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Can anyone explain how a variac *really* works as a current limiter?
Hi all,
While I am at it (not understanding something), let me
continue ;o)). How does (actually: can) a variac work
as a limiting inductor ??
For my questions, let us use a predefined variac
"schematic" (saves me those awfull ASCII drawings).
1.) The lower connection is labeled "0"
2.) The upper connection is labeled "Ph"
3.) The wiper is labeled "W"
4.) The variac has NO additional turns for increased output voltage.
Normally connected (i.e. as a variable autotransformer), the current
flows (no load, yet) from Ph to 0. This current draw is the magnetizing
current, which is integral to the variac design. It flows through the
WHOLE core. This current flow will always have to be added to the
current draw our load takes.
If I now place a load across N and W, the magnetizing current STILL
flows from Ph to N (as a predetermined size). An additional current
flows through the windings (from Ph to W) thru the load and back to N.
The average magnetic flux through the whole(!) core doesnīt increase
too much. As long as I stay within the limits of the wire (winding on
the variac) ampacity and the variac is properly designed (i.e. the total
magnetic flux doesnīt reach the saturation level of the core), I stay
out of trouble, and it works wonderfully.
Let us (now) look at a variac used as a variable inductor: In this case
the connections are somewhat different. N goes straight through to the
load (no connection to the variac). The phase enters the variac at point
Ph and the load is connected to the (W)iper. Essentially, the variac is
in series with the load. Obviously, as long as the load isnīt connected,
no current flows through the variac and the core flux density is zero.
If I now set the variac at itīs lowest position (wiper is near or at
the N terminal), the maximum current that can flow is the nominal
current draw that the variac sees, when connected normally (minus
the resistance of my load of course) across the mains. The core
flux is easily within the engineered values. So far so good and it
works.
If I now set the variac at itīs highest position (wiper is near the Ph
terminal), the density of the magnetic field is at itīs maximum, BUT
through a VERY small cross-sectional core area. There is NO current
flowing through the windings from W to N, so this core area see no
or very little (stray losses) magnetic flux (& current). The density of
the magnetic flux flowing through that minute core area has got to
be massive. So, in my eyes, the core MUST saturate and it will no
longer truly work as a limiting inductor. That poor (current limiting)
variac is going to start growling like crazy. Of course, I realize the
magnetic field is a result of amps and volts per turn (on the variac),
so if you run a controller containing a second variac (connected in
the normal fashion), which you use as a voltage control, you may
not experience this problem at low power (voltage control set low
to medium and current control set at mid to high). 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.
Comments and explanations are welcome, because I am in
"no comprehende" mode right now.
Coiler greets from Germany,
Reinhard