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Hi, I am thinking of doing something similar. I have a Toyote estima with a mid undrfloor mounted engine and a drive shaft to the accessories in the front. I have the shaft off at the moment as the couplings were bad, I am thinking of leaving it off.
an alternator has about a 3:1 ratio to the engine, so when the engine is doing 3300 rpm the alternator is doing around 10,000 RPM. at this speed the fan on the alternator as well as the windage is a major loss, also the iron goes though reversal of magnetic feild at a very high rate, as they are 6 pole the frequency is P/2XRPM/60 = 500Hz, and will incur high losses.
an alternator with a "feild wire", often designated "WL" for "warning light" does not get it's feild current through that wire but only uses that to initiate generation, at least in the older ones I have worked on. May be different now, not sure. the warning lamp is connected from that wire to the battery through the ignition switch, the field winding through the slip rings is connected to that wire in the alternator, the other end of the feild winding connects to the regulator and then to ground. that is the path the current takes before the engine is running. That is why if the warning lamp is blown it will not charge, it needs the tiny current through the warning lamp to "kick start" the alternator, once the alternator has started generating tha voltage on that "warning lamp" wire increases to be the same as the battery voltage and as the lamp now has the same voltage both sides it goes out.
There are 3 diodes from the main windings to the "warning light" wire as well, when the egine is running curent flows from the windings through these diodes to power the feild, so that wire never carries the full feild current, and breaking that wire does not cut the feild current or stop the iron loss. An electric clutch like you talking about will cut those losses. I have seen one on a Opel (GM product, call different names in different countries.)
I am thinking about a device that raises the voltage on the feild wire (WL, warning lamp) to about 16V, that would cause the regulator to cut the current to the feild and stop the alternator generating. not for my project, though, I want to remove the alternator, aircon, fan, and P/S pump.
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