Variac nonsense

The main issue is overheating and deterioration of insulation. A "variac" is unique in that it has only a single layer of windings, which are very tightly bound to the core, thus providing a fairly good thermal heatsink, and the exterior is exposed to air which provides convection cooling. The hottest part of the winding is likely to be inside the core of the toroid, which is not well ventilated, and often has wires bunched up for more of a combined heating effect. Very likely the lack of insulation on the machined surface allows more cooling, and the slight amount of additional power in the small portion with higher resistance will create only a small increase in temperature.

Variacs are designed quite conservatively (except for the Staco 1520), and will run very cool at rated current. The usual failure mode on units used at normal currents is brush wear that causes more turns to be shorted, or uneven contact which causes hot spots and pitting. Sometimes brushes can be restored with a file, and the winding surface can be cleaned with ScotchBrite, but some units have a light gold plating that reduces tarnish and makes for longer life.

Paul

NONE of which changes the declared ampacity of the wire as defined by the maker of said wire and the NFPA. I do not care how well makers have decided to soak the heat out of the wire, nor what they decided to soak it into. It still doesn't change the fact that reduced cross section means reduced ampacity, and that is a 100% linear reduction.

You're an idiot.

The purpose of a quick and sloppy calculation is to see if the effect under consideration is serious enough to deserve detailed analysis. If an effect is small, you can ignore it even if the calculation isn't exact. But it still has to be quantitative; you know, involve numbers.

We do this all the time. Somebody will ask whether the resistance of a pcb trace will create too much voltage drop. If a quick in-your-head calc indicates the drop is, say, 50:1 less than might cause trouble, the calc is good enough.

Like: "that trace is 20 mils wide and about 2 inches long. That's 100 squares at 500 uohms per, 50 milliohms. It's in series with a 50 ohm resistor, so it's a 0.1% error. No sweat."

No, I'm an engineer.

John

The capacity of the variac is declared. If someone is using one at its max rating, and expects to be able to make voltage changes at that current level without brush pitting means to me that you do not know what is happening at that interface. It is to be expected with high currents.

Of course brushes can be refaced flat. Up to a point, and that relates to the brush's total mass and the push spring behind it still pushing correctly after it loses height.

Not likely considering the difficulty presented with such a plating operation.

A machined face on a round shaft (length of wire) = definite QUANTATIVE reduction of the cross section of the wire, you retarded f*ck. And yes, it IS a considerable amount. You lose... again.

Bong sucking ?:-)

...Jim Thompson

"John Larkin" "Phil Allison"

A secondary IS wound all over the primary - essential for minimising I leakage.

This is NOT the case with a few turns in the middle of a variac.

A variac can work with DC too.

..... Phil

Cool. We can get rid of those expensive complicated magnetic cores and just do everything with wirewound potentiometers.

John

How's that?

Sylvia.

Only used as a large rheostat.

"John Fields"

Long as you don't snip the original context out of sight, ignore it totally and then make up an irrelevant example that proves nothing - like this f****it just did.

My 2amp / 240 volt unit has 16 ohms resistance - so could be used at up to

The original point has gone way over JF's head.

..... Phil

He means it works as a potentiometer, using the copper resistance. He didn't estimate the power efficiency.

John

Potentiometer, too.

John

How about a lossy inductor.

greg

I have not made anything lately, but I have used a few variac cores for transformers, winding turns over the core and orginal primary. I even made a choke for a welder. Had to cut one side of the core. That was about 8 inches across the diameter. Good to clean out the grooves in the track and give a coat of enamel. You can also use it as an autotransformer with better efficiency than the variac.

greg