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winter project '07

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Kristian

Sun Nov 11 2007, 02:52PM

Registered Member #611 Joined: Wed Mar 28 2007, 11:40PM
Location: Hudsonville, Michigan
Posts: 79

Itâ€™s starting to snow around here and my wife wants the garage back, so I took down my 4â€ coil and picked up the tools and trash that was taking up most of the garage. I finally got around to sorting and setting up my workshop in the basement of our new house.

I think its time for a winter project.

This post is a journal of the construction of my 6â€ SRG Tesla coil. I intend to update this post with more pictures and descriptions as I progress through this project. Target completion date is 4/1/2008, but may take much longer

Project Description

6.5" diameter PVC secondary 33" wound with 22 gauge magnet wire
12 - 15 turn primary 3/8" OD copper
MMC, sixty 942c20p15k capacitors, variable capacitance
6 microwave oven transformers
Variable inductive ballast (hopefully)
120BPS synchronous rotary spark gap

Secondary

I have not wound the secondary yet. I have a 6â€ diameter acrylic tube 36â€ long. Iâ€™m not sure I want to use it for this project. I might just get some 6â€ pvc instead. The wire will be 22 gauge magnet wire with heavy insulation.

Primary

I have two options. First option would be to cut a couple of turns off the inside of my 4â€ primary. I donâ€™t like this idea at all. Second option, wind a new primary. That sounds a lot better. I havenâ€™t locked down any dimensions or chosen the materials to use for this yet. I have been trying to find some 1â€ copper strap of a decent thickness that wouldnâ€™t be to stiff to coil or too flimsy to hold its shape. I think this would be cool because it would stay flat, and not get all bent out of shape from moving the tap and general wear and tear. Also strap would make more turns in the same overall diameter as tube. If I can't find any, or it is ridiculously expensive I'll pick up some 3/8" copper tube and make 12 turns or so.

Power supply

The goal of this component is to have a large power supply that is durable, transportable, and can cover a range of applications. Iâ€™ve been scrounging broken microwave ovens for a few months now and have compiled a couple of shelves full of transformers.

Iâ€™ve gone through and picked out 3 pairs to use for the Tesla coil power supply. The MOTs were chosen primarily by the amount of insulation and the distance between the lead from the inside turn of the secondary and the core. They were paired up by roughly matching the coil sizes and overall size. They all vary some, but the pairs are as closely matched by these standards as it gets from the stock I have to pick from.

The pairs are mounted on a piece of cutting board sized to fit inside an ammo can. All of the wiring from the transformers is secured inside the cans. I put standoffs made from pieces of 1/2â€ pvc and caps with bolts running through them in the lid. This design was inspired by the mot power supplies shown on Gregâ€™s Garage Tesla Coil Site @

My design differs slightly in the way the transformers are wired inside the box. By extending all of the transformer leads to the outside of the box I can easily change the wiring around. I can switch the input from 120 to 240, and use any number of the 6 MOTs in any configuration.

I have one can wired so far and I still need to source some oil. I was thinking about asking at the feed store if they sell castor oil (cow laxative?) by the gallon. Otherwise, Iâ€™ll have to think of something else. Anyone have any ideas or suggestions?

Ballast

I would like to make a variable inductive ballast. I have not even begun to figure out how Iâ€™m going to do this.

MMC

The capacitor array is made up of sixty 942C20P15K-F capacitors arranged in 5 strings of 12. These are mounted on two cutting boards that will be stacked one on top of the other (with adequate spacing) in some kind of support structure.

The cutting boards were drilled every three inches and bolts were inserted. The capacitor leads are wrapped around the bolts and held in place with nuts. The thought here is that by using the spacers and buss bars I made from hammering flat some Â¼â€ copper tube, I can vary the voltage and capacitance by tapping across different bolts.
For example, 7 strings of 8

And 5 strings of 10

Etc.

By the way, you must imagine that the strings are connected across the two boards.
I donâ€™t know if this will work. I wonder if the caps that are only connected at one end will effect anything. Like in the 7x8 picture, there are 4 caps that are not being used but are still connected at one end. And the 5x10 arrangement has 2 unused caps connected at the end of each string.

I donâ€™t know if this will be the final design. I would like to switch the bolts out for brass ones. I might get rid of the bolts all together and just solder it all together and be done with it. I just threw this together to see what it would look like and to get a better visual idea of the things I was thinking about trying for this MMC.
Eventually I will add another 60 caps to bring the total to 10 strings of 12. Hopefully I can come up with them by spring.

Synchronous Rotary Spark Gap

I guess salient pole motors are hard to come by. The Tesla store guy on ebay had some, but the price seemed a little steep. Well, after months of asking around and looking on line, I finally decided to go the ebay route. The item I won was a 3200RPM Teletype motor, just the motor and an included capacitor. The item that came in the mail was an 1800rpm Teletype. Also included was a little 4â€ G 10 rotor with 4 tungsten flying electrodes and a few different sized motor capacitors. It turns out that he sent me the wrong package. I was a little bummed that it wasnâ€™t the bigger motor, but the rotor is kind of a bonus. I was going to make a little bigger rotor that would be able to be run with two, four, or eight flying electrodes for different break rates, but now I can move that project further down the list as it is no longer a necessary component for the coil to operate.

I threw together a lash up just to check it out on my 4â€ coil. I adjusted the phase by turning the motor and screwing it back down. Once I finally got the thing kind of synced up, I noticed the current draw on my ammeter was only reading a few amps. I decided to investigate this. I backed off the ballast and eventually didnâ€™t use it at all. I was quite surprised that it was drawing less than 15 amps (the 4 MO transformers would draw over 50 amps if I were running it like this with a static gap). Without the ballast, the top load erupted with multiple streamers in all directions, even with a 1â€ spade drill bit for a break out point. As I brought the power up to full, some nasty arcs started between the primary and the base of the secondary. Fortunately, I shut it down immediately and saved the secondary from damage.

Anyway, here is a picture of the motor and the stationary electrodes that I hobbled together.

The piece of 2x4 and the straps were just a throw together lash up. If I canâ€™t find a proper pre fab mount for this motor, I guess Iâ€™ll have to make one. I have a scrap Â½â€ thick piece of acrylic that might end up being used for this, or whatever I can come up with that works proper. Does anyone have any suggestions for this?

I think the electrode holders came out kind of cool. They are made from stuff I found around my workshop. The electrodes are Â¼â€ brass rod. A pair of Â¼â€œ copper lugs with set screws hold the electrodes in place. The â€œheat sinksâ€ are square pieces I cut from some aluminum bar stock. There is a 1/4â€ threaded brass rod down the center to the insulator. The aluminum pieces are spaced with a brass nut in between each square. I donâ€™t know how efficient it is as a heat sink but I think the idea is good. I might rebuild this better. Maybe make the squares from thinner al stock so more pieces will fit in the allotted space. Maybe Iâ€™ll cut threads in the pieces also to get better surface contact between the rod and the squares. I donâ€™t know.

RF filter and Safety gap

I scored a bag of some cool bolts for making safety gaps. They are brass and they are pretty big @ 3/8â€ diameter x 2 Â½â€ long. The coolest thing about them is the head on the bolt is rounded. These bolts are made specifically for the legs of pinball machines.

These are the 100ohm 200watt power resistors. These are for the R part of the low pass filter. I still donâ€™t know what Iâ€™m doing for capacitors. I was thinking about getting 20 of these capacitors

and using 10 in series on each side. The only reason Iâ€™m thinking about using these capacitors is that they are the same kind of caps used in the â€œTerry filterâ€. I have seen others using ceramic doorknob caps but I donâ€™t know enough about caps to know specifically what to look for. I donâ€™t know if a filter is even necessary or beneficial for MOT power supplies.

Control Box

Iâ€™ve gutted my old control box and Iâ€™m starting over. Iâ€™ve been watching ebay for a 30A or more variac in my price range. The trick is finding one at the same time I have some cash. Iâ€™ll be using the 7.5A variac I have now for the phase controller for the motor.

I cooked the 30A EMI filter in my old control box. When the filter blew, everything connected to the ground wire went to 120 volts. I got bit a couple of times and trying to turn it off. It must have been a capacitor failing short to ground or something. I got a much bigger(55A) one to use in the new controller.

So it begins. If there is interest, Iâ€™ll keep this updated as I progress and find time. Suggestions and comments are welcome.

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