variable autotransformer, higher voltage than nameplate

It's not the insulation, running any tranny at twice rated voltage is gonna let the smoke out :-)

Rheilly P

No almost twice the rated voltage to any normally designed mains transformer will cause the transformer core to heavily saturate. This will burn the fuse or the transformer itself if it is not properly fused.

It is not the insulation but the transformer core+coil design that made for lower voltage. Most propably the transformer will not arc at twice the normal voltage...

You might saturate the core, in which case it will draw a large current. In any event you are exceeding its design voltage. As others pointed out, it has nothing to do with insulation. You could run it from L-N at 120V and then feed a 2:1 step up transformer to get your 208 volts.

Ben Miller

| It is not the insulation but the transformer core+coil design | that made for lower voltage. Most propably the transformer | will not arc at twice the normal voltage...

Unless the overheating due to the overcurrent due to a combination of the greater voltage and the reduced inductance resulting from saturation were to melt the insulation enough to compromise their the insulation integrity.

In which case the problem would not be the insulation, but rather the overheating due to overload or saturation. The point was that it is unlikely that the inulation would arc from double the voltage, especially when you consider that the transformer is designed to pass a hi-pot test of several kilovolts.

Ben Miller

- Benjamin D Miller, PE

The insulation at this voltage level will have a factor of safety in the order of 5-6. The problem is that 208V vs 120V will drive the transformer into heavy saturation where the no load current will greatly exceed the rated current- so I^2R losses and heating will be much greater than normal resulting in, as Rheilly indicates; insulation has temperature limits after which it starts to fail or burn and

"is gonna let the smoke out"

The idea of "letting the smoke out" has been around around for a long time. For a number of years experianced engineers have told me that to do so usually results in the piece of equipment becoming unservicable. I have considered undertaking research into the exact nature of this smoke to find out in what way it somehow contributes to the correct operation of electrical equipment and am seeking fanancial backing from interested parties. I think there are uncountable riches to be made if I can successfully manufacture this "smoke" and the potential returns to any backers would be significant. If you would like to contribute to my research program please feel free to contact me. Daniel

You will only make money if you find an easy way to put the smoke back in. Most things today are sealed so securely that it is almost impossible to get the smoke in unless you hire an expensive service technician from the manufacturer of the object.

John G.