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Old 12-01-2002, 04:51 PM
Richard Wooldridge Richard Wooldridge is offline
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Join Date: Dec 2000
Location: Battle Ground, WA
Posts: 576

Hi there,
Medical Dave, I must take violent exception to a comment you made - you stated that you can reverse charge your battery with an alternator - IMPOSSIBLE! The diodes in an alternator ONLY allow it to put out a positive voltage! I suspect you have forgotten a statement taught in electrical "machine" classes - at least it was in my college class - that a DC generator can be field flashed to put out an opposite polarity voltage, due to the fact that it only has the residual magnetism to get it started and if the magnetic field is reversed by applying the opposite voltage to the field winding for a few seconds, the generator (and regulator) will happily put out the opposite voltage. I have personally done this, so I know it works. If you attempt to produce the opposite voltage with an alternator, which is an AC machine, the diodes will rectify the sine wave and always produce a positive output, unless the rectifiers are physically resoldered into the circuit in the opposite direction, which is usually impossible due to their construction. Automobile alternators are all 3 phase AC machines, with 6 diodes connected so that the output is a positive DC voltage with very low ripple, due to the 3 phase design. Their field winding must have a low voltage (around 3 volts for most alternators) applied, usually through the idiot light circuit, to function, although typically the integrated regulator designs don't even need the lamp circuit connected to operate.
I'm still confident that Josh MB's actual problem is that the alternator belt is glazed and allowing the alternator pulley to slip on startup, a condition that only a new belt will fix. I have seen this problem many times, and it really isn't obvious sometimes. Usually, if neglected, it causes the alternator to overheat and burn the diode stacks after awhile, because the alternator is putting out maximum amps at a low speed, even though it's unable to put out the proper voltage, which means the fan cannot cool it properly. Please replace the belt, JoshMB!
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