TAMING MEGAVOLT OUTPUT

Recently, our group has been experimenting with some very large (200 to 1800 pounds) transformers with around a 500:1 step-up ratio at 240VAC input. With outputs at the secondary between 0.1 and 0.2 MegaVolt DC at between 0.2 and 1.3 Amperes, these transformers have a truly awesome potential. Having been manufactured for the medical industry for intermittent pulse type duty, they are not current limited and the challenge has been to devise a control circuit that will allow running the secondary to dead short or air gap without internal arcing or overheating. Since the instantaneous current draw for these units is between 100 and 400 Amperes in their normal mode of use, one must also consider the strain on one’s connection to the power grid – specifically blowing all the breakers and fuses at once.

To this end, we have experimented with various types of inductive ballast - use of resistive types having been discarded as an idea a priori. Initially, a large EI frame shunt transformer, possessing 3 separate single windings on the three legs of the ‘E’ intended to accept the A, B, and C, phases of a 3-phase input was tried.

This monster weighs over 100 pounds and was assumed to be more than adequate. In reality, it had no discernable effect when placed in series with one leg of a standard 240 VAC circuit with all 3 windings in series with respect to each other (melted conductor on heavy #8 hookup wiring).

A good LC meter is almost indispensable when conducting these types of experiments. A member of our group had such an instrument and we began taking readings on every coil and transformer we have. An extremely useful configuration of the above shunt transformer was identified. With all windings in parallel with respect to each other, it has a series inductance of 5.1 mH and a 60Hz reactance of 1.9 Ohms. Our idea was to create a series/parallel arrangement of several large transformers and a welder to allow precise current control in the 48 to 60 Amp range at the highest voltage output levels of the large Variac stack below (about 308 Volts), while having enough excess iron core weight to eliminate the heating problems seen with smaller inductive ballast. We settled on an arrangement of 2 transformers and a welder with total core weight of about 200 lbs. which gives the desired range of reactance (3.5 – 5 Ohms).

Although we burned up one 26-lb. ballast transformer as the windings were not quite stout enough, this approach has ultimately been successful. We were able to power the Megavolt unit actually beyond its rated output without overheating in any part of the circuit. An 8-foot tall Jacob’s Ladder was constructed for the Halloween’s entertainment, running at close to 0.2 Megavolt. The schematics of our design and related calculations may be found here.