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Re: new power transformer (variac as ballast)
Further to previous comments,
<SNIP>
>> > > Did you have to cut a slot in the core to prevent saturation?
<SNIP>
>> it should not saturate with less than that through it, correct? Of
course
<SNIP>
the ampere-turn product for the xfmr is the value which will be exceeded and
thus cause saturation, if it isn't slotted. With a variac run in
common-mode, the "primary" current and "secondary" current flow in the same
winding but in opposite phase. The total ampere-turns is kept balanced so
for example, with the variac set to half of the input voltage, the current
into the load is equal to the current sourced from the supply, but
out-of-phase (standard xfmr theory ie application of Lenz's Law). If you now
consider the situation where the variac is connected in series mode, the
balancing current is non-existent. Thus the variac is likely to saturate at
half its' name-plate rating.
Gapping the core will drop the inducatnce drastically: an eigth-gap will
drop it by half, from memory. You can cut it with a saw and then pry it
open a bit with a screwdriver or chisel. The laminations are soft and easily
bent. Once you've opened it up, wound it and measured it, you can squeeze
the gap down to optimise towards your desired value.
Richard Craven, Malvern, England