From: medin@catbyte.b30.ingr.com (Dave Medin)
Subject: Re: How do constant voltage (ferroresonant) transformers work?
Reply-To: medin@catbyte.b30.ingr.com
Organization: Intergraph Corporation, Huntsville AL
Date: Wed, 18 Nov 1992 18:58:34 GMT

In article <1992Nov18.052939.25480@u.washington.edu>, whit@carson.u.washington.edu (John Whitmore) writes:
|> In article <1992Nov15.031727.25053@imax.imax.com> dave@imax.imax.com (Dave Martindale) writes:
|> >I just bought an old Sola CVS-series constant-voltage transformer.
|> 
|> >1. How does it WORK?  I can't explain how it does its voltage regulation
|> >   trick with just a transformer and a capacitor. 

<Good description on how it works deleted>

|> >2. The label says that the transformer is "harmonic neutralized".  What
|> >   would the output waveform look like without this?
|> 
|> 	Because saturation is a kind of regulation in itself, it
|> is possible to make a third-harmonic resonant circuit to give
|> roughly square-wave output from a ferroresonant transformer (the
|> design is slightly different from what I described above, I think).

Harmonic neutralization means that the output secondary is resonant,
too (or nearly so), such that distortion products of the regulation
process are removed. Typical THD at full load is about %3 for a
harmonic-neutralized version. This is as clean or cleaner than
your average utility line. The un-neutralized ones have a
very ugly-looking output...

|> >3. What sort of winding temperature would the transformer be designed for?
|> >   I've had it running at a little under 1/2 rated load for a few hours,
|> >   with the input voltage at the lower end of the regulation range,
|> >   and the temperature inside the core is about 70 degrees Celcius.
|> >   This seems a bit high to me for only half load.
|> 
|> 	Losses in the iron are circa 20% of full rated load, and they
|> DO NOT GO DOWN when the load is lessened.  You are still running
|> a lot of quiescent current in the resonant secondary, after all.
|> Temperature limit on these devices is roughly that of the varnish
|> that insulates the wiring, unless it has soldered connections that
|> would melt first.  

The varnish is good for about 120 degrees C, and yes, they run hot
AND hum (magnetostrictive motion in the core).

|> 	Reliability is very high.  I have black crinkle-finish
|> CV transformers that work just fine (estimated 40 years old).
|> Also, as surge-protectors go, this kind of device has
|> my vote: it absorbs a surge with a few dozen pounds of steel and
|> copper, not just a wimpy ten-gram MOV.

Yeah, a ferro-resonant transformer is about the best form of
power conditioning you can get. Unfortunately, they're real
expensive unless you can get them surplus. I thought I'd died
and gone to heaven when I got an auction lot of 20 of the 1KW
models. I was vacuumed clean when local HAMs found out about them.

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