The Lost Art of Rebuilding Generators Part III COLUMN BY BOB THOMAS The first time I came across a three- brush generator, I had no idea what purpose the extra brush served. It is true there are not many of these generators left today, but the ones that remain are from antique vehicles that belong to potential cus- tomers. They want them right and have the money to pay you to make them that way. Understanding three-brush generators can put some of that money into your pocket. There is nothing mysterious about that extra brush. It is just a secondary voltage source, used to supply current for the field, similar to the diode trio used in alternators we are more familiar with. Because the third brush is offset from the neutral point (the brush position at which maximum benefit of the field magnets can be obtained), the voltage being picked up is reduced. In this way, the maximum current output of the generator is controlled internally. For this reason, three-brush generators are used with cutout relays only, like Model A generators, or with two-unit regulators (one cutout and one to control voltage), like some 1930s cars and many tractors. Three-brush generators came on the scene in the early days of automotive electrical systems. There were no high demand devices to power back then. The battery needed to be maintained for the cranking motor, and there were headlamps for those brave enough to drive at night. It was a simple generator for a simple time. If you test and adjust a three-brush generator on your test bench, you will notice that as you increase the speed, amperage will climb (as you would expect), but only to a point. Once you pass the mid-range rpm, amperage will begin, drop off slightly and then stabi- lize. This is a normal phenomenon, and is caused by the distortion of the mag- netic lines being cut by the armature (see Figure 1). As the speed is increased and current flow within the coils of the rotating armature increases, the magnet- ic lines between the field poles are twisted. This distortion is caused by the interaction of the magnetic lines of the field poles with the magnetic lines of the armature windings (which are carrying the full amperage output of the genera- tor). This desired characteristic serves to limit the amperage output of the three- brush generator. Most field brushes are adjustable, and to make the adjustment properly, you need to know the electrical load of the system. You also need to know if a voltage regulator is being used. You should make the adjustment to match the amperage requirements of the vehicle. Moving the field brush closer to the hot brush will increase the output (because you are getting closer to the neutral position). If the system uses only a cutout, set the brush to produce only a few amps beyond the normal electrical load. A common mistake is to make the adjustment to see 7 volts (nearly all of these were on 6 volt systems). You can do this if the adjustment is made on the vehicle, with a fully charged battery. But on your test bench, use amperage to guide your adjustment. I have found that 9 amps is about right for a Model A generator, used for normal daytime “Sunday drives.” If the car is making a road trip, the driver should be prepared to monitor the charge and possibly make an adjustment if needed. Experienced A Model owners carry a wooded stick for this purpose. If your three-brush generator came off a tractor, or later model car, it may have a regulator to control voltage. Do not be fooled into thinking that proper adjustment is not needed. You must still limit current to prevent it from being overheated. Adjustment is made the same way. Try to set the current to just a few amps over what the system requires. Successful adjusting of the three- brush generator depends upon communi- cation with the customer. They probably FIGURE 1 Low Load Low Speed Medium Load Medium Speed High Load High Speed won’t know their amperage require- ments, but you can ask questions like: Does it have headlights and how often do you use them? How often do you start and run the vehicle, and for how long? Taking some extra time to ensure your product will last adds value to your work and builds confidence in your cus- tomer. Thanks to Henry Henke, United Starter & Alternator, Lowell, IN, for providing some of the information I have shared. Please feel free to email me at: elebob@aug.com. Bob Thomas is owner of B & H Electric, Jacksonville, FL.