High Voltage Forum
Tesla coils => Spark Gap Tesla Coils (SGTC) => Topic started by: MRMILSTAR on August 30, 2019, 06:17:05 AM
-
Here are some pictures of the ballast which I made to limit the current to my pole transformer. It is an adjustable air gap ballast. The maximum current is set by the thickness of the gap between the E and I sections. G10 or other hard plastic spacers are used to set the gap distance. The design specs are:
1. 240 volt operation
2. Maximum current can be set over the range of 4 amps through 40 amps without saturation
3. The center leg of the E sections is 3.75" x 3.75"
4. The E and I sections alone weigh 80 pounds
5. 73 turns of 8 AWG THHN stranded wire
I obtained the E and I sections from a dry-type 25 KVA 480/240/120 volt distribution transformer that weighed about 250 pounds. I individually de-interleaved a stack of E and I sections 12 inches high from the donor transformer. It was a lot of work. I used my lathe as a winding jig. Winding 8 AWG is extremely difficult. I had to stop each 1/4 turn to bend the wire cleanly around the corners of the bobbin. The finished ballast weighs about 100 pounds.
The ballast is mounted in the bottom of my control cabinet. It is very efficient. I have tested it up to 30 amps (the limit of my variac) and it runs completely cool to the touch.
-
Nice work there.
I bet not your first exercise in coil-and-core transformer winding.
A thing I've done a couple of times, but yours is bigger. :-)
-
Actually, it was my first attempt at core-and-coil. Though I will say that I studied the theory and building techniques thoroughly before I started and I had excellent tools. After having it done for over a year now, I can honestly say that I wouldn't change a thing about the design or construction, which is unusual for me. The hardest part was getting the E and I laminations. The hardest part of the construction was building a stiff and strong bobbin. Thanks for the generous compliment.
-
Are you familiar with the How It's Made video about pole transformers?
/>
I've never even touched a pole-type transformer myself. Interesting that the one in video, perhaps a 50 kVA, has low voltage winding made from aluminum strip instead of wire. One turn per layer.
Speaking of which ... When I measured the sheet resistance of Al alloy in beverage can bodies, it was between 50% and 100% higher than pure Al of the same thickness. Have forgotten the actual value, and the alloy number found on Internet. This really belongs in a general can-crushing thread. With one helical cut, anybody can turn an empty can into a long, narrow strip to get a more easily measured resistance. Anybody else here done that?
-
I have not seen that video. Thanks for posting it. Interestingly, the donor distribution transformer that I delaminated had coils made only of thick aluminum strip about 0.5 inch x 0.125 inch tapped at a few points. There was no copper. I was able to extract the entire coil in one piece just by removing the tightening wedges. No cutting was required. I still have the coil. I have been trying to figure out something to do with it. Of coarse since it was a 480/240/120 volt transformer, the turns ratio was almost nothing (2 and 4). The coil had to carry big current though. It was a 25 KVA unit.
-
Would love to have that for my night stand. Great piece of work, I would be proud. Its like a big magnetic heart. Love it