variac question?

Just a WAG, but maybe the sliding contact has worn "flat" and is touching too many turns of wire, creating a multiturn shorted winding when set at the 30% position, thus causing the problem.

You didn't say whether your variac is a 0-100% kind or the kind which can output more than 100% of the input voltage.

If it's the 100% kind, then maybe when it's set to 100% the contact arm is no longer resting on the winding but is on a broader piece of metal connected to the hot input.

More details please. If you plug it in at 100% and then lower it to 30% do things stay OK?

Jeff

--Well I'm clueless but this might be a good question for sci.electronics.basics

it sounds like you have a short, or have the autoformer wired wrong.

The slider connects to the output side. The input goes across the windings of the toroid, either at the ends for a 0 to 100% transformer or one end and a tap for the 0 to 140% ones.

The slider should have no effect on the transformer at no load otherwise.

It doesn't pop the breaker at 100%, and at the moment it's turned down to

60% and doing an electrolytic derusting job. After that's done I'll be able to experiment more.

Mine can be wired either to 0-100 or 0-120%, I believe. I've had it both ways. I used to use it to overdrive hot air popcorn poppers to get more power out of them when using them to roast coffee beans. But now it's wired 0-100%. It's possible there's a wiring error, but I don't think so.

Grant

Yet your experimental evidence suggests otherwise. With no load, the variac should draw negligible current at any setting.

Grant, can you post of a picture of how it is wired exactly.

i

Do you have a GFI somewhere in the 20A circuit?

By the way, check your USPS mailbox, I sen't you something.

cheers T.Alan

Magnetizing current doesn't depend on the position of the slider. Sounds like a short.

my guess is that his output terminal is connected to hot terminal.

That would explain why it does not pop breaker at full voltage, but pops the breaker at middle voltage.

i

Many people have suggested to me that there is a short.

If you think of the primary side as having terminals L1 and L2, let's for the sake of argument say that there are 100 coils between L1 and L2, numbered 1, 2,

3, ... 99, 100. The output terminals are connected between S (for slider) and L2. As you vary the control, S should physically contact one of the coils e.g. coil number 50.

Let's imagine that it is instead contacting coils 50 through 54. This would be the short that people tell me I probably have, right? As I see it, that would be equivalent to having a primary with 95 coils, and I *still* don't see where a large increase in magnetizing current would happen.

I'm sure the problem is in my thinking - please show me where!

GWE

If connecting coil 50 connects to itself, then you have the equivalent of a secondary consisting of one turn connected to a dead short.

Dan

Will happen if you hook the sucker up backwards. The "primary" goes on the 2 ends. The "secondary" is one end to the wiper.

I think this is very likely what is going on. If you have the primary wired through the wiper, the current drawn will increase dramatically as you turn the variac down to low values; at low values, it means that the primary is essentially a short (although possibly not the kind of short you were looking for).

Open the variac and check the wiring as soon as you can, because it might overheat and destroy the insulation.

Best wishes,

Chris

It sounds like you win the prize, Chris!

I think this is very likely what is going on. If you have the primary wired through the wiper, the current drawn will increase dramatically as you turn the variac down to low values; at low values, it means that the primary is essentially a short (although possibly not the kind of short you were looking for).

Open the variac and check the wiring as soon as you can, because it might overheat and destroy the insulation.

Best wishes,

Chris

We used a lot of 20A variacs in the calibration labs where I worked. They will occasionally trip the breaker when energized and I think this depends on the part of the AC cycle present at the instant the switch closes.

It is not possible that you have the variac wired backwards as it would dramatically raise the output voltage and the input current when turned down.

It is likely that you have the input leads connected backwards. In that case, when turned down both output leads (the hot and the neutral) would be

120volts to ground and this might cause an input overcurrent depending on what you have connected and its isolation from ground. I strongly suspect this is the case if you can reliably reproduce the problem. If so, it creates a serious unrecognized hazard and should be corrected immediately. When the output is turned to zero, both output leads can kill you if you contact one of them and are grounded.

Don Young

My bet you have the inputs on the wrong points.

The 'Mains' go across the whole coil. The white wire connected to one end of the coil is zero volts out the wiper. When the wiper (the output) is turned to the other end it is at the max.

The variac should easily idle on the power supply at any wiper value when no load is attached. The output is the wiper.

Mart> I have a 20 amp variac. Yesterday I tried plugging it into a 20A circuit

Treat it like a Pot (volume adjustment) - don't put it in series.

It is a transformer that has an input winding that one end is common to the output. Often the input is not across the 100% winding but a few turns short. That gives you the 120% more than the input because the wiper sees ''120 windings'' when the input is press across ''100 windings''.

Martin

Mart> Jeff Wisnia wrote:

Was there any load on the output? ...lew...

No, no load whatever.

GWE