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I've always been puzzled why folks would go to all the trouble of disabling a perfectly good internal regulator so as to install an external unit. If the Field circuit gets somehow internally shorted to the output circuit, the alternator is going to run away and create an over voltage condition. In that event, there's no way that turning off current to the Field is going to help you. Other than a short described, I don't know how an over voltage can occur. So my question is: is an internally regulated system all that much more apt to have an over voltage type failure, and if so why? I've had lots of trouble with externally regulated alternators, very little with internal regulators. My crowbar over voltage system is bullet proof. It's the only scheme that prevents damage caused by an internal short in the alternator.
What, exactly, is the big attraction of external regulators? ... Jim S.
kenpowell@comcast.net wrote:
Bernie, as you know (I sure wish Tracy lived close to me!!!) it really pays to have friends with the skills to do those tasks that we know nothing about. Kelly Troyer modified my stock Mazda (mitsubishi) alternator to be externally regulated so I can now use the common over-voltage schemes/modules that interupt the field wire.
Ken Powell
Bryant, Arkansas
501-847-4721
C150 / RV-4 under construction
-------------- Original 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.
Ed
----- Original Message -----
*From:* WALTER B KERR <mailto:jbker@juno.com>
*To:* Rotary motors in aircraft
<mailto:flyrotary@lancaironline.net>
*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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