ulator. The two unit regulator consists The alternator also has residual magof a voltage regulator, and reverse curnetism, and it also builds up a small volrent cutout relay. The three unit reg- tage in the stator coil. However, when ulator has both of these units plus a cur- this current tries to flow through the rent regulator. diodes, it is stopped cold. Most diodes need 0.7 volts across them in order for The DC generator generates electricity by the action of a coil cutting across current to flow through them in the forthe magnetic field, which is created by ward direction. Unfortunately, the vol-

tage generated by the residual magnetism in an alternator is much lower, and no output will occur from the alternator until the field receives current from an external source, namely the battery. The alternator that is not putting out power due to a failed battery can also be "flashed" by taking a smaller battery, attaching the negative pole to the alternator case and momentarily touching the positive contact from the battery to the field input, hopefully, "inspiring" the battery into output. The use of the alternator did away with boiled batteries and burned out wiring and regulators, if everything is wired correctly. Alternators are self current limiting and do not need current regulators as generators do. The alternator will provide 14.5 volts even though the engine is idling. This is accomplished by its voltage regulator which may be separate from the alternator or located inside it. depending on the model. Therefore, the battery wil not back-feed into the alternator (due to the diodes) and it always puts out 14.5 volts to keep the battery charged and operate the electrical equipment. Why then do batteries still boil over on occasion? The reason is due to the infrequent use of our aircraft. If an aircraft sits on the ground unused, the battery will slowly discharge due to internal impurities. Add to this the starting of an engine which has not run for awhile, and you have the ingredients for a potential battery boil. The battery is more likely to boil over if the aircraft becomes airborne very soon after start up. Let us not forget that alternators need some power before they give you any. If after start up with an almost dead battery and the heavy use of electricity, such as at night at a controlled field, you takeoff within a short period of time, the electrical system might fail completely. The reason for this is that there is insufficient power left in the battery to supply all of the needs and the alternator field as well. You just might find yourself airborne and have everything go dead. This can be a harrowing experience on a dark night or on an IFR takeoff into overcast at night. SPORT AVIATION 63