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Message
On alternators with internal regulators (most
Auto alternators), breaking the low current Field circuit may not (will likely
not) stop the alternator from producing current. Most of the alternators
need the field coil current to "bootleg" their start in producing voltage, but
once generating - do not need the external field coil current to
continue. That is why if you have an alternator with an internal
regulator, you can not depend on opening the low current field coil circuit to
kill the power and why you would need a method similar to the
one described in this thread of breaking the high current (Power lead usually
referred to as the "B" lead) through a high capacity relay (Contactor) or a
"fuse".
There are always side effects when interrupting a high
current flow (less if the current flow is minimal). If there is a
large inductive load (such as a large DC motor) then the back EFM can
produce a high voltage spike. There are reports that alternators may
increase their voltage dramatically if there is a sudden loss of load
-but, its never been clear to me if that is a malfunctioning generator or caused
by the lost of load - never been tempted to experiment. However, I
once had two batteries which connected to the alternator through their
contactors. I could switch the batteries on and off the alternator line
without any ill effects - not quite the same as dumping a large load - but,
after starting on just one battery, the load was fairly high when put back on
line. However, one battery was always on-line.
So not certain what effect breaking the "B" lead would be
(since it would depend on conditions) but since many designs have a "fuse"
in the "B" lead for that purpose, I am going to hazard my opinion that the
effects of breaking the "B" lead may be less serious than what-ever bad
situation is causing you to want to do that in the first place.
Ideally, of course, is to have an externally regulator alternator (or a modified
auto alternator with the internal regulator disabled so unable to provide the
field coil internally) and a low current circuit breaker tripping mechanism for
the field coil.
Ed
----- Original Message -----
Sent: Thursday, September 08, 2005 2:14
PM
Subject: [FlyRotary] Re: Overvoltage
control (help Ed A)
If you
don't have a low current way to break the field current, and need to
break the high current line, then you have to used this OV module to trip
a breaker that's holding in a main contactor for
your high current "B" lead. If any of that was confusing, Ed can explain it
:-)
Cheers,
Rusty
======================================
Help Ed. I almost understand Rusty's comments, but
Mark S left me in the dust. Us old dogs are sometimes hard to
train!
I think that with an internal regulator; just
because you break the field current supply , a failed
regulator may keep powering the field with the internal alternator
voltage. The continous breaker in the big alternator line can protect the
battery and airplane except what happens to the alternator in the meantime
if the interal regulator is applying a large field current to the field will
the alternator burn up quiet rapidly and possible start a
fire?
Bernie, soggy Treasure Coast although we had a
bonanza just taxi in so must be drying out a
little
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