1: The donor transformer: 8.25KV 0.25 Amps
The original transformer had a burnt-out secondary winding when I was kindly given it for free. My aim with this project was to provide a robust tesla coil power source, allowing me to move away from the fragile Neon Sign Transformers.
You may wish to skip this first section if your not interested in the theory.
3a: The cores are in matched pairs.
It is most important that you keep the pairs (top and bottom) together, as they will have the best fit on their mating surfaces, important if you want a consistent value from the inductor.
3b: Making the two secondary bobbins
I was also given a large inductor that used the same cores and centre leg bobbin as the radar transformer. As I was going to wind the new transformer with two secondaries, one on each outside leg, I now needed two extra bobbins. I decided therefore to modify the bobbin that came from the inductor, by cutting it in half and making two, as can be seen above on the right.
4a / 4b: The phases relationship of the two secondaries
Another reason for this arrangement is less voltage stress on the windings. NSTs also use this system to keep their construction costs down, but they also ground the centre tap to the core, which I didn't.
To wind this out of phase arrangement, imagine you are looking at the coils from the top or plan view. Starting the windings at the top, I wound one winding clockwise and the other anti-clockwise, with the primary sitting in the middle. This arrangement, shown in picture 4b, allows you to have the inner low voltage ends of both windings at the same physical end, meaning you only need a short link between them to join the zero ends together.
Some transformers will automatically limit their current when the secondary is short-circuit. This feature occurs if there is a flux path created by the primary that doesn't interact with the secondary. This then causes a current limiting effect to occur that would be the same as placing an inductor in series with the secondary. Transformers that have this feature built in include NSTs, welders, MOTs etc, and it is achieved by having so called shunts (metal inserts), bridging the gaps between the core's legs. This characteristic can also occur if there is an excessive leakage flux, which is the case with my own transformer.
8: The setup on my lathe
This rather cumbersome setup (8) was needed as the centre height of the lathe is not enough to allow for a fully wound coil. The chuck of the lathe is driving another chuck which is raised above the lathe bed, allowing more room for the coil to swing around in.
I have used the same method to wind several Secondary coils as well. In those cases I removed the tail stock of the lathe and fixed it to the bench further away, this then allows any length of secondary to be wound.
9: The Primary bifilar winding. AWG #14
Because the thick wire needed some two-handed work I left the side pieces off when winding this for better access, and just positioned the paper by eye.
10: One of the Secondary AWG #26 windings
In each case the lathe's screw feed was engaged and a suitable gear was employed that would, on each turn of the mandrel, only move the saddle the same distance as the wire's diameter.
The lathe was run at around 30rpm to 60 rpm. I am fortunate to have a variable frequency drive with a foot operated switch, therefore making the whole procedure far easier to handle. I also did a couple of test runs with some old magnet wire to fine tune my procedure first though.