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Re: Can anyone explain how a variac *really* works as a current limiter?
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
Hi Rienhard,
Comment below:
> <snip> However, looking
> at what I wrote above, ANYONE using this dual variac setup must
> run into a similar problem at some combination of voltage and
> current setting. If I take a step further (and make it even more
> complex ;o}), ANYONE using this combo has got to have a sweet
> spot on their voltage and current variacs, where they hit mains
> resonance (in combo with the TC primary cap) and all limiting is
> essentially "out the window" and they get maximum spark length
> and / or a blown fuse.
> The above, of course, wonīt hold true for a modified variac (a slit
> cut into the core). However most websites, I have seen, donīt
> mention they did this, so I gather they didnīt modify their variac.
I agree with your analysis here. I use a variacs for voltage and ballast
control. The variacs are both 30A (pretty good size). If I turn up the
ballast even at low voltage, I can hear it want to growl. However, I won't
run or start a coil in this mode using a variac method of control. I will
turn my voltage up to a mid setting, then increase ballast until sparks
begin. and find the best sparks with the ballast setting. Once this is done,
I will go back to voltage control and increase under the same observations,
and go back to the ballast, etc... Typically, my ballast ends up about mid
setting. All this time, I keep an eye on the current and current swing. I
had current limits, so maybe I haven't reached the line resonance on my coil
that you described and others have commented on.
I am one of those guys that slotted the core of my ballast variac to
increase the saturation point. If I had a larger variac (say 40A or so), I
would try to get away without the slot (only slot it if needed attitude).
There was a time I was using smaller variacs, and I did growl the variacs
and eventually saturated and broke loose one from it's epoxy setting. This
caused it to hum loadly under normal conditions. (This was slotted too. Even
slotted variacs "can" saturate).
I won't even attempt to pull my ballast to full swing - as you anaylized,
the magnetizing current would be huge. I have no doubt I would kill a variac
under that condition. This is part of what is nice about using a welder for
ballast. There should be a lot more ballast variance and control. Like most
of us, I am no different in that I used what was available to me. I do like
it because what I'm using works for "my coil".
What I've experienced with "sweet spot" is that not only is the current and
voltage setting part of obtaining it, it is also dependent on the coils
processing of power into sparks and the gap rate. At 120 bps, I have
different "sweet spot" settings (and control) vs. a 240 bps setting. For my
coil, 240 bps with my variac combination (V & Ballast + 2.5R) is the best
setting I've found for max spark at least input power using a SRSG gap and
pole pig. With a DC drive gap, I've found a higher BPS worked well, and
maybe it had something to do with the variac control (limitations).
I guess what I'm saying here is that your post was accurate in my mind, and
it all depends on more than just the type of ballast control. There is also
the rest of the system to consider in line with what your trying to process.
The variac, if used, must be capable of limiting the current, and if not,
modify it or consider other alternatives. A current limiter must be capable
of limiting the "total" current that you plan to process through your
system. Variac size and control swing must be considered. In all cases, when
using a variac, I would never recommend taking it to (or near) it's limit
or you may short the variac.
Bart