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From: Lehanover@aol.com
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Date: Sat, 22 Jan 2005 21:26:32 EST
Subject: Re: [FlyRotary] Re: alternator problem fixed
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Could you install a diode in the line so that no energy from the 
 battery could be used by the alternator, along with a fuse? 

Lynn E. Hanover

 That would defeat the purpose of the fuse.  If battery current can't 
 flow toward the shorted B+ or grounded alt field winding such as Paul 
 had, what is your power source for blowing the fuse?  A diode would 
 result in say a grounded B+ to result in the alternator thrashing its 
 brains out pumping all the current it was capable of straight to 
 ground.  Fuse exceeds alt output so it wouldn't blow, and you'd be on 
 battery power while the alt tries valiantly, but in vain, to blow the 
 fuse by itself.
What would be the benefit from that? ... 
Jim S.
 
I see a number of situations where the rather high value of the proposed
fuses might come back to haunt you.

Any failure the produces a load that is less than the fuse value, can still 
be a big load on the battery. 

If you believe that the fuse is designed to protect the wire and the aircraft 
from a fire hazard, I would think that the fuse would be mounted on or very 
close by the alternator. The diode would be at the other end mounted on or 
close by the battery terminal. Should wire to the battery be damaged or short to 
ground, the battery cannot "see" it. No energy from the battery is expended, 
nothing that the battery is supporting gets damaged or even shuts down for a 
second.The wire from the short to the alternator is protected by the fuse. Fuse 
blows, idiot light comes on, you go home.

An alternator failure that results in any voltage lower than the battery 
voltage has no affect other than loss of charging voltage. Idiot light comes on, 
you go home. 

Battery suffers internal short from over charging. Bad smell, idiot light 
comes on. Alternator continues to charge (now a 10 volt battery) with 14.2 volts, 
smell gets worse, as battery gets hotter. The 100 amp fuse gets hot, but 
hangs in there. 

You flip marine battery management switch to backup battery, you head for 
airport, any airport. You drop GPS, it goes under the seat, damn......... 
Alternator turns primary battery into hydrogen bomb, you cross the fence way too fast 
trying to remember how to forward slip, and wondering why there is a 100 amp 
fuse in that system. 

I can weld with 100 amps. You can too.

I suspect that sizing the fuze or better yet the breaker, requires more 
thinking. I suspect that a small drycell backup to run just the all electronic 
engine is a must have piece. 

If you cruise at 5,500 RPM instead of the 1,900 RPM in the street car, I 
would think about slowing the alternator speed with a bigger pulley. The 
alternator should not be functional at cranking speeds, and should not be a factor in 
the voltage reduction due to cranking. I think any switch that moves engine 
systems to the backup battery should not only disconnect the alternator but also 
cut the field circuit. 

Lynn E. Hanover