Arc Welder & Melter
Before the King of Random YouTube channel began to greatly decline in terms of the quality and utillity of its content, Grant, its creator, released quite a few useful and cheap home-build instructional videos. A few of my favorites include his construction of a transformer capable of delivering up to 1600 amps, an AC arc welder, his 2000-watt sun-focusing Fresnel lens, and his soft-metal foundry. I appreciated these projects and their reward-to-expense ratio so much that I ended up constructing all of them for myself after pulling ideas from his designs.
The King of Random channel introduced me to microwave transformers and the many ways in which they can be used to construct useful devices. Using the transformers that I scavenged from two broken microwaves with the addition of some 6-gauge copper wire, I constructed my own 36-volt 100-amp AC arc welder. Though admittedly not nearly as clean as a store-bought welder, it gets every job done that I currently need it to. Welding opens a lot of doors in the way of constructing larger machinery and repairing metal structures.
Similarly, using a single transformer core and primary copper coil, I was able to replicate and improve upon King of Random’s “Metal Melter” project. Along with the 6-gauge copper wire used to build the welder, I also acquired a four-foot-long section of 000-gauge or three-aught (effective solid diameter of 0.41 inches) gauge cable. I wrapped this around the transformer core above the primary coil, forming one and a half turns around the core. Using this new transformer to lower the input voltage and increase the current by a factor of 80, I can now deliver hundreds of amps through conductive materials.
The welder and melter make arc and spot welding possible to perform at home as well as melting metal in an insulated arc furnace using the welder’s transformers. These projects require some knowledge of current induction and any other basic mechanisms behind transformers, which makes them educational in addition to producing a useful product. Transformers are so easy to salvage from broken microwaves that I see it as a major waste when one is thrown away.
Both the welder and melter designs take advantage of the basic principles of high-efficiency electrical transformers. The welder is designed to stack two in-phase 18-volt transformer outputs in series to achieve a 36-volt output capable of realistically peaking at 100 amps. The primary coils of both transformers contain 120 turns, and the secondary coils both contain 18 coils. This decreases the voltage by a factor of 6.7 from 120 volts to 18 volts in each transformer and increases the output current by the same factor to maintain transmitted power, resulting in a ~100-amp current.
A bit of research shows that, with the 3/32-inch 6011 type welding rods that I use, the optimal amperage is between 40 and 85 amps. My welder exceeds this which tends to make the arc more violent, causing it to break often, so I plan to fine-tune it so as to improve the quality of my welding and the ease-of-use of the welder. Frankly, the interrupted arcs make operating my welder extremely difficult, and I used to attribute it to my inexperience, but I no longer believe that to be the case. I think that experimentally testing different amperages and adjusting the welder to match the best settings that I settle on will help immensely.
My melter project was more of a fun project than anything else. I have no real use for a spot welder at the moment, nor a device capable of melting steel with excessive current for any purpose. It is safer to operate the melter than the welder, and it can technically only deliver half of the power that the welder can, but operating it is still scary business. It is designed to transform 120 volts to 1.5 volts, and 15-20 amps to 1200-1600 amps of AC current. Microwave transformers are constructed with magnetic shunts located between the primary and secondary coils which are designed to reduce the power output through the secondary without largely decreasing the efficiency of the transformer. Removing these shunts allows the transformer to reach its full potential output wattage.
Using the resistivity value for copper and the gauge and estimated length of a typical transformer’s primary winding, I found that microwave transformers tend to have about a 3% copper loss, or heat dissipated due to resistance in the copper wire. I have not calculated the hysteresis or eddy current losses in the cores of my transformers, but based on what I’ve seen online, I estimate them to be another couple percent. This leads me to believe that the total efficiency of a microwave transformer with its shunts removed is around 94%, which is quite good in my opinion.
While the welder operates on 240 volts and around 15 amps, the melter is a single transformer, and is designed to operate with a 120-volt input. When connected at each end to a 120-volt outlet, its primary winding, which is made from around 120 turns of copper wire, induces a 60-Hz sinusoidally oscillating magnetic field in its iron core and through the secondary 1.5-turn winding. The field strength produced by each loop is proportional to the current through each loop at a given time. The average of this current peaks at between 15 and 20 amperes which is designed to be slightly below the limit of the breaker allowing power through the primary winding.
When the three-aught cable is brought into a closed loop, shorting the ends with each other or another piece of conductive material, the electromotive force or EMF induced within it drives a current through it that will induce a magnetic field through the transformer’s core to oppose the changing magnetic field generated by the primary coil. In a perfect transformer, this would result in a net-0 magnetic field outside of the transformer.
With its much lower number of turns, the current required in the secondary coil to match and oppose the strength of the primary winding’s changing field is much higher at a value proportional to the ratio of the number of turns of the primary and secondary coils. In this case, the secondary is made from one and a half turns and the primary from 120. This means that the current induced in the secondary winding necessary to oppose the changing magnetic field through the core is (120/1.5) *(~20 amps), which yields an immense current of ~1600 amps through the secondary winding. This high current can only be delivered whilst the resistance of the circuit allows it, and by increasing the resistance of the secondary loop by shorting it across a thinner metal object like a screw, nail, or in spot welding, the gap between plates, one limits the current flow when compared to that through a perfect dead short.
Surprisingly, using the given resistivity in 000-gauge copper cable of 0.0618 milliohms per foot and the approximate length of the secondary cable being four feet, the calculated resistance through the whole secondary coil is found to be a mere 0.0002472 ohms. This extremely low resistance combined with the still quite low resistance of a nail or screw yields a very small total resistance and allows hundreds of amps to flow through both the secondary winding and the load. The secondary winding in my melter can handle such high currents, having a known current rating allowing 2700 amps for around 10 seconds without fusing from excessive heat buildup in three-aught copper wire. Screws and nails, however, fail easily in just a few seconds, becoming molten puddles of steel.
As the temperature of the load metal rises, its resistance increases proportionally, so the induced current does decrease with time when melting an object. The melter is capable of providing current capable of melting metal as well as spot welding through extreme heat, and without exposing the user to dangerous voltages. However, the temperatures reached by melting steel are dangerous in their own way. Once I have need for a spot welder, which is certainly a useful tool in certain manufacturing applications, I definitely plan to construct one using my melter as its power source.