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When building a Phase controller for a SRSG I found that using a 2 Amp 400 Hz variac works just as well as using a large 5 to 7 Amp 60Hz variac. Since the variac is used as a variable inductor its normal operating frequency doesn't matter. Typically, one wants something in the 2 Henry range or less. Because of the higher frequency, the much smaller 400 Hz variac has about the same inductance as a much larger 60Hz variac. With a 2 Amp General radio M2 and a 7.5uF capacitor I can get more than 45 degrees of control on a small Oriental synchronous motor. The variac is useable over its entire rotary range with a maximum voltage rise of somewhere between 5% and 10%.

Dennis E. Bahr, P.E.

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