nuckollsr(at)cox.net
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 Posted: Sun Jun 11, 2006 6:13 am Post subject: one Main switch or 2 spst switches? Alt. field circuit br |
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At 01:39 PM 6/10/2006 -0700, you wrote:
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Bob:
Your diagrams all show a 2-10 Main switch which is apparently hooked
up to give us OFF, Battery Only, and Battery+Alt field. I can see the
merits of this. But I happen to have a number of high quality, sealed,
mil spec. Cutler-Hammer SPST switches that I'm itching to use. Is there
any reason not to use one for the battery and one for the alt. field coil
instead of the 2-10?
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| Quote: | This combination would allow having the alt. field ON while the battery is
OFF, which is likely to make the alternator unhappy, but is it
dangerous? I'm thinking that would come under the "don't do that"
heading, like so many other things about flying.
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You may use any switches you wish. It's your airplane. The rationale
for two poles in the DC power master switch of alternator-fitted aircraft
was to PREVENT alternator-only operations where the system's operating
characteristics under these conditions were not fully explored.
The Bonanza's and Barons have separate switches . . . in fact, those
alternators will come on line self-excited without a battery
(but that's another story). I was not privy to any testing Beech may
have done at the time this system was certified to demonstrate
performance. I do know that a regulator I designed for that system
was required not to degrade the alternator's ability to self-excite.
Special cases aside, the vast majority of aircraft have been produced
with variations on the infamous split-rocker switch. A device
designed to provide control of the battery-alternator combination
while specifically preventing alternator-only operations. Hence
the use of a 2-3 or 2-10 switch in my drawings.
| Quote: | Also, you mention in chapter 10 of your book that you like to have a
circuit breaker on the alt. field rather than a fuse because of over
voltage situations, but you don't explain why it's actually handy to be
able to reset this one circuit.
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If you choose to incorporate crow-bar ov protection there
is an operational desire to be able to reset it one time.
There is also a chance of nuisance triping where being able
to reset is useful. Hence a breaker is used in lieu of fuses
and also located on the switch panel. See:
http://www.aeroelectric.com/PPS/Switch_Panels/Switches.pdf
Other forms of ov protection may not benefit from this
configuration and could drive the field circuit directly
from the bus through a fuse.
Bob . . .
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