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Low frequency tesla coils?

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Artlav

Wed Apr 09 2014, 12:34PM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

Were there any tesla coils that ran at low resonant frequencies, like 5-10kHz?
I've only heard of and seen ones running at 100s of kHz.

For typical designs that would mean huge topload and/or huge coil, but are these the only problems?
Would there be any advantages to low frequency?

Steve Conner

Wed Apr 09 2014, 01:09PM

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

As far as I know, the optimum characteristic impedance for a Tesla coil is about 50k ohms. That establishes a relationship between the size of the coil and the operating frequency. So, low frequencies are only usable for really large coils.

Below a certain frequency (which due to the above relationship, also implies above a certain power level) GTO thyristors will become more cost-effective than IGBTs in solid-state coils. I estimate this happens around 10kHz and 100kW average power.

20Products

Uspring

Wed Apr 09 2014, 04:14PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Artlav wrote:

Would there be any advantages to low frequency?

From a theoretical point of view, a lower frequency allows for more arc length for a given power. This is due to the effect, that arc lengthwise resistance can be higher since the capacitive load due to the arc capacitance is lower. Arcs can therefore be less conductive for a given arc length and thus require less heat.

Whether a match between the arc load and the bridges voltage and current capabilities can be obtained, depends mostly on the secondary to primary inductance ratio. A large secondary inductance would require a correspondingly larger primary inductance, careful tuning and as much coupling as possible. Arc impedance is probably higher at low frequencies, so the match is probably simplified somewhat. My guess is, that a low frequency, moderate power coil can be made to work.

Dr. Slack

Wed Apr 09 2014, 04:21PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

OK Steve, tease/troll successful. What is that bit of power pron from, and what are its specs?

Ash Small

Wed Apr 09 2014, 05:14PM

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

Dr. Slack wrote ...

OK Steve, tease/troll successful. What is that bit of power pron from, and what are its specs?

It's one stage from a pulse generator, it consists of a toroidal core with a one turn primary. The secondary is a shaft which fits through the hole in the centre. These stages are then stacked.

There is a description of one of these here:

(it's the link in the second post on that page)

Artlav

Wed Apr 09 2014, 05:32PM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

Now i seem to remember reading about a lighthouse-sized tesla coil (modern days one), that ran at something in the low 10s of kHz, but Google failed me here.
Anyone heard about it?

Dr. Slack wrote ...
OK Steve, tease/troll successful. What is that bit of power pron from, and what are its specs?

That looks like a part from a locomotive drive.

Steve Conner wrote ...
Below a certain frequency (which due to the above relationship, also implies above a certain power level) GTO thyristors will become more cost-effective than IGBTs in solid-state coils. I estimate this happens around 10kHz and 100kW average power.

Hm?
I was under impression that SCRs don't work well or at all above a few hundred hertz, which is a couple orders of magnitude too low.

Uspring

Wed Apr 09 2014, 05:51PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Anyone heard about it?

Artlav

Wed Apr 09 2014, 06:05PM

Registered Member #8120 Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94

Uspring wrote ...

Anyone heard about it?

, that's it.
38kHz as i remembered.
Not DRSSTC as i thought, but a spark gap one.

And it looks a lot smaller in the video -

than on the pictures.

So, with a big enough coil low frequencies are possible.

Hydron

Wed Apr 09 2014, 07:30PM

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

It is indeed 38kHz, specs are here:

I've actually been to the sculpture farm it's located on, sadly it's only visible in the distance as it's in a private area, so I can't comment on what it looks like in person. It's a shame it's one of the few sculptures (most of which are extremely impressive) that aren't able to be experienced by the public on an open day.

Steve Conner

Thu Apr 10 2014, 10:33AM

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

Dr. Slack wrote ...

OK Steve, tease/troll successful. What is that bit of power pron from, and what are its specs?

It's a GTO thyristor assembly from an electric locomotive. I don't know the exact spec, but it will be thousands of amps at a good few kV. The large round object is the thyristor in a hockey puck package, and the smaller semiconductors clustered around it are the gate drive. A GTO is easy enough to turn on, but needs a huge pulse of current to turn it off.

GTOs switch somewhat slower than even the slowest IGBTs, but they're still a lot faster than 50Hz. In a DRSSTC application, the resonant load would help a lot with turn-off.

I will get my hands on an H-bridge of these one day. >:)

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