Yes, the laminations are high-grade silicon steel and are individually coated. They were taken from a scrap transformer. I used to go through the scrap dumpster at the place I used to work before retirement. One day I found 3 low voltage (440/220/110) distribution transformers in there! Each one weighed about 250 pounds and had a steel core about 12" thick. I wrestled one of those monsters home and disassembled it which was a lot of work itself. After disassembling and unraveling all of the core laminations I ended up with a stack of silicon steel E and I laminations about 12 inches thick perfect for making inductors. I used a stack of these laminations about 4 inches thick to make my variable air gap ballast.
The ballast is extremely efficient and runs cool to the touch even after running with a short circuit for many minutes. The only place that I get any heating is in a small area of the low-grade thick steel clamps holding it all together. This localized heating only occurs very near the gap where the magnetic flux lines leak out into the steel clamps causing eddy currents in the clamps. The heating is only in an area about 1/4" to either side of the gap and is not extreme. I should mention that the 60 Hz hum is appreciable when you are passing 20 amps through it. I reduced the hum a lot by pressing the gap tightly together with a wooden clamp. It turns out that my obsession with the 60 Hz hum was pointless because when the tesla coil starts firing you can't hear anything but the streamers!
I designed this ballast to handle up to 40 amps. Winding the bobbin with 8-guage wire was a real chore. My lathe as a winding jig helped but I still had to wind it 1/4 turn at a time. Each bend on the first 2 layers had to be made with pliers to get it to fit tightly around the bobbin. My hands were sore for 2 days when I finished it. The core consists of 72 turns of stranded 8-guage THHN wire. I made the bobbin from bakelite and G10. The finished ballast weighs about 100 pounds. It sits nicely at the bottom of my 19" rack power control cabinet.
The only thing that I would do differently is to fill the bobbin with as many turns of wire as it would hold. This would have allowed me to use thicker air gap spacers to achieve the same current limit. This is important because thicker spacers allow you to have finer current control because you can combine various thinner spacers to achieve a very precise thicker spacer. My ballast uses a 0.030" sheet of G10 to give the required gap for 20 amps. While readily available, the next sheets down available that I have seen are 0.010 and 0.015 inches. These 3 thicknesses only allow for a rather coarse selection of gap widths. My ballast with no gap passes 5 amps. This is really not a problem because why would I want to go below 20 amps? I only mention this so that anyone contemplating building such an adjustable ballast may want finer control over the current limiting.
Here is some test data that I have taken.
Gap Thickness (inches) Current (amps)
0 5
0.015 12
0.030 20
0.045 24
0.060 30
----- Original Message -----
From: "Yurtle Turtle via Tesla" <tesla@xxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Friday, August 11, 2017 3:30:07 AM
Subject: Re: [TCML] First coil
Nice work. Are your ballast laminations individually coated?
From: Chip Atkinson <chip@xxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Thursday, August 10, 2017 11:59 PM
Subject: Re: [TCML] First coil
Hi Steve,
I put the pictures that you sent up on pupman.com.
http://www.pupman.com/current/swhite/index.html
My fancy thumbnail generating script got lost in the server move a long
time ago, so it's just file names.
Chip
On Thu, 10 Aug 2017, Steve White wrote:
> I just completed my first tesla coil. It is a large one. Here are the prime specs.
>
> * 8.6" secondary with 1410 turns
> * 8 x 10 toroid
> * Center line of toroid is 71" above ground plane as defined by javatc
> * 12-turn primary currently tapped at turn 10.5 ( all primary circuit "wiring" is done with 3/8" copper tubing)
> * 45 nf of capacitance using 6 Maxwell energy discharge capacitors in a series-parallel configuration with load balancing resistors
> * 10 kva pole pig running at 14,400 volts
> * Sophisticated control cabinet including 25-amp variac, ballast, RSG phase control, breakers, contactor, PFC capacitors (currently about 150 uf), volt meters, current meters, EMI filters, MOVs, lockout switches, indicator lamps
> * Hand-made ballast with adjustable air gap currently set for 20 amps (similar to Richie Burnett's)
> * Rotary spark gap with one stationary 5/16" tunsten electrode and 4 flying 1/8" tungsten electrodes turning at 3600 RPM
> * HV leads made from 10,000 volt AC Belden test lead wire threaded through PVC tubing for additional insulation. This is all surrounded by grounded copper braid for uniform field distribution and to protect against streamer strikes.
>
> I made extensive use of javatc to design this coil. I was confident in starting with a large coil because I am a retired electrical engineer thus I had a lot of knowledge about electrical theory. I also own my own mill, lathe, bandsaw, oscilloscope, and signal generator. Finally, I had the funds to muy and/or build quality parts.
>
> Even though this was my first coil, surprisingly it worked the first time that I fired it up. I am currently getting 6.5 foot power arcs. It should be getting up to about 8 feet according to javatc. I am continuing to adjust the primary tuning. I think I need to go a little lower in frequency to account for streamer capacitance.
>
> I do have a few questions concerning RF grounding. I currently have a copper-clad steel rod driven 3 feet into the ground for the RF ground. This is as deep as I could drive it. I may try a water boring rod to try and get deeper. I currently have the following connected to the RF ground.
>
> * Strike rail
> * RSG motor chassis
> * Copper braid shield around HV leads
> * Copper braid around RSG power cord
> * Pole pig case
>
> Does this sound about right?
>
> Steve White
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
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