# core saturation in variac

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Hello Malcolm, Ed and all,

I am struggling with the idea of using variacs as current
limiting reactors.

Firstly, a variac rated as, say, a 10A variac is one which
can supply a _load_ current of 10A I would say. If you
consider a variac to be an autotransformer, this 10A is
made, if you like, of a 5A current flowing down the winding,
and a 5A current flowing up the winding in the opposite
sense. The sum of these two flows out of the tap into the
load ( this assumes that the variac is set mid-way, for ease
of discussion. If it was set at 25% then the current from
the top would be lower and the current from the bottom to
the tap would be proportionately higher).. remember the
phase shift that occurs. The ampere-turn product for each
half of the autotransformer is made to balance.

If you use a variac in series mode i.e. as a choke, you will
try to drag, say, 10A through the whole of the winding.
Unless you set the variac brush (tap point) close to the top
end, the ampere-turns product will be very big and the core
will saturate. I know that the magnetic path length is very
long in a toroidal core, but even so the core will saturate
if the current is pulled though all of the windings.

If you now cut a slot in the variac, and "gap" it, the core
will not saturate until the ampere-turn product is much
higher, but now the reactance of the variac has dropped to,
say, a third of its previous ungapped value.

The point is, if you saturate the core, the current through
the LV winding of your pole transformer will become
decidedly non sinusoidal and the harmonic power that you end
up creating as a result of the non-linear B-H curve of the
core will be wasted. You can't charge the capacitors with
the harmonic currents, in other words. Not to mention the
increased EMI problems, Joule losses, iron losses in the
pole transformers.

Today, I gapped all three of my variacs and I will remove
the 250 turns of 22swg wire, and replace them with 100 turns
approx. of 16swg (1.6mm diameter). I will thus end up with a
compact reactor that won't saturate and will be good for 40A
( I can then switch the three in series/parallel
combinations to progressively wreck the regulation of my
pole transformer).

The thing that bothers me is that my understanding of
magnetic circuits is limited, and I know that lots of people
have successfully use variac limiters. Where have I gone
wrong?. Have I gone wrong? Theory and explanations would be
appreciated.

As a final remark, I realise that the RWS approach avoids
this by using an air-core and lots of turns, so this is my
viable alternative. I might just go and buy a reel of 2.5mm
squared single core cable and measure its inductance on the
reel. This would be a nice lazy way of building up inductor
banks. Also you don't add to the magnetising current that
you already need for the pole transformer.

Cheers

Richard Craven
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CMPQwk #1.42-21 UNREGISTERED EVALUATION COPY
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