Solid State Voltage Regulators For 3 Brush Motorcycle Generators


Home Up Next




The three brush generator design required removing the cover and adjusting the position of the third brush in order to change the charging rate. By moving the position of the third brush the voltage applied to the field windings was changed. This in turn changed the strength of the magnetic field applied to the rotating armature and thus set (or limited) the maximum charging rate, or current output, of the generator. The generator would continue to charge at this fixed current rate all the time, regardless if the battery was boiling over or dead, lights on or off, it didnít matter. This was fine if the electrical load on the system remained constant, but motorcycle loads usually do not.  If the battery was low, or additional loads such as lights were turned on, the generator output remained the same.  In short, the 3 brush generator cannot automatically compensate for any type of varying electrical loads.

The 3 brush generator did not have any type of voltage regulator built into it. Adjusting the 3rd brush only changed the current output of the generator. The battery essentially became the voltage regulator in the system. As a result, the generator consistently applied an excessively high voltage to the battery (on the order of 8 Ė 8.5 volts). In fact, as the battery reached a fully charged state, the generator current output actually increased slightly as did the battery voltage! Over prolonged periods, this would cause overcharging of the battery.  Many batteries and fine paint jobs have suffered from this poor regulating system!

If for some reason the battery were to be dropped out of the circuit, the generator output voltage would now be unregulated.  If allowed to run unregulated, the generator voltage would quickly rise and could easily get up to 40 or 50 volts.  This would in turn burn out the light bulbs and other electrical equipment, such as ignition coils.  Excessive heat would be generated internally and quickly damaged the generator as well. 



In an effort to eliminate over and under charging problems, both Harley Davidson and Indian adopted their own version of a two charge system. Both systems still used a 3 brush generator but provided a high output as well as a low output setting.  Moving the 3rd brush would change both settings simultaneously, proportionately limiting the maximum current output of each setting.

 Harley Davidson models were equipped with the model 32 E generator starting in 1932. This remained standard equipment through 1954. Operation consisted of energizing only one smaller field winding during engine operation only.  This provided enough current output from the generator to run the ignition system and place a small constant charge into the battery. When the lights were turned on, the second larger field winding was energized, resulting in a much higher output.  This high output could run the lights and ignition as well as a small charge into the battery. Properly adjusted, the Harley system was simple, reliable and effective, as long as standard lighting equipment was retained.

Starting in 1938 as an option, Indian Chief, Scout and 4 cylinder models could be equipped with the TC regulator and a model GDE high output generator.  This system became standard equipment in 1948- 1953. However, the verticals were never equipped with this system.  Operation consisted of energizing both field coils simultaneously. During low battery voltage conditions, all of the voltage provided by the 3rd brush was applied directly to the field windings, resulting in a high charge rate.  When battery voltage rose above a set point, a relay inside the regulator would switch and insert a large carbon resistor into the field circuit.  This dropped the field voltage considerably which resulted in a much lower generator output.  In theory, if adjusted properly, these 2 charge rates would match the electrical demands when running with lights on, or off.  This system worked well but only with standard lighting equipment and when adjusted properly. However, it did require routine maintenance and very careful adjustment of the relay points and spring tension within the TC regulator. The large carbon resistors could break or overheat and burn out, which would result in a constant high charge rate.



The cutout relay, or reverse current relay, is a switch located in line between the generator and the battery.  The purpose of this relay is to prevent battery discharge through the generator whenever the engine is shut off. If this relay were not in the circuit, the battery would energize the generator and attempt to turn the generator like a DC motor. This would quickly result in a dead battery and a very hot or possibly damaged generator! 

Some early cutout switches were centrifugally operated. Most cutouts however are electro-mechanical relay switches. Regardless of the design, both types required periodic maintenance and adjustment. Over time they became prone to sticking closed at times. This would of course drain the battery in a very short time.

Email Inquiries to: