If you want to take an alternator off line remove the field current to
that alternator. Do NOT open the output of the alternator. The
alternator is a rotary current amplifier. Most 12 volt 50 to 60 amp
alternators have about a 2 amp field current which implies the current
GAIN is around 30 amps per amp.
On a piston engine the cranking speed is much too low to generate
enough output voltage to get much current generated until the engine
starts. No output current means no torque required to spin it.
However you will be drawing full field current of 2 amps or so which
is about 1% to 2% of the cranking current. This additional current
must be supplied by the battery which does not help but don't hurt
much either. At idle speed and full field current ,which the
regulator will give since the battery voltage is low, if the output is
not connected to the battery the alternator voltage will rise to 50
volts or more. The open circuit alternator output voltage depends on
RPM and field current. The voltage is proportional to the magnetic
flux and the rate of change of that flux. which is the alternator
field current and RPM. This extra voltage will not help extend the
life of the diodes in the alternator or the circuit breaker contacts
as you will get a good arc when the breaker contacts bounce open upon
closing of the breaker

A turbine engine is an entirely different story. At the end of a
turbine start cycle the turbine has to be running at about 50% - 66%
RPM before the engine loop gain exceeds one. At this speed the
alternator would be spinning fast enough to be generating a lot of
current if it had field current. If the alternator were 100%
efficient it world not matter if it was generating or not however I
have yet to see one that even approaches this efficiency in an
aircraft. This means that if the alternator field is enabled the
starter may not be able to spin the engine past the point where the
engine gain is at least one. This will yield a hung or hot start.
Some turbines have separate starters and alternators. However in
many cases the alternator/generator on a turbine is also the starter
motor so this is a moot point as it is impossible to have both
functions at the same time.

On Mon, 20 Oct 2003 23:33:16 -0400, "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.