“How Generators & Regulators Work”
Once you understand the basics of how a battery works and how it is constructed, we can move on to the generator, which is the second most important parts of the electrical system. To sound bona fide, I might as well give you the official job description of the generator. It is “a machine that converts mechanical energy, supplied by the engine, into electrical energy used for either recharging the battery or supplying power to the electrical system.” While the description seems a little confusing, if you follow along a little further we will make sense out of it all. Come on, it’ll be better than you think. THE WORK SCHEDULE FOR THE GENERATOR FAMILY When the engine speed is at idle or at low rpm, the generator has little or no output, and the battery provides all the electrical energy needed for the electrical system. When vehicle speed reaches about 20 mph or engine rpm reaches about 1200, the generator will begin to charge. The output will help the battery with some of the electrical load. (This speed is known as the generator “cut-in” speed.) At higher engine rpm of about 1800, the generator is capable of providing all of the electrical current needed to run the accessories, as well as recharge the battery as needed. Generators will usually provide their maximum output at about 1800 to 2300 rpm engine speed. Normally the pulley diameter of a generator is designed so the engine will spin the generator at, or close to, its ideal rpm, (the rpm at which the generator operates most efficiently.) This rpm is matched to the rpm at which the engine will spend most of its time.
IN MOST OLDER CAR APPLICATIONS, THE GENERATOR ARMATURE TURNS ABOUT TWICE FOR EVERY RPM THE ENGINE TURNS.
When a generator spins at high speeds (above 3500 rpm engine speed) the output of the generator will actually drop off quite a bit, as the brushes are lifted off of the armature by centrifugal force. If heavier brush springs were used (a great idea), it would cause excessive brush wear at the slow speeds. An interesting note: Did you ever wonder why over the road trucks get such long life out of their generator brushes as compared to a car? Here are the reasons. One is the constant rpm that make it easy to match the correct engine to generator speed. The other factor is called air gap. This is when the brushes lift off of the commutator just slightly due to the centrifugal force. The brushes will then experience minimum wear because the brushes are not physically touching the commutator and the loss in output will be slight. Cars driven in town will wear out generator brushes at a much faster rate than those that spend their life traveling up and down the highway. The same principle applies.

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