they work well,
but there is loss in the variacs, more sag under load
than without one.
I think part of the reason is the carbon brush contact. You cannot use a
metal contact because it would short out adjacent turns as the variac is
rotated and overheat the coil and burn up the contacts as well. The
graphite brush acts as a current limiting resistor to protect the winding,
but also introduces losses. I also found the voltage regulation to be
noticeably better without the variac. The best tapped autotransformer I
ever had was the voltage control from an old x-ray machine. It had huge
contacts on a rotary switch, and there was no detectable increase in voltage
sag with the transformer in line. Unfortunately, I swapped it for some
other gear after I had acquired a nice new G-R 20 amp variac.
Don K4KYV
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