Mark II

An attempt at DC melting

The Mark II iteration of the mini arc furnace was an attempt at creating a DC arc furnace with one electrode at the top and one beneath the crucible making contact with the metal inside. Every arc furnace I built after this used this approach as it is the most efficient method for an arc furnace operating at low open-circuit voltage. For my transformer designed furnace, the voltages in question could be as only be as high as maybe 75V DC, produced by linking the windings series. Linking them in parallel would never start the arc with the unsmoothed voltage dipping to zero so often.

The basis for this attempt was the Mark I furnace design. I added a few more components to this one. Namely, a 150A full bridge rectifier. This effectively converted the AC to an unsmoothed DC current which dropped back down to 0V every 120th of a second.

The problem with this design still had to do with the fact that, despite the arc length being minutely adjustable via the threaded rod, and only very slowly at that, the furnace wouldn't maintain an arc for more than a few seconds. In my mind, I figured that being able to very slowly and precisely control the arc length was going to work. The problem with this approach (as I was to figure out later), is that arcs need to be started rather quickly at low amperage and OCV and need to be judiciously maintained. The diagram below shows the anatomy of mini arc furnace mark II.

This also had no way to control current other than the inductance of the AC transformer powering it. This proved insufficient (surprise, surprise), and tripped the breaker repeatedly.

Following this failure, I slowly began to understand some of the intricacies of how an arc furnace worked. The second issue presented itself in this furnace as well. Mainly, that the anode gets seriously hot. I always knew that large arc furnace anodes were water cooled, but I didn’t believe it was necessary as I was running this furnace at low kilowatt numbers, not megawatts. I was wrong. The electrode, when contact was made, would almost immediately begin glowing red and get brighter and brighter until the graphite began to sublime away from the portion of it which was exposed outside the furnace lid. I have no pictures of these failures, because, at the time, I didn’t know I would someday wish to document this on a website. So that would need to be addressed in the next iteration of the mini arc furnace… the Mark III, which would prove enough of a success, to keep the desire to keep going alive.