The correct way to take an alternator off line is to open the field circuit, not
the output circuit. Electro-mechanical regulators do exactly that many times a
second. The alternator can only produce an output if the field is energized.
Basically the field (which is actually the rotating part of the alternator) when
energized is a rotating magnet which then induces currents in the stator
windings. Open field means no induced currents, which mean no output. This also
avoids the load dump problem.

You can help extend the life of your alternator by not having the alternator field

energized while starting the airplane. The starter presents a load of over a
hundred
amperes when it is cranking the engine. If the alternator field is energized, the

alternator is trying to supply current into a load that far exceeds its capacity,
which
stresses the diodes in the alternator. If the alternator field is open during
starting,
the alternator produces no current, therefore the diodes remain unstressed. Your
car charging circuit is similar to an airplane's circuit except that in a car,
turning the
key to the start position deenergizes the alternator field so that it does not
produce
any output while the starter is being cranked.

If you have a split master, you can approximate the action of the automotive
starter
lockout by leaving the alternator side of the master off until the engine is
started, then
turning it on. This also has the benefit of not putting the field load (up to a
couple of
amperes) on the battery before and during the start. If you don't have a split
master,
I wouldn't advise using the field circuit breaker to turn it off, as those are not
designed
for the amount of operations you'd subject them to pulling it on every engine
start.
The other thing, is make sure you turn the alternator side of the master on once
you do
have the engine running, otherwise you'll find yourself with a dead battery before
long.


"G.R. Patterson III" wrote:

wrote:

If you open the alternator output breaker while the alternator is
generating power then you get what is known as a "Load Dump".

So, Based on this (and the rest of the info in your post), it sounds like it
would be perfectly ok to start the aircraft with the alternator out of the
circuit. I'm not sure it would be a good idea to cut one out in a twin. Does
this load dump occur if there are two alternators feeding the system and you
cut one out?

George Patterson
To a pilot, altitude is like money - it is possible that having too much
could prove embarassing, but having too little is always fatal.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

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