BIG variacs

I've got one of those that has considerable output droop under load above the tap. IIRC the max current falls from 10A at the tap to about 6A at the top end. I don't have it wired up in an enclosure, and I'm not about to power it up exposed on the bench.

If the feed is at a 120 V tap, and the maximum voltage is 140 V, I would expect the maximum output current to drop to 8.57A, not all the way to 6A.

If the input tap is nominally 110 V, then the 140 V output should be 7.86 A.

If the maximum output is something other than 140 V, then it all needs re-calculation. :-)

But essentially, you divide the nominal current by the ratio between maximum and input tap to get the safe output current.

Essentially, make sure that neither the input current or the output current exceeds the nominal 10A -- check *both* to be sure. (This is why a fuse on each. :-)

Enjoy, DoN.

It's a 240/280V 10A Variac. I used it with 120V on the center tap to check a motor I wanted to turn into a three-phase converter, because I don't have 240V on the bench. .

The 240V output wouldn't keep the 240V motor spinning. I checked voltage vs current with resistive loads and found that the voltage drooped badly above 6A.

jsw

Hmmm, maybe some type of inductive feedback is causing that? I have never had any trouble running my variacs at a high current, with no significant voltage droop. This is with running a resistive or an inductive (transformer) load, but not with a motor.

You probably already know this, but it should be mentioned that a variac can be, and is actually designed to, run at a higher current than is specified by the nameplate, when you run it on a duty cycle (like a welder). The limit here is the heat generated by the windings because of the overcurrent, not any saturation in the core.

This duty cycle is normally provided on a chart, but is generally similar among different models. I'm too lazy to pull the picture from the .pdf file, but if you look at "Curve 3" under "Overload Capacity" on page three, you will find it:

It should also be mentioned that the weak link in any variac is the wiper brush, and any overcurrent capability assumes that the wiper brush is in satisfactory condition.

Jon

O.K. So you were using it as a voltage doubler. This means that the input current would also be doubled in the other direction relative to the load current.

And at 6 A out of the 240 V tap, you would be pulling 12 A into to the 120 V input tap tap -- well above the nominal current, and probably saturating the transformer core.

If you had fused both input and output at the nominal 10 A, you would have blown the input fuse, which would have given you a clue what your problem was. (Or an ammeter on both input and output as well as a voltmeter on the output.

Enjoy, DoN.

I'm fairly sure of what happened but didn't scope the waveform for clipping to prove it, so I just reported the observation that a center-tapped 240V Variac won't deliver the nameplate current as a voltage doubler. jsw

That one would be for a constant 120V line (lights) (motor...) and the variable zero to 240 is the output for a 240 load.

On this one but mostly others one end is common and the 240 is max. zero to max. if you hook the supply to the tap - then the transformer works in 'auto' mode and generates more. Normally it is 110 to 125 or the like and is used for that voltage only.

Mart>> >>> ...

Just make sure you get some XC40s to keep it fast. Check at 'EJOT'