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Originally Posted by david riley
Quote:
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Originally Posted by jazzbass
basically there is not enough excitation current to the alt at low RPMs
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What do you mean  As previously stated alternators require outside stimulation before they can produce the correct response. Should I have touched briefly on how the whole system is based on switching back & fourth between AC/DC  |
Right... not sure what the question is - I think we're saying the same thing. The excitation current is the outside stimulation. An alternators is a 3 phase AC generator, in essence, with a set of diodes that rectify the AC into DC. The stator of the alt is what produces the 14V+ of AC (then turned into DC) the car runs on.
The rotor needs an excitation current of up to 7A to produce the rotating magnetic field necessary to induce the current on the stator. Once the alternator is spinning high enough, it will feedback current from the output to "self-energize" the rotor. However, this feedback condition needs a "bootstrap" current to get started, and this is the current supplied through the alternator warning light. This is the reason why your system will not charge if that light is burned out. The issue with the older cars was that the resistance of the bulb was too high to produce enough pre-excitation current. So, putting a resistor in parallel with the bulb lowers the overall resistance of the pre-excitation circuit, producing more current (because, of course, I = V/R).