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Re: [TCML] Low Frequency AC



David Speck wrote:
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


That's a lot like a cycloconverter.. same basic idea. Obviously, if you have 3 or 6 phases in, you can get a lot smoother
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