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- From: bills@inqmind.bison.mb.ca (Bill Shymanski)
- Newsgroups: sci.electronics
- Subject: Re: How do constant voltage (ferroresonant) transformers work?
- Message-ID: <4yN9TB3w165w@inqmind.bison.mb.ca>
- Date: Sat, 14 Nov 92 11:05:02 CST
- References: <1992Nov15.031727.25053@imax.imax.com>
- Organization: The Inquiring Mind BBS 1 204 488-1607
- Lines: 62
-
- 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
-