Odd alternator behavior

Jerry Springer wrote:
: A car doesn't even have such a switch and you can't show me that the
: transients are any different in an airplane. Remember, mags aren't even
: connected to your electrical system while the old automotive Kettering
: ignition system was. I defy one to say there is any different between the
: starting system either.

: So correct Cy, it is an old wives tale about turning the alternator on and off
: before start and shutdown.

: Jerry

I will concede that almost always there will be no problem. I have looked at
the problem in great detail working on my masters on a prototype hybrid electric
vehicle. The inductance of the armature on a car alternator (plane is usually the
same) is quite high. The back EMF running full rated output is often 80-100V. If the
load disappears, the voltage will get very large. The OWT has merit for
protectionless alternator circuits and people disconnecting a dead battery to see if
the car will run on the alternator. If the alt was dumping 100A into the battery,
it's unloaded like that, the inductive energy spikes the bus.

Regarding turning alternator on and off, I prefer not to run my equipment at
its maximum unless necessary. I think having the regulator running field current
through the brushes at shutdown and startup is the electrical equivalent of yanking
full flaps on the instant you decellerate to the edge of the white arc on the ASI.
YMMV, it's your plane and I'm sure it'll be fine.

-Cory

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering Ph.D. Graduate Student *
* Virginia Polytechnic Institute and State University *
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