Frank,
Can't help you directly, but I saw a potential solution in a different
application a while back.
I have an in-floor heating system that uses three pumps to mix and
balance the circulating water temperature. The metering pump has a
nameplate rating for 120 VAC, 60 Hz, and no brushes. Still it runs at a
ride range of speeds depending on the heat demand.
Intrigued, I put a small filament transformer for isolation, and a scope
across the controller feed to the pump motor. I found a very
interesting wave form being generated apparently by selectively
triggering a triac. The character of the pulse train varies over a wide
range of values depending on the intended pump sped.
The voltage impressed on the pump motor was a series of amplitude and
phase modulated 60 Hz pulses. The net result was the synthesis of a
sinusoidal waveform at much less than 60 Hz. I wonder if such an
arrangement applied to a small iron core NST would work for you.
I'll try ASCII art, view in fixed font for best results:
| |
| | | | | |
| | | | | | | | | | |
| | | | | | | | | | |
| | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | |
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | | | | |
| | | | | | | | | | | | | |
| | | | | | | | | |
| | | | | | | | | |
| | | | | |
| |
Each column of "|" characters represents one half of a 60 Hz cycle,
whose amplitude and polarity are determined by the firing timing of a
triac.
If you could design a similar controller, it might work for your
application.
Dave