Archive for August 1st, 2009
History & Principles of Cars Alternator
Electricity is produced by moving coils of wire through a magnetic field, thus producing a current flow in
the coils of wire. Two different devices have been used on cars to produce electricity: Generators and
Alternators.
Early GM cars, up through 1962, used a Generator to produce electricity. Generators, for those of you too young to have seen one, are about the size, weight and shape of a GM starter. They use permanent magnets to produce Direct Current (DC). The magnets are located stationary around the case, and the currentproducing coils are spun on a shaft in the center of the generator. Generators are neat in that they do not need any external source of power (a battery) to begin producing electricity: all you have to do is to spin them, and they produce a DC output. But they are heavy, and they do not produce much output at low rpm: you’ll typically see the headlights on older cars with generators go noticeably dim at idle.
In 1963, GM introduced Alternators on its cars. Alternators do not have permanent magnets, but rather send a small current through a series of coils to produce an electrically-induced magnetic field. In an alternator, the magnetic field is created by spinning the electrically-induced magnetic field in the center of the alternator, producing current in the stationary, case-mounted coil. This makes an alternator much smaller and lighter, and its output at low rpm can be maintained by increasing the strength of the magnetic field. An alternator, however, does not produce DC output: Due to its design, an alternator, as the name implies, produces Alternating Current (AC). This AC must be changed to DC before it can be utilized in an automotive electrical system.
Alternating current, if visualized, is like a wave moving up and down: it cycles from positive to negative. It the mid point between positive and negative, there is no current flow at all. Obviously, then, if we only had a single coil producing AC power at low rpm, this cycling and “dead spot” would make our lights and electrical system blink on and off very quickly. Not good for our application. An alternator, then, typically has three separate coils, each producing its own “wave.” These waves are set as far opposite each other aspossible, so by the time they “overlap,” they are producing a steady stream of AC power. But now we must convert it to DC.
Each of the three current producing coils is attached to two diodes. A diode is an electrical component that allows current to flow one way, but not the other. It’s like a one-way door. One of the “one-way doors” is set to “open” in one direction, while the other one “opens” in the opposite direction. Thus, when the alternating current is flowing in the “positive” direction, the positive output is shuttled out of the one diode.
When the current shifts to the “negative” direction, it is allowed to go only out of the other diode. Thus we have separated out the two elements of the alternating current into a positive and negative DC power output. With all three of the coils doing this at staggered times, a steady stream of DC power is realized.
Pretty simply, huh?
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