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burtgummer

Sun Sept 09 2007, 06:03AM

Registered Member #991 Joined: Sun Sept 09 2007, 05:47AM
Location:
Posts: 6

Well, to steer from use of of NST power, i decided to incorporate 2 mots that i had sitting around. So, I hooked them up and actually didn't trip the breaker of my 120v outlet. I'm also running an async gap that runs at 1725rpm with 8 flying electrodes and 1 stationary gap.

Even though it gets power, it seems to run sparatically. It might run 'normally' for about half a second, and then stop arcing at the spark gap for half a second, then have lower power arcs across the gap (not in that order, but all of those randomely while power is applied).

In theory, I should be getting at least 4200kv from the mots. I closed the gap on the arsg, so the electrodes are almost touching, but it will still stop arcing periodically.

The mots have their cores connected together and grounded, and i'm using each high voltage output for the tesla circuit.

My capacitor is a .02uf 20,000vdc MMC. I know the capacitance isnt as high as it should be, but would that be the problem of sparatic firing?

Thanks for the help!

sparky

Mon Sept 10 2007, 05:03AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I'd be using at least a .08uF cap just because of the lower bang voltages present in the gap. You might also want to use a voltage doubler too to kick the voltage up to prevent power arcing in the gap. Too much current and not enough voltage will cause huge problems that way.

J. Aaron Holmes

Tue Sept 11 2007, 12:06AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

burtgummer wrote ...

My capacitor is a .02uf 20,000vdc MMC. I know the capacitance isnt as high as it should be, but would that be the problem of sparatic firing?

MOTs are hard to use well. A pair gives you ~4200V. At this voltage, with your .02uF cap, a typical 120bps break rate on the spark gap will yield only 50W power throughput. Yes, *fifty*. By the Freau equation, this means sparks about a foot long. And that's assuming everything else is reasonably efficient. Efficiency tends to drop considerably with lower-voltage spark gap coils, however, suggesting that foot-long sparks are probably wishful. I'd expect even less.

~4kV makes spark gap design hard. I'd personally be terrified to have electrodes spinning at 1000's of RPM whilst being only a few mm apart. The huge current potential of MOTs also means that quenching will be difficult, a problem that's exacerbated by the close gap spacing.

If you have access to a NST or two, you'll doubtless find it MUCH easier to exploit the available power. MOTs, however more powerful they may be, are correspondingly much more difficult to use well. For a spark gap coil, I'd probably use a minimum of four just because 4kV is almost hopelessly low, IMO. Terry Fritz has produce some great sparks with only *one* MOT, but that's using a gapless design.

Cheers,
Aaron, N7OE

Kristian

Tue Sept 11 2007, 12:58AM

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

I tried to run my coil off two mots straight up at 4200V (if I'm not mistaken, that should be about 6,000 peak, right?) and I couldn't get it to go at all. I put a voltage doubler on it. It performs much better than the 15KV 60ma NST power supply I was previously using. This is with a static gap, by the way. The tank capacitance is .06uf.

J. Aaron Holmes

Tue Sept 11 2007, 02:04AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Kristian wrote ...

I tried to run my coil off two mots straight up at 4200V (if I'm not mistaken, that should be about 6,000 peak, right?) and I couldn't get it to go at all. I put a voltage doubler on it. It performs much better than the 15KV 60ma NST power supply I was previously using. This is with a static gap, by the way. The tank capacitance is .06uf.

When you say "voltage doubler", are you talking about DC resonant charging? 8400V RMS with .06uF cap and 120bps on the gap would only be 500W. You were getting ~3' streamers, it looks like? Seems right. But a 15/60 NST is 900W, which ought to get you about 1.5' more than that. If it wasn't doing better than the MOTs, I'm thinking something wasn't set up correctly.

Cheers,
Aaron, N7OE

burtgummer

Tue Sept 11 2007, 04:50AM

Registered Member #991 Joined: Sun Sept 09 2007, 05:47AM
Location:
Posts: 6

Thanks for all the helpful info! I have been using 2 NSTs that I've had for about a year. They are rated at 12kv 30ma each, so i run them in parallel to give 720 watts. However, they are made by "Actown" and are the "Guardian II" Model. They have Secondary Ground Fault Protection on them along with other 'special' features. In other words, they don't enjoy running in a tesla system. One of the transformers constantly cuts its power because it feels it is shorting out, which is mostly true for what it considers shorting. So I end up getting half power, although sometimes I can get them both to run....

In the next week or so I'm going to go on a hunt again to get some less pickier transformers. I'm also working on a new rotary spark gap, a synchronous one. My Async gap would give me 233 bps, so of course I'd have a 'wave' in the coil as it running of high-low power, due to the ac waveform. I THINK I found a sync motor in a garage door opener, which was driving a worm gear. I'll be testing it soon to see if it is a sync motor or not. If not, the hunt continues.

J. Aaron Holmes

Tue Sept 11 2007, 05:43AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Consensus seems to be that running async rotary with NSTs is a bad idea; misfires can result in overvoltage conditions that can damage your NST's fragile secondary. You're better off waiting for the sync motor, or else building an actively-quenched static gap. I've run a so-called "leaf blower" gap (actually two in series) up to a couple of kVA and had fine results. I think a lot of people jump off the static gap train too soon. If I were going to be running < 1kVA at 12- or 15kV, I wouldn't waste my time on a rotary. I'd just keep my resonant frequency down (plenty of secondary turns, a good topload) so as to minimize gap losses by forcing the primary tank to be as inductive as possible, and I'd expect to be fine. At MOT voltages, the need for a rotary gap is much more obvious. This is because the higher resistance of a static gap, combined with the hideous pulse currents necessary to get any kind of power out of a MOT-based coil, means a tremendous amount of power will be wasted in the gap (not to mention that a static gap operating at low voltage and high current is much, much harder to quench).

That's too bad about the GFCI in your NSTs. I've seen that kind start to show up in the "free" pile at my friend's neon sign shop. I thought about taking one so that I could dissect it and see what was involved with disabling the GFCI, but then decided I had enough projects running simultaneously.

Safe coiling,
Aaron, N7OE

Dr. Drone

Tue Sept 11 2007, 06:28AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Kristian

Tue Sept 11 2007, 05:27PM

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

J. Aaron Holmes wrote ...

When you say "voltage doubler", are you talking about DC resonant charging? 8400V RMS with .06uF cap and 120bps on the gap would only be 500W. You were getting ~3' streamers, it looks like? Seems right. But a 15/60 NST is 900W, which ought to get you about 1.5' more than that. If it wasn't doing better than the MOTs, I'm thinking something wasn't set up correctly.

By voltage doubler I mean I put a MO cap in series with each HV output and put a couple of MO diodes across them. Should be 12,000V peak pulsed DC.

I was getting about 40" arcs with breakout point to grounded target using the NST power supply and .015uf cap. Without the breakout point I would get a few weak looking 2 foot streamers at a time. I'm getting about 42" arcs using the MOTs, but the arcs are much brighter and thicker. Without the breakout, the arks are numerous and about 3 feet with a few longer streamers breaking out every so often . As far as my observations go, the MOT supply is definitely out performing the NST supply.

Dr. Dark Current

Tue Sept 11 2007, 07:07PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Just to clear things up, a 15/60 NST will NEVER output 900W or anything close that! The 60mA is short-circuit current, the maximum output power will be achieved with 1/2 output voltage but with the current also just at around 1/2 (guess) - so you will get about 250W out maximum.

Edit- Steve's probably right, this works only for resistive loads...

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