Re: DANGER! Strange transformer problem

Hi John, Terry, all,


>I know something strange is going on here but I can't figure out
>what.  Saturation would be the first thought but the variac should
>handle it since it's rated for 25 amps.  I thought the variac might
>be breaking down but today I borrowed a 5KV hipot set and tested
>it.  Nadda.  Some sort of resonance comes to mind but I can't
>magine where, given only stray capacitance in the circuit.  I've
>hought of slotting the variac core but it's going to be a pain in
>he a** to get to plus I hate to hack this nice variac until I know
>what's going on.

> Some more tidbits of information.  The pig nameplate impedance is
> 2.1%.  I removed the variac and choke from the circuit and hooked up
> a welder transformer with the shorted secondary as the current
> regulator.  Worked fine.  Smoothly controlled the shorted secondary
> current from 200 ma to about 2 amps.  Open circuit performance is
> smooth with no bumps across the range of the welder.  Putting the
> variac back in line with the welder set on max and the pig sec open,
> the growling and high voltage returns.  This is pointing back to the
> variac, I think.  This variac is the type where  the whole outside
> circumference is the slider contact circuit with a bank of brushes
> spanning the height of the core.  Could the turn that the brushes
> short possibly be doing this?

If I read it correctly you have the pig, a choke and the variac all
connected in SERIES. Is this true? If so, and if the variac is
unmodified (slotted), this won´t work.

Think of how a variac is normally hooked up: The top and bottom
of the main winding are connected across the mains. Manufacturers
want to invest the least amount of money possible. The inductance
of the variac has got to (just) be high enough, so that the core does
not saturate at mains input voltage. This is the nominal, no load
current flow, ranging from a few 100mA to a few amps, depending
on variac size, actually voltage applied, etc. Normally, you apply the
load from the wiper to N, so the TOTAL core, variac wire, etc
(i.e: resulting flux strength) never sees the complete load, you are
presenting (which is one of the reasons you can overload variacs).

The way you hooked it up, your variac´s core ALWAYS sees the
full current flow (i.e: the full magnetizing current flows through the
winding, core, etc). Try this: measure the no load current draw
of the variac itself with the variac connected normally across the
mains. Compare this value with the current value (variac connected
your way) where your variac starts to growl. I´ll bet they are similar
(with some leaway built in due to manufacturer´s "overbuild").

The only way you can use the variac as a variable inductor (and not
as a variable transformer) is to slot the core. That way, the leakage
inductance is high enough to prevent saturation and the only limit will
be the ampacity of the wire gauge used to wind the variac. The way
you are using it now, is limited to the saturation current of the core.

My question to you: Why did you chose this route and not the "normal"
way? I.E: variac across the mains, choke and pig in series, etc.
Another question: Is your variac (sorry if you already posted this) a
220V or a 120V model? If it is a 120V model, then you are really
driving the core "nuts" connecting the variac the way you did.

Coiler greets from germany,