[AMRadio] Ferroresonant transformer revisited

Bob Bruhns bbruhns at erols.com
Sun May 7 07:41:49 EDT 2006

My own experience with a ferroresonant transformer
was with a simple battery charger probably 10A at
14V (140W).  I don't recall that it got noticeably
hot.  If dissipation is about 20% of full load,
that would have been about 28 watts - but the unit
was bolted to a steel vehicle body, which took
away a lot of heat.

But this baby that Rick is talking about...  24V
at 50 to 75 amps?  Assuming 24V at 65 Amps, that's
about 1500 watts, and 20% of that would be about
300 watts, which can make something very hot.  If
the unit was not bolted down to something that
would take the heat away, then air convection
would have to do it, and the thing could get
pretty darn hot.  Probably it was just on the
floor, or on a wooden bench for this test.

I would figure that the unit is designed to be
mounted on some chassis and in some cabinet, which
would act as a heat sink.  If it gets too hot to
comfortably hold, then put a fan on it.  300 watts
dissipation... Rick, you said it drew about 0.9A
unloaded, is that at 240V?  That could still be as
much as 200 watts dissipation, although I don't
know about the phase angle / power factor.

One thing to watch out for with a ferroresonant
transformer is line frequency.  The output voltage
is approximately proportional to line frequency,
so if you're on a generator, it could vary a bit.
And don't expect good charge regulation on 50Hz if
the transformer was designed for 60 Hz, and vice
versa.  I learned that the hard way - and I am
glad that I wasn't the one who designed that
system, because I might have made the same
mistake.  There I was looking at the system that
had worked OK Fine in the USA, and it just didn't
work overseas - and then I noticed that the
musical pitch of the AC hum was wrong.  Arrrgggh,
50 Hz!!!  And the system was going to be used in
some places that had 50 Hz power, and other places
that had 60 Hz power...  Fortunately there were
switching battery chargers available at that time,
and the project survived.

  Bacon, WA3WDR

----- Original Message ----- 
From: "W5OMR/Geoff" <w5omr at satx.rr.com>
To: "Discussion of AM Radio"
<amradio at mailman.qth.net>
Sent: Sunday, May 07, 2006 7:14 AM
Subject: Re: [AMRadio] Ferroresonant transformer

> Jim Candela wrote:
> >Rick,
> >
> >    The ferroresonant transformer is not really
> >resonant at all since the winding that feeds
> >capacitor is wrapped around a portion of the
> >that is a saturable reactor. Resonance and
> >reactors don't really go together, but in this
case we
> >do have circulating energy in a ferroresonant
> >transformer which consumes about 20% of the VA
> >of the device. So for a 1 KVA ferroresonant
> >transformer the power draw when unloaded may be
> >200 watts, and this is pretty much a constant.
> >Therefore with a 1 KVA load, the power input
would be
> >1.2 kva. Every line cycle runs the core into
> >saturation, and this creates eddie core losses,
> >buzz, and a quasi square wave output. The
output is
> >not a sine wave unless yours has a harmonic
> >Therefore a true RMS meter is needed to
> >measure the AC voltage.
> >
> >These are really neat devices using very old
> >technology.
> >
> which covers everything *except* what Rick
asked, Jim (grinz).
> How hot is the darn thing -supposed- to get?  I,
too, wouldn't think
> that it's hot enough just sitting there, with no
load, that you couldn't
> put your hand on it..
> --
> -Geoff/W5OMR
> AMRadio mailing list
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