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Carbon brush in DRSSTC primary?

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Daniel Uhrenholt

Tue Nov 24 2009, 07:10AM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Killa-X wrote ...

heh funny, Long ago when i was doing SGTC, i thought it would be cool to find a way to make a adjustable warm, with maybe a wheel that goes around the copper flat primary, so when you turn it, even by motor (adjust while running) it would ride the rails, and extend the arm out...

Finn Hammer did this years ago:-) Unfortunately it seems that his old website is gone, so I cant give you a link for it...

doctor electrons,

I already designed all the parts in SolidWorks, this include some spur gears that I have to mill this weekend. But all of this mechanical stuff is better suited under my project, so you have to wait until the weekend to see more of my design:-)

Cheers, Daniel

Herr Zapp

Tue Nov 24 2009, 07:18AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Daniel -

At the Nor-Cal Teslathon held in San Francisco in 2001, there was a fair-sized spark-gap coil that had a motor-driven rotating flat spiral primary coil, with a fixed brass roller as the "tap point".

There was some discussion of this on the TCML just after the Teslathon:

At around the same time, Ken Herrick was also designing a primary with a remotely-adjustable tap point consisting of multiple phospher-bronze contact springs:

Because of the high current through the connection, low resistance is essential to prevent eventual catastrophic failure. A slightly different approach might be to use a miniature air cylinder and a cam mechanism to clamp a pair of contoured copper contact blocks around the primary conductor. During adjustment, the cam would be released, so there was zero pressure between the contact blocks and the primary, and no "lubricity" of the contact block (graphite, etc) would be needed since there would be essentially no sliding friction. At the desired tap point, the cam would be actuated, firmly clamping the copper blocks around the primary conductor. There would be nearly a full 360 degree contact around the OD of the primary conductor, much better than just the limited contact area that could be obtained with any type of "sliding" contact arrangement.

Regards,
Herr Zapp

Finn Hammer

Tue Nov 24 2009, 09:43AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

The website where I hosted my old tesla coil pages was discontinued by my ISP, but luckily I grabbed a copy the same morning they took it down.
Here is a link to the Ambassador coil, the one with the realtime adjustable primary, and realtime positioning of the 3 stationary RSG electrodes.

This was back in december 2000

I am happy that you are reviving the RAT technology, and am sure it will find itÂ´s way into the Thumper as well, in due course!

Enjoy, Finn Hammer

Finn Hammer

Tue Nov 24 2009, 03:55PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,
There is a rule saying that new discoveries warrant double posts, right?

It just struck me, that the perfect tapping point for our goals: The RAT coil, consists of a hard rubber block, shaped to fit the surface of the primary coil. This block is used to press a short length of ground strap towards the coil.
A connection to the strap goes to the inverter, and the combination of the rubber and strap "giving" will provide a multitude of contact points to the coil.

Cheers, Finn Hammer
-Starting to feel a wee creative again.......

Daniel Uhrenholt

Tue Nov 24 2009, 06:30PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Great idea Finn! I'll defiantly give this a try

This may work both the places where there will be some sliding contacts in my design...

Cheers, Daniel

BTW,

Its great that you feel creative again:-) I have the same feeling right now... I'm reorganizing my â€œlabâ€ right now, to fit all the new gear from the University:-)

Steve Conner

Tue Nov 24 2009, 08:36PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Daniel: I wouldn't worry too much about the exact method of making a sliding contact. You and Finn have enough mechanical skills between you to make it work, whatever it is. If you do choose carbon brushes, the only thing I'd want to check out carefully is the skin effect in graphite. It might be especially bad for some reason.

Another worry would be that a bad sliding contact might cause rapid interruptions of the current that could drive your feedback system crazy and blow the IGBTs. But that's only a worry if you have a bad contact, which you won't.

The thing to do is calculate the RMS primary current. RMS is what causes the heating, so it's the "I" you have to use in your I2R heating calculations, but it isn't the same as the peak current. Here's a formula that I worked out to estimate it:

Irms = 0.5*Ipk*sqrt(Burst length * BPS)

Normally the RMS of a sine wave is 0.7 times the peak. I used 0.5 because the primary current has ringups, notches and so on, which can most easily be represented as interference between two sine waves of different frequencies. I've seen DRSSTCs that ring up quickly and settle to a single frequency, though, and 0.7 would be closer in this case.

Also note burst length * BPS is just a convenient expression for duty cycle.

BPS = 1/(Ton+Toff) therefore (Burst length * BPS) = (Ton/(Ton+Toff)) = duty cycle

Finn Hammer

Tue Nov 24 2009, 10:11PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,
Another thing to keep in mind, while unnerving visions of smokingly hot tapping points fly around your scull, is, that the sliding of the contact is only being used during research and setup. So relatively short bursts where data is retrieved, or when a desired operating point is dialled in. After this condition has been met, the tapping point will be clamped securely for prolonged runs.
Having had a look at AntonioÂ´s funky program for DRSSTC tuning, a program that is designed to take the "Hairiness" out of tuning ("A Brazilian for your tesla coil"), both Daniel and I feel it is time to make coils where the coupling as well as the tuning can be altered in real time, so that we can get a quick feel for things while eyes are trained on the scope.

Imagine a situation where you can sweep the primary coil through half or whole turns while streamer loaded, and monitor primary waveform simultaneously. This must be every coilers dream.

WeÂ´ll see how it works out.

Cheers, Finn Hammer

doctor electrons

Tue Nov 24 2009, 10:16PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Daniel Uhrenholt wrote ...

I cant wait to see some photos! You do some really cool stuff!!

GeordieBoy

Wed Nov 25 2009, 12:46AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I'd still go with fixed connections and vary the mutual inductance by changing the separation between coils myself. It has just been my experience that high RF currents are really fussy about conductors having a good rock solid connection.

> Having had a look at AntonioÂ´s funky program for DRSSTC tuning, a program that is designed to take the "Hairiness" out of tuning ("A Brazilian for your tesla coil")...

This forum gets more entertaining by the day!

-Richie,

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