Will,If you are getting 12" sparks with a single MOT for a power transformer, I'd say that you are really doing very well. The general consensus on the list is that MOTs are difficult for TC use because their voltage is so low that it's hard to get a gap to fire without very finicky adjustments.
You didn't mention what kind of gap you use (static single or static multiple gap, or rotary (synch or asynch)) and what you are using for a primary cap.
If you are using a static gap, just blowing a lot of air from a beefy fan or shop vacuum on it will improve your quenching and make your sparks longer.
I think you misspoke -- if the primary capacitor was in parallel with the spark gap, all of the energy in the cap would be wasted in the gap, and little would end up in the secondary.
These days, the consensus seems to be that the power transformer secondary and the gap are in parallel, feeding the cap and the primary coil which are wired in series. However, that will require good ballasting of the MOT so that it will have some degree of current limiting while the gap is firing. While the gap is firing, the transformer current must be limited by the ballast, as the firing gap is pretty much a short circuit.
A circuit breaker is definitely not equivalent to a ballast. A ballast limits the current through the transformer on a cycle by cycle basis. A breaker just shuts the system down if an instantaneous overcurrent situation happens. There's a big difference between a throttle and a kill switch.
You didn't mention the length of the secondary. Current thinking says it should be between 4 and 5 times the secondary diameter.
Might want to look up JavaTC, a comprehensive online calculator that can optimize all sorts of TC parameters.
Just adding a second MOT in series with the one you already have would double your primary voltage, and give yo more spark. Of course, that will require a better rated primary cap.
Just be aware that unlike NSTs, contact with a MOT output can easily be lethal. NSTs bite hard, but MOTs play for keeps. You don't get second chances when working with MOT circuits.
Varnishing the secondary will help keep the wire in place. If the wire slips down, you will get a shorted turn which will significantly reduce your performance.
Dave On 12/23/2013 10:09 PM, Will wrote:
Hello, My name is Will, I am 14 and have a fair bit of experience with tesla coils for my age (I have built several smaller coils with current limited Neon sigh transformers with no GFCI's on them). My science teacher wanted me to construct a rather large SGTC for him. So far I have constructed a basic coil (the capacitors are in parallel with the spark gap, and the spark gap is in series with the secondary and the transformer.) It has ~3500 turns of 28 gauge magnet wire on 6 inch white PVC for the secondary. The torroid is comprised of a metal dryer duct pipe curled into a circle and measures about 18 inches in diameter. The primary has 8 turns of copper tubing. The coil is powered by a 2100v Microwave oven transformer. The capacitor bank has a total capacitance of 1uF at 2100v. Please note that the transformer is not ballasted, instead I simply ran it through a 15A breaker switch. I know that this is not exactly the same as a ballast but it keeps the coil from tripping other circuits in my house. The secondary is not varnished (I am not sure if this effects overall performance of the coil, but thought I would mention it anyway.) I have not tuned or measured either of the coils frequencies, but would like to. Guidance on this would be appreciated. The coil throws out ~12 inch arks at the moment, and generally there are no problems with the coil. I would like to tweak it's performance as much as I can. Suggestions for improvements would be appreciated .
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