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Message
Ed,
I wrote up step-by-step instructions a
while back on converting an ND alternator from internal regulator to external.
If anyone is interested, let me know and I will send it to them directly.
This conversion makes it so that you can shut the thing down by cutting the
power to the field wire as is the method used by the Perihelion Design
OVM.
Mark S.
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Ed Anderson
Sent: Thursday, September 08, 2005
2:24 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Overvoltage control (help Ed A)
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.
----- 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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