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Re: core saturation in variac
Richard:
I haven't even considered cutting across (gapping)
the core of a variac to make a variable inductor out of it.
Much too stingy! However, the first thing that comes to mind
is that the core is sized to withstand full voltage (135
volts for a "115 volt" variac, as I recall). So, regardless
of the reactance (as it is adjusted by changing the air gap),
if you are using only half the turns you can only withstand
half the voltage without saturating.
If someone were eager enough he could calculate the
inductance knowing the cross-sectional area of the core, the
width of the air gap (assuming all of the mmf drop is acroos
[across!] the gap), and the number of turns; I'm not...
I see no reason, subject to core saturation problems, why
a variac wouldn't work as a variable inductor. Will be
interested in your results.
I have some ballasts which appear to be for
mercury vapor lamps. They are regular E-I core chokes,
with a mechanical arrangement to vary the spacing between
the E and the I. This is a simple arrangement using a
threaded stud on each side of the I. The I is connected
to a casting which fits over the bolts, and the position
(air gap) is adjusted by means of nuts which can be
screwed up and down. It stikes me that this arrangement
might be a whole lot simpler (and cheaper) than cutting up
expensive, good, valuable, and useful variacs! Certainly
rewinding would be a lot easier, and you could use almost
any old transformer core of suitable cross section.
Please let us hear about your results with the
variacs. By the way, a very simple way to measure the
reactance (at least below saturation) would be to hook
the reactor in series with an ammeter and a 100 watt light
bulb, the whole across the ac power line. You could just
measure the voltagte across the reactor and the current
through it and calculate the rest.
Good luck,
Ed Phillips