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First tesla coil thread - Dual Mot

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LAN_403

Hydron

Mon Oct 28 2013, 07:59PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

PET is also fairly lossy, though I don't have the experience to tell you whether it's too lossy to work. The reasons it is used as a common dielectric is because it has a fairly high dielectric constant and can withstand higher temperatures than some other plastics.
As for the bottle caps, I don't know about the corona thing but every one I've read about just used normal salt. NaOH will likely cause issues if it gets in contact with aluminium.

MrFlatox

Sat Nov 23 2013, 11:46AM

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

Hi, here i am again !

I finished my PET bottle capacitor bank, and as expected, it does not work. I have not the faintest trace of a spark on the output, even when testing various taping point on the primary coil.

I am starting to wonder if there is a construction problem. In order to "troubleshoot" this mess, I will post below pictures of the actual setup and configuration, any help and comments are welcome. As it is my first TC, I might have done mistakes that would have been obvious to experienced coilers, but not for me.

ground connection

HV end conncection

The capacitor bank is measured to be 133nF, which is approximately what Javatc gave me for a 165khz secondary, and 4.75 primary turns.

As a sidenote, my triac based speed controler does not like the EMI emitted by the primay, and cannot maintain a constant speed, it has a rather eratic behavior.

Thanks in advance for your help.

redruM69

Sun Nov 24 2013, 06:24AM

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

Can you record a video and do a tour of its construction? Its difficult to see how its wired. Show the sparkgap working too. The sound of it may actually help diagnose whats going on.

MrFlatox

Sun Nov 24 2013, 09:43AM

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

I can do that, i will post videos this evening (at my time, wich means in about 8 hours).

In the meantime, i did some low light tests yesterday at night, and i saw some 5cm violet sparks at the breakout point, the sparks were the same lenght no matter where i put my tapping point on the primary.

Kizmo

Sun Nov 24 2013, 10:07AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

This result is not very surprising. Your tank circuit Q is very poor due to capacitor losses and almost all RF energy goes into heating of your lossy capacitor :)

Dr. Dark Current

Sun Nov 24 2013, 10:24AM

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

Except for what was mentioned, did you calculate resonances?

Shrad

Sun Nov 24 2013, 10:47AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

to reduce losses you can (and you should) solder the ends of all your multi-lead cables to make some uniform conduction between leads and connecting medium, as well as carefully clean the areas where contact takes place

but the guess is your capacitor is the troublemaker here... maybe you would be luckier using a stack of alu foil and plates...

MrFlatox

Sun Nov 24 2013, 11:48AM

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

I did tried al foil and plastic sheets with not much success.

I did use javatc to calculate all the parameters of the coil including resonance frequency. I also physically measured the secondary resonance frequency to make some corrections on the javatc file.

To sort out the problem of primary capacitor, i will probably go with mmc thanks to Kizmo.

Ash Small

Sun Nov 24 2013, 11:57AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I can't see a great deal in your photos' of your capacitors, but I think you need to use more copper in order to reduce losses.

Ideally you want Litz wire or foil, but multi-stranded will do.

coil the wires up inside the bottles. This reduces resistance. The more copper surface area in contact with the electrolyte, the lower the losses. Also, the more surface area on your conductors/busbars, the lower the 'skin effect' losses.

EDIT: Also, temperature is a factor with salt-water capacitors. Electrolyte must be 'super-saturated', and warming the capacitors increases dissolved salt thus increasing capacitance and efficiency. (this is based on personal experience, rather than 'conventional' capacitor theory.)

MrFlatox

Sun Nov 24 2013, 02:13PM

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

Here is the video :

The spark gap speed and break rate is really eratic. This is something I have not seen in any other TC video on youtube, but it seems rather normal to me : when the spark gap is not breaking, it's because it's rotational speed is "synch" to the 50 Hz mains, so my moving electrode can go in front of the static ones when the mains sine is at 0v. To overcome this issue, should I run the spark between two mains frequency multiple ?

As for the capacitor, I mixed salt and water in a big container, then shake it for about 10 minutes. I then poured the water in the bottles, and fill the container. When looking at the solubility sodium chloride solubility( 357g/L ) I think I might be far from that, because I used 3 kg of salt, for at least 20 to 30 liters of water.

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