Regarding constant voltage transformers, or CVTs. 

I have been using them for over 30 years, and have always found
a sinewave output. There are some better designs that use a 
larger high voltage AC capacitor for the resonant circuit that
are "harmonic neutralized" and regulate to 1% over a +,- 20%
input change. These cost about 10% to 20% more than the 3% regulation
"line conditioners", which DO put out a sinewave, but with higher
distortion. The very nature of CVT design is that it is a true
transformer (total isolation of the input from the output), that
only passes a sinewave to the output winding at the resonant
frequency of the high voltage secondary/capacitor circuit.

If you play with one with a variac (variable transformer) input,
you can see this: as you increase the input, there is little
output until there is enough voltage (about 45 volts AC) to resonant
the high voltage secondary, at which point the output jumps to within
a few percent of the regulated level. As you drop the input voltage,
to as low as 20 VAC, the output remains near 110V as long as the
transformer resonants (test with a moderate load, like a lightbulb).

SOLA is a major manufacturer, and they claim that their standard
conditioners (MCR series) will output a sinewave with 3% THD (total
harmonic distortion) with a square wave input. I use a 120 watt CVT
with my large frequency controlled converter in my field van to 
convert the square wave to a sine wave for electronic loads. This 
is similar to common marine use for AC operated electronics.

Since CVTs are passive, they last a long time; the main problem being
the high operating temperature of the transformer core that eventually
cooks the insulation, and the high voltage (660 volt) AC capacitors that
eventually fail causing a voltage drop. The capacitors are replaceable.

Since CVTs ONLY pass a completely isolated sinewave, I have always
used them as a primary protection of the AC line from lightning. They 
always work, except in one case when the lightning arced thruout the 
case to the secondary line. I have had one at a shared tower facility
where lightning has frequently smoked the varistor-based line protection
of the other equipment, while the seismic station and transmitter were
never harmed. They are an expensive ($350 for 250 watts) but permanent
solution for protecting AC loads.

Regards,
Sean-Thomas
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