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Jim,
I first explored this issue after Bob Knuckols expressed a dislike of
internally regulated alternators...because they can fail in such a way
that you cannot shut them down. Yes, you can interrupt the "B" lead,
but this will not shut down the alternator. I imagine it could get hot
enough to self-destruct and possibly catch fire.
The modification I use removes the internal regulator assembly and puts
a direct (external) feed line to one of the brushes. The other brush is
tied to ground. Nothing else is connected to the field. With this
arrangement, can you explain how removing power from one of the brushes
will allow the alternator to continue producing power? I've tested my
modified alternators with the engine running and it does in fact stop
producing power when I cut the power to this lead.
Another benefit to an external regulator is that you can precisely
adjust the voltage, provided you use an adjustable regulator.
Mark S.
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Jim Sower
Sent: Thursday, September 08, 2005 5:31 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Overvoltage control (help Ed A)
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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