Driving the primary with other than 60Hz

[Robert3z]

Suppose you just happened to have a pair of 5KVA 40:1 step up matched transformers rated to 5000V each. 20GA wire on the secondaries so they are good to app. 27KHz. Well matched enough to connect primaries in parallel and run the output grounded center tap. Think a 1A 10KV "NST." Yes, they would make a scary full wave multiplier.

Then suppose you had access to higher frequency than mains, say a Variable Freq Drive or a full bridge converter.

Could you drive an SGTC at higher than mains frequency and get any sort of gain from it?

I would assume an SGTC would really benefit from 400Hz or so with lots more breaks and a more annoying buzz, but that going up to the KHz would just create a constant arc unless there was a mechanical break device.

Since I do have the two supposed transformers on the way, suggestions?

Cheers / Robert

[davekni]

Not clear where 27kHz comes from.  I'd guess upper useful transformer frequency is limited first by secondary winding capacitance and leakage inductance.

For a given SGTC design (given MMC value and spark gap voltage), transformer RMS current increases with frequency, as it must charge the MMC more often.  That's one limit for SGTC use.

Do you have a ballast (series inductance) for your transformers?  I'm guessing they don't have intentional internal series inductance as NSTs do.  At high enough frequency, transformer's leakage inductance might be enough for ballast.  However, that may be too high a frequency for transformer power rating.

I would assume an SGTC would really benefit from 400Hz or so with lots more breaks and a more annoying buzz, but that going up to the KHz would just create a constant arc unless there was a mechanical break device.

Synchronous rotary spark gap (SRSGTC) would be a great option.  I'd guess that 400Hz could be achieved with a static spark gap and fast air flow through the gap.  If you added arc-extinguishing features as found in circuit breakers along with fast air flow, I'd guess 1kHz is possible.  I don't have any specific experience to justify this guess.  Closest is my 120Hz Marx generator using static gaps and rather gentle air flow.  (Limited to <50Hz without any air blowing at spark gaps.)

[Hydron]

You could also rectify it and run it as a DC resonant charging coil - if you run it at >60Hz (taking care that the diodes are fast enough - 400Hz would be likely fine though) then you can be less generous with the smoothing capacitance, possibly to the point that a bunch of microwave oven caps could be enough. In this case an ASRSG lets you change the break rate independently of mains frequency (thinking about it, you could use your VFD for running a ASRSG motor!). You also get double the capacitor voltage, so if you want >10kV from your transformers then that's an option.

https://www.richieburnett.co.uk/dcreschg.html

I'm with Dave with not being clear where 27kHz is coming from, unless you're thinking of skin effect? I also suspect you'd be limited by other parasitics before that!

[Robert3z]

The transformers are rated to 5KV and 20KHz by spec sheet (and I talked to the engineer) - Ferrite cored like the one I am making for a friend's project. When they show up I will post. Plans for these are modular - I would like to get as much usefulness out of them as possible.

The DC resonant link is interesting and makes use of the freq response for DC power which I'm sure these were originally intended for.
A single unit could be a 5KV DCPS just like the article linked - the cap(s) would definitely need to be guarded - doom in that. The large inductor is an issue - they're readily available for drive chokes but not at the voltages mentioned. Wonder if there's a giant input line choke out there for a 4160VAC motor drive used???

The HV DCPS makes sense in a module too as does the Asynchronous Gap - Thank you!

[Hydron]

If they're ferrite cored, are they even able to be run at sub-kHz frequencies?
If not, a full bridge for transformer drive, fast rectifiers and DC resonant charging is probably your only option, though you're right about the choke being a pain to find/make - it's one of the reasons I never got around to building a non-DRSSTC coil. If you'd have to build a full bridge to drive the transformer anyway the question arises about whether it's worth continuing down the spark-gap route though.
If they can handle lower frequencies though then yeah you've got more options.

[Robert3z]

Hydron: I don't think so. I'm hand-building a ferrite core grounded CT transformer for HF high voltage and it doesn't look like running it low is an option. I could do a small signal test but that will only measure coupling, not saturation or heating effects from being run at low freqs. Too much work and winding to risk  smacking it with 60 or 400Hz! Big learning experience that I'd prefer not to repeat.
Running a Variac / full bridge / step-up transformer / rotary gap does seem sideways reasoning : )  especially given the DRSSTC design polish and losing the expensive transformers.

A VFD would drive the Richie Burnett 3 phase version but wouldn't be voltage-adjustable other than soft start. You always seem to end up back at the Variac : ) or a big switch!

Thank you / Good Conversation!

[Hydron]

To be fair I currently use a variac to run my DRSSTC, which kinda defeats the idea of ditching the big iron transformers! But it does work without it (with the UD3 controller doing soft-start etc), just with a bit less control of the spark output (can only adjust via on-time/BPS/current-limit, rather than both that and bridge voltage).
At some point I hope to have a boost PFC converter to replace my voltage-doubler rectifier, which would give control of the bridge voltage in the range of ~325V DC (rectified 230V) up to the limits of the bridge/bus-caps/PFC (probably around the 800V mark).

On the topic of DRSSTC vs ASRSG SGTC I do think that you get a bit of a different character to the sparks with the SGTC, plus the howling of the ASRSG, though as I said I haven't built one myself.

[davekni]

Too much work and winding to risk  smacking it with 60 or 400Hz!

I quite agree.  Given ferrite core and 20kHz design frequency, it is certain to function poorly at 60Hz or 400Hz.  Output voltage capability before saturation will be roughly proportional to frequency.  At 400Hz, max output voltage will be about 100V rather than 5kV.  (I'd misunderstood initially.  Thought you were asking about running a 60Hz transformer at higher frequencies.)

[Robert3z]

I really LIKE the idea of running the 3 phase DC Primary circuit Coil Richie Burnett wrote up but will move it to the proper list : )

[Robert3z]

This is the proper subforum! 

I spent some time going through the DC schematic on Richie Burnett's website and realized the charging inductor is basically a boost-mode SMPS setup. 2xDC is Awesome! Here are all the thoughts on this Coil idea. My dry-erase board in the workshop is full - attached sketch.

http://www.richieburnett.co.uk/dcreschg.html
Below is from Richie's Website:
"It should be realised that the function of the de-Q-ing diode can be performed by the DC bridge rectifier in 3-phase designs where no HVDC smoothing capacitor is used. The lack of a separate de-Q-ing diode and potentially lethal DC smoothing capacitor make such 3-phase supplies ideally suited to variable speed DC resonant charging."

This is a good idea! So here is Project "10-Horse," a DC Primary "Boost Mode" Asynchronous Rotary Spark Gap Tesla Coil.

You can buy a cheap 7.5KW (7500VA) or 10HP VFD ($300.00 US) that runs on 240VAC mains at 35A and outputs 208/240VAC 3 Phase from 1-400Hz, 1-600Hz or in a few cases, 1-999Hz. "You get the three-phase source without paying the power company for it."  It will require adequate impedance in the transformer primaries or the drive will fault as it will be "expecting" to see 10 horsepower motor windings or the equivalent.

I have a box of 30 12KV 1A NTE rectifiers (block mount on heatsink)
https://www.digikey.com/en/products/detail/nte-electronics-inc/NTE541/11655505

RECTIFIERS
1.) Actual Rectifer output will be 1.414 x Transformer Output; diode voltage ratings must take this into account and have leeway above it.
6000VAC transformer output (25.0 : 1 Step Up) = 8.5KVpp rectifier output. DC doubling = at least 12KV
6500VAC transformer output (27.1 : 1 Step Up) = 9.1KVpp rectifier output. DC doubling = at least 13KV
7000VAC transformer output (29.2 : 1 Step Up) = 9.9KVpp rectifier output. DC doubling = at least 14KV - near rectifier diode limit
7500VAC transformer output (31.2 : 1 Step Up) = 10.6KVpp rectifier output. DC doubling = at least 15KV - rectifier limit

NTE Rectifiers chosen for the project because I have them and they need a home. If a pair of NTE diodes are used in parallel for 2A, then 7-7.5KV are tops.
If a pair are in series, then higher primary voltage may be utilized with lower current.

- 2 NTE 12KV Diodes in series = 1A at app. 13-18KV, possibly 20KV - assumes voltage drops are balanced between the 2 series diodes. All diodes series or parallel will be tested, I don't desire to use balancing resistors. Defeats the DC de-Q-ing diode!

- 2 NTE 12KV Diodes in parallel = 2A at app. 9-10KV max - more current but less voltage

First question on primary circuit - better higher current or voltage?
   
2.) Step-Up Transformers (3)
Drive output is 240VAC at 0-400H, possibly 600Hz. Using higher output frequency allows a smaller step-up transformer to be utilized. Possibly cheaper?
For 6000VAC, a 25 to 1 step-up
For 6500 VAC, a 27 to 1 step-up
For 7000VAC, a 29.2 to 1 step-up
For 7500VAC, a 31.25 to 1 step-up


Design difficulties are the step-up transformers and the choke. I'd like to think I could wind the choke myself given the correct core, but transformers in that power range will probably have to be ordered.

Difficulties:

1.)  3 strong step-up transformers

3.) Choke for resonant charging: high value, high current, high voltage

Has anyone built this design?

In my build queue I have the current Jacob's Ladder with a friend and a McCauley 8.0 DRSSTC to learn that aspect of the hobby. I prefer to study lots and then build a kit or proven design before I try stuff on my own.

For a home-designed project, the above sounds pretty awesome. Choke protects the transformers, diodes and DC keeps the resonant rise in the primary down.

And an awesome rotary gap : )

If you see anything wrong, please post!

[davekni]

It will require adequate impedance in the transformer primaries or the drive will fault as it will be "expecting" to see 10 horsepower motor windings or the equivalent.

This is likely your most difficult challenge.  My guess is that you need inductors between VFD and transformers.  HV transformers are not likely to handle VFD switching-frequency voltage swings, and the VFD is likely to have trouble with HV transformers secondary winding capacitance if not isolated by primary inductors.

I have a box of 30 12KV 1A NTE rectifiers (block mount on heatsink)

At <1kHz, series connection may work.  At 30kHz, the biggest issue I found with diode series strings is variation in reverse recovery charge.  The first diode to recover gets the entire string's reverse voltage, conducting recovery current of other diodes in avalanche breakdown.  Sorting diodes by recovery charge helped, as did using diodes with avalanche energy handling specifications.

BTW, if transformer secondaries are wired as 3-phase, only 6 diodes are required, not 12 as shown in your diagram.

3 strong step-up transformers

If you can find a 3-phase transformer, that will be lighter and more efficient than 3 separate 1-phase transformers.  Not sure where you will find either, nor the HV inductor.  Good luck with that.  (I used resonant charging on my SRSGTC, using intentional inductance of MOTs and a bit more added inductance in primary since MOT inductance was not quite enough.  Charging tuned to 120Hz to match my 60Hz line frequency.  Synchronous, so no diodes.)

[Robert3z]

I think 400Hz is probably a good target, it is common in low cost drives, also a common value in older electronics before the SMPS and associated static frequency converters. I'll stick with that.

Loading the outputs of the drive at 240VAC/400Hz/30A or so may be in the scope of home-building an inductor - you need the step-up primary values first though.

Yes - 6 diodes in three full wave rectifiers AB/AC/BC - I just copied Richie's schematic x 3 - it is easier to explain to a non-electrical person. I've found that Deltas and Wyes make most people's eyes cross : ) though I think it's a more attractive way to draw an industrial schematic.

The other good reason for 400Hz: they might appear on the surplus market - first stage of older 400Hz powered radar or comm gear. We'll see about that. Lots of the wrong kinds at Sales and Surplus of Nebraska...but at least 400Hz is there in a large population, mostly filament transformers.

Transformer(s) and Inductor(s) continue to be difficult, and you really can't go forward with the design until they're locked down. I'm going to keep it in mind and watch for inductance for sale!