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Originally Posted by George Tyler
as that document shows, the diodes are a big loss at high currents, at low current there is the magnetic losses and also the fan etc. I have been thinking of doing the mosfet rectifier thing , but using an international rectifier chip that does the same thing.
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Do you have a part number for this chip? The only thing I have seen is reference to something they designed for another company which isn't for sale to others.
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de-energising the alternator removes the magnetic losses, but the windage losses are still there. Some cars now have electromagnetic clutches on their alternators, they would not do this if it did not reduce consumption. Failing this, how about removing the alternator fan and using a controlled electric fan instead, and also disconnecting the feild winding when it would help?
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Actually most alternator designs use Pulse Width Modulation changing the percentage of on time to off time to vary the field strength. This way it controls the output voltage of the alternator and creates only as much electrical/magnetic related drag as needed. This is why an alternator is more efficient than a generator.
From the papers I have read so far mechanical losses and fan losses only make up 5 to 10% of total losses in an alternator. However I agree with you and do think that there is room for improvement. I'm not so sure an electrical fan is the way to go. You want it running almost all the time if not all the time to keep the copper coils cool and thus keep their electrical resistance low. An electric fan is going to suffer an penalty efficiency drop equal to the drop of the alternator loss itself vs a directly driven one. Looking at most alternator fans its obvious that their shape could be improved quite a bit.
You mentioned some cars now have electrical clutches on their alternators. Do you know of a particular model? I might be interested in getting one or two from a junkyard or maybe new if I have too for experimental A/B testing.