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First tesla coil thread - Dual Mot

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MrFlatox

Sun Sept 22 2013, 08:14AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi 4HV,

I started building my first tesla coil, and I have a few questions.

So far I have built the topload, secondary, primary, control box, tesla coil base. The power supply, the rotary spark gap and the main capacitor bank are missing. All the parts already built or planned to be soon were made according to a simulation with javaTC. So far so good.

For the power supply, I plan on going with a dual MOT supply, with no ballast. Here is my first question : Would it be better to go straight AC with just the two MOTs in series or do some kind of voltage doubler and go with DC (I already have enough microwave oven diodes and caps to do the multiplier). In the second case, I would need a charging inductor (another MOT might work ?) and a de q-ing diode. I know that both concepts work, and I am wondering, as a beginner, what is the best one to go with ?

Second question, about the rotary spark gap. If I understood correctly, its main paramater is the achievable BPS, and the dwell time. The dwell time must be as low as possible, or in other words, as close as possible to the energy transfer time (javatc) ? Right ? So why most of the RSG uses four electrodes on a disc instead of two ? Dwell time is divided by two if the speed of the motor is increases by two right ? So... Why ?

I have a universal brushed motor (160W 230V 20.000RPM) driven by a triac based variator, but the RPM might be too high for my appliccation, I may use an angle grinder instead (also triac driven) to power up the RSG.

Thank you for your time and input.

MrFlatox

Sun Sept 22 2013, 05:57PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

What it looks like so far :

redruM69

Mon Sept 23 2013, 06:04AM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

My 5.25" coil here:

Its running dual mots, no ballast, no filter, no DC doubler. Just straight AC.
The MOT's are wired parallel primaries with series secondaries with opposing phases.

I occasionally see 60" streamers out of this thing :D

Voltage doubling is not required or desired in a ARSGTC as you can gap your electrodes very close with no issues. My gaps are about 1mm per side.

MrFlatox

Mon Sept 23 2013, 06:26AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Thanks for the input,

I know both AC and DC designs work (I contacted you on YT), but my question is : What is the best between AC and DC to have the bigger "bang for the buck" ?

An example of a great working dual mot TC with doubler on this forum is here :

Going from AC to DC essentially means that you have 4 times more volts with DC (with doubler and charging choke), but obviously less current, so you might not be able to put a main capacitor as big as you could go with straight AC. How does it affect the results (streamers) ?

redruM69

Mon Sept 23 2013, 03:20PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

Well, you could also go AC to get it running, then add a doubler in to see how it compares. I'm curious to your results.

MrFlatox

Mon Sept 23 2013, 04:53PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

That sounds good, I might try both solutions and see the differences if any.

Tetris

Wed Sept 25 2013, 03:03AM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Mmm don't be like me. If you say you're going to build it, build it. Most people have the trouble of the parents yelling at you for building a Tesla Coil. My parents are yelling at me because I've been dragging mine out for years...

MrFlatox

Thu Sept 26 2013, 07:37AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Any thoughts on the dwell time thing ?

MrFlatox wrote ...

Second question, about the rotary spark gap. If I understood correctly, its main paramater is the achievable BPS, and the dwell time. The dwell time must be as low as possible, or in other words, as close as possible to the energy transfer time (javatc) ? Right ? So why most of the RSG uses four electrodes on a disc instead of two ? Dwell time is divided by two if the speed of the motor is increases by two right ? So... Why ?

Thank you for your time and input.

Hazmatt_(The Underdog)

Thu Sept 26 2013, 09:57PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Four offers more balance for the flywheel. You could do two, but the speed at which the wheel turns could make things dangerous and unbalanced.

All in all, the exacts are not too critical. I was just doing a basic functionality test with my coil, with the wrong sized secondary and it still performs well enough, its just not optimal. (low by 6 mH)

make sure your strike rail has a decent gap. You don't want that to be a shorted turn.

redruM69

Fri Sept 27 2013, 04:21PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

To be honest, I didn't put any thought into calculations when I built my RSG. I completely winged it based on what I saw others doing. Its a 5" phenolic disc, 1/2" thick. It has 4x 1/8" tungsten flying electrodes, and 2x 1/4" stationary electrodes with some small heatsinks thrown on them. I use a large DC motor with a variac connected through a full bridge rectifier to control speed.
Regardless of lack of calculations, it performs excellent and gets the job done.
I've never tried with only 2 electrodes installed to decrease dwell, but I don't know if I want to spin my disc that fast. The disc is not that well balanced and things would get scary.

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