>>There must be about 100,000,000 automobles out there running around
without a
>>switch on the alternator field. Why is a switch so common on our little
>>airplanes. Or is it a case where I can't find mine on all my cars and
>>motorcycles. Someone tell me the problem of not having the switch.
>
> Excellent question . . . and sure, the answer is simple. In airplanes,
> the prudent way to architecture and operate an electrical system is to
> assume that at some point, each and every electrical system component
> is capable of failing in flight. After deducing ALL the ways an alternator
> system can fail, one finds several conditions under which you need to have
> separate and total control over the alternator field current. (1) the
> alternator has become disconnected from the bus but is still drawing
> maximum field current because the regulator thinks the alternator is
> simply shirking . . . it applies full field current in hopes of
> flogging the alternator to life. In flight, this is a LARGE and useless
> drain on the battery that you'd like to conserve for running really useful
> things. (2) You're on short final to an unplanned arrival with the earth
> and you want a way to get all electrical sources shut down before
> contact with the rocks, trees, whatever . . . (3) If for any reason,
> one might turn the battery master off before stopping the engine, the
> alternator may continue to run, self-excited but poorly regulated
> and be a real bother to other electrical sytem parts that may object
> to the poor quality power. There might be others but these are pretty
> compelling.
>
P.S. Actually, most automotive alternators DO have a switch. It's
the one your key is stuck into. There's often a control line running
from the keyswitch to the alternator's built in regulator that
kills the alternator when the key is turned off . . . and 99.9%
of the time, it works. Problem that this line is not THE single
supply source of field current . . . simply a control line to some
itty-bitty transistors and stuff inside the regulator. If we're
prudend in the design of our electrical system, we have to concede
that those devices can fail to funtion . . . therefore, the stock
control line into an automotive alternator cannot be depended upon
for 100% authority over field current. This is the MAIN reason
why B&C takes perfectly good, brand new Nipon-Dienso alternators
apart to remove and bypass the built in regulators. When you open
the alternator field switch for a B&C alternator (as most others
flying around our there) the alternator is OFF.
Bob . . .
AeroElectric Connection
////
(o o)
| |
| Go ahead, make my day . . . . |
| Show me where I'm wrong. |
|