From: bills@inqmind.bison.mb.ca (Bill Shymanski)
Newsgroups: sci.electronics
Subject: Re: How do constant voltage (ferroresonant) transformers work?
Date: Sat, 14 Nov 92 11:05:02 CST
Organization: The Inquiring Mind BBS  1 204 488-1607

dave@imax.imax.com (Dave Martindale) writes:

> I just bought an old Sola CVS-series constant-voltage transformer.
> It works as advertised - as the input voltage is varied from 95 to 130 V,
> the output voltage changes about 2% (or +-1%, if you prefer to think of
> it that way).
> 
> Of course, no documentation of any kind came with it. So I have a couple
> of questions I hope someone can answer:
> 
> 1. How does it WORK?  I can't explain how it does its voltage regulation
>    trick with just a transformer and a capacitor.  None of the reference
>    books I have around discuss it either.
>    
>    I have noticed that if you bring the input voltage up from zero,
>    the device acts at first like a normal isolation transformer - output
>    voltage varies with input voltage, and in addition the thing is
>    absolutely quiet.  Then, at somewhat below half of nominal input
>    voltage, the transformer suddenly starts buzzing, and the output
>    voltage jumps to almost the regulated range - it's almost as if
>    an oscillator was starting up.  The phase is even a bit unsteady
>    for a few cycles - you can hear the pitch "bounce" as it starts up.
> 
> 2. The label says that the transformer is "harmonic neutralized".  What
>    would the output waveform look like without this?
> 
> 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.
> 
> 	Dave

   
Its a ferro-resonant transformer; there's a tuned circuit in there
resonant at 60 HZ.   The amplitude of response is limited by saturation
of the magnetic core; since the core is constantly switching from
+ve to -ve saturation on each cycle, the flux change is about constant
and so the secondary voltage remains about the same. This explains your
other observations; you see it behave as a "linear" transformer 
until the applied voltage is high enough to saturate the core.  It
buzzes, because of a magnetostricive effect ( also if any of the 
laminations are loose) due to the saturated field switching on each
line cycle. It gets hot, because of higher hysteresis losses in the
core - don't worry, they are designed for this and will run for years
at a high temperature.   
   Not sure exactly what the output waveform would look like but I
expect it would be almost square or possibly a heavily clipped
triangular wave; the compensated unit probably has enough filtering
on the output to bring the waveform to an approximate sine wave.
   
  Useful units, these; for example, people who run enlargers 
like them because it makes the line voltage to the enlarger independant
of line voltage fluctuations.   Not terribly energy-efficient, of
course, as you can guess; but before the advent of the $10 switch
mode supply, this was the only way to go. 
     Bill


bills@inqmind.bison.mb.ca
The Inquiring Mind BBS, Winnipeg, Manitoba  204 488-1607