🚀 go-pugleaf

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. 

	That isn't a transformer.  It's a constant voltage
transformer and the two are VERY DIFFERENT.  The constant voltage
transformer has two separate, loosely-coupled magnetic circuits,
the second of which is capacitor-tuned to the line frequency.
Saturation of the core limits the amplitude of the resonant
secondary, and this secondary usually has 90 degrees phase
shift from the primary.

	Amplitude change on the primary causes the secondary
to change its phase with respect to the primary (because
the saturated link does NOT transmit an undistorted waveform),
and one can make a tertiary winding which sums the voltages 
on the primary and the secondary: this sum (actually a weighted
sum, the secondary usually runs at conveniently high voltage
for inexpensive power capacitors) can then be made independent
of the input voltage, to first order.

>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).

>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.  

	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.

	John Whitmore