mikem wrote:
: Try this:
: http://home.utah.edu/~mgm17160/Alternator/

I see you didn't try to model the load dump transient... Trouble with that
one is that it's dependent on all sorts of other loads on the bus.

For those that didn't look up the potential damage of load dump transients,
the trouble is the tremendous amount of inductance in automotive claw-pole
alternators. Yes they're 3-phase synchronous machines, with six-pack diode rectifiers
to get "DC.".... very crappy 3-phase machines. When charging a battery that's sucking
down tens of amps, the backEMF necessary to overcome the AC voltage drop across the
(large) stator inductance can often measure 60-90V. If that battery load suddenly
goes away, all that magnetic energy stored in the inductor tries to keep the same
current flowing by increasing the output voltage. Then all your avionics go *poof*.

Now, if you've been flying for awhile and your battery is all charged up, it
will only be taking an amp or two. All other loads remaining constant, you could
disconnect the battery and run on just the alternator without hurting anything.
Trouble is, any other transient load shedding (gear motor, pitot heat, etc) will not
have the battery there to absorb it.

It seems like it might be useful as an emergency limp-home mode if the battery
somehow fails. I'm having a hard time envision a scenerio where it would be
safe/useful to do (other than a fried master relay).

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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