Author Topic: DRSSTC no sparks  (Read 1253 times)

Offline Radek

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DRSSTC no sparks
« on: April 21, 2019, 09:47:28 PM »
Hello,

managed to get into placing the secondary in, however, got almost no sparks. I was trying to tune the primary circuit to receive the proper frequency but no results. The secondary frequency (approx 74 kHz) was measured with a floating scope's probe whilst a 555 oscillator connected to the coil. I pushed the Vbus up to 150 VDC and primary current to 200 apk but only a 'ticking' was heard without any sparks to be seen. I also tried to attach a grounded wire to the topload to find out whether some hv is there and I got small sparks at a distance about 1 cm from the toroid.

Does anybody have an idea? Is it due to a weak coupling? I can provide a photo of anything needed so feel free to ask.

Thanks for your help in advance.

Offline Mads Barnkob

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Re: DRSSTC no sparks
« Reply #1 on: April 21, 2019, 10:14:51 PM »
Hi Radek and welcome to HVF

What is the calculated secondary frequency on something like JavaTC?

What makes you think that your coupling is too weak? JavaTC should also give you this, if its in the range of 0.1 to 0.2 it is not bad and can be adjusted accordingly by raising the secondary if you need to lower it.

You are getting sparks, that is great! It could be something simple as a phase issue, like flipping polarity of your feedback CT or primary coil connections that should get the system into phase, maybe your feedback CT and primary circuit is 180 degrees out of phase?

Please do post some more details and images :)
« Last Edit: April 22, 2019, 08:46:50 PM by Mads Barnkob »
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Offline Radek

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Re: DRSSTC no sparks
« Reply #2 on: April 21, 2019, 11:13:07 PM »
What is the calculated secondary frequency on something like JavaTC?

According to the JavaTC, the frequency should be around 84 kHz, but when I used the 555 oscillator, the scope showed me 74 kHz. In fact, when I tried to go higher with the primary frequency up to about 80-90 Khz, that was the only time I could see really tiny sparks coming from the topload electrode without a grounded wire attaching close to the topload (shorting it). At the same time, the current and voltage waveforms changed a lot without adjusting ontime. The comparison of both cases are included in the attachments.

Details of the coil expect the parameters that can be seen in a javaTC attachment.
Half-Bridge - IGBT - BSM 50GB 120DN
Max. current 200 apk
Primary coil: flat spiral coil starting at diameter of 25 cm, ending at a diameter of about 43 cm with 8.75 turns and 0.6 cm copper tube
Primary capacitors: 3 strings of 4 pieces: 0.47 uF and 2 kV each (ESR 4,8 mOhm), overall capacitance approx 352 nF
Driver: UD2.7C
CT ratio: 1:20:10 for FB and OCD (3 cores) and one more CT (2 cores) for scoping 1:20:10 with 5.1 Ohm resistor
BUS capacitors: 2x 4700 uF 400 VDC in paralell
Snubber over the IGBT: 2 uF, 1 kVDC
Voltage capacitive divider for the primary LC: 2x 2,5 uF snubbers 1kVDC
Vbus: up to 350 VDC, according to the current limit -> 200 VDC max
Interrupter: oneTesla - ontime approx 12 - 50 us


Offline Teravolt

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Re: DRSSTC no sparks
« Reply #3 on: April 22, 2019, 07:30:18 PM »
that last waveform has a phase inversion baby there is something wrong with your feadback coil. do the primary and secondary have the same resonant fequency?

Offline Radek

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Re: DRSSTC no sparks
« Reply #4 on: April 23, 2019, 09:03:48 PM »
The frequency difference is about 1-3 kHz now, I suppose. That would not affect the spark's length that much I think.

I tried to reverse the primary connections and also the feedback CT afterward. I did all the combinations possible. However, it only resulted in the same spark's length or no primary current at all. But I noticed quite a strong magnetic field around the coil so my laptop even 2 meters away started to work properly. That's why it is weird that the spark's length is not proportional to that.

What about the low capacitance of the voltage divider mentioned in a previous post? Otherwise, I have no clue.

I took some pictures of the DRSSTC itself and also some waveforms again so you could better imagine.

Thanks for every tip. This project is supposed to work on Thursday, so I'm quite desperate.
« Last Edit: April 23, 2019, 09:14:35 PM by Radek »

Offline Mads Barnkob

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Re: DRSSTC no sparks
« Reply #5 on: April 23, 2019, 09:47:50 PM »
I see no problems with your voltage divider capacitance, I have pulled plenty of power and made long sparks with similar capacitance in a divider.

Are you sure your IGBT module works?

12-50uS on-time is not much to generate some actual break out, you never tried running it for longer than that? My DRSSTC1, which is similar in size to yours, needs a good 150uS to make 1 meter sparks, up until then there is not much, it has to do with the detuning of the primary coil, that it needs a certain load to pull the secondary circuit down in frequency to match the lower tuned primary and once it comes down there, sparks are flying. It really runs best at 200uS for sparks and 300uS for MIDI :)

I hope you find the problem or can do with very little sleep until thursday. I know my last advice sucks, but try to go through all the simple things that you take for granted to work, it could just be some minor detail that is causing the trouble.
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Offline Radek

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Re: DRSSTC no sparks
« Reply #6 on: April 24, 2019, 01:06:37 AM »
Are you sure your IGBT module works?

It is not a brand new piece, so there is a chance that it is not in its best condition. However, if it did not work, I would not see a primary current flowing through the primary LC, would I?


12-50uS on-time is not much to generate some actual break out, you never tried running it for longer than that? My DRSSTC1, which is similar in size to yours, needs a good 150uS to make 1 meter sparks, up until then there is not much, it has to do with the detuning of the primary coil, that it needs a certain load to pull the secondary circuit down in frequency to match the lower tuned primary and once it comes down there, sparks are flying. It really runs best at 200uS for sparks and 300uS for MIDI :)


Well, I can try to set the interrupter for longer on-time, but I'm kinda limited by the highest current (200 apk). I've decreased the MMC capacitance down to 152 nF and increased the primary inductance for higher surge impedance and therefore slowest current rise hoping it would help. It did not, unfortunately. The only thing it allowed me to do, was putting higher Vbus on the bridge (up to 250 VDC) at 50 us so I got about 2 cm long sparks which is still not an improvement. So can I try to get 150 us on-time at lower Vbus (100 VDC) so I won't overcome the maximal current. In terms of detuning, I agree with that the secondary need a certain load to pull down its frequency, but it would be still about 5 kHz or so on. I don't think this small frequency difference would end up at spark's length to 2 cm. I've not even managed to tune the primary to accurate frequency so far since it is impossible to recognize it. I can only recognize I'm quite near that point by listening to the coil. I can also see on the scope when I hit the point that is the closest to the resonance since I get the weird waveform where the primary current rises in two parts (see an attachment).

Anyway, I cannot get rid of thoughts, that this problem has something to do with the geometry of the coil (either of the primary coil or the secondary). Can the secondary be too huge (15x80cm approx 2200 turns)? Can the coupling be truly weak regardless of what javaTC calculated (0.128)?

What about the feedback CT? Can there be any problem with its design? Should I scope the FB input of the UD whilst the coil running?

Thanks you, Mads. I'm just slowly but surely losing hope, though.

Offline Netzpfuscher

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Re: DRSSTC no sparks
« Reply #7 on: April 24, 2019, 07:36:46 AM »
Don't let the primary tapping wires run in the opposite way of the primary. On the pictures it looks like that's the case, it cancels out parts of the field.

Offline Radek

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Re: DRSSTC no sparks
« Reply #8 on: April 24, 2019, 12:58:19 PM »
Don't let the primary tapping wires run in the opposite way of the primary. On the pictures it looks like that's the case, it cancels out parts of the field.

Okey, I’ll keep both of them in a same direction. Do you think the cancelled field would be that enormous so it would cause such small sparks?

Offline Teravolt

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Re: DRSSTC no sparks
« Reply #9 on: April 24, 2019, 09:38:41 PM »
is the phase correct from your feedback transformer going into the univesal driver. have you tried feeding a sqare wave into the feedback input with a low voltage at your mains cap to see if your gate drive and bridge is working as expected? once you get that sorted out you can see if your feedback transfromer is coupling the current properly into a resistive load

Offline Uspring

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Re: DRSSTC no sparks
« Reply #10 on: April 25, 2019, 01:30:40 PM »
It's probably a bit late for your project, but there are two points, that caught my attention in your design. You wrote:
Quote
Well, I can try to set the interrupter for longer on-time, but I'm kinda limited by the highest current (200 apk). I've decreased the MMC capacitance down to 152 nF and increased the primary inductance for higher surge impedance and therefore slowest current rise hoping it would help. It did not, unfortunately.
I think, you are on the right track here. The bridges power capabilities should be matched to your primary tank impedance. A ballpark estimate for the best primary impedance is given by:

Vmax/Imax ~ 0.5 * k  / (2*pi*f*C)

where Vmax and Imax are the bridges peak output voltages and currents, f the primary resonance frequency and C the MMC value. A bit more on this can be found at https://highvoltageforum.net/index.php?topic=113.msg657#msg657

For your primary we get for the right side (k=0.14, f=80kHz, C=0.35uF) a value of about 0.4 ohms. The left side comes out to 1.25 ohms using Vmax=250V and Imax=200A. There is a big discrepancy there, which implies, that the bridge will be overloaded unless you shorten the burst. But that will reduce the possible energy input to the primary. You need to reduce the capacitance of the MMC and increase the number of primary turns to keep the resonance frequency.

The drawback of a higher impedance primary is, that the slower primary current rampup will require a longer burst period.

The other point is, that in your bridge voltage scope shot, the voltage does not look like a square wave but drops considerably at the top. A likely cause is a too thin wiring between the bus caps and the bridge.

Offline Radek

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Re: DRSSTC no sparks
« Reply #11 on: April 26, 2019, 04:52:26 PM »
is the phase correct from your feedback transformer going into the universal driver? have you tried feeding a square wave into the feedback input with a low voltage at your mains cap to see if your gate drive and bridge is working as expected? once you get that sorted out you can see if your feedback transformer is coupling the current properly into a resistive load

In terms of feedback ct, I've tried to reverse it many times as well as the phase jumpers and primary connections (mentioned in previous posts). If the phase of the feedback ct was wrong, I'd not seen any primary current on the scope which I'm able to see now. Hope this is what you meant.

I did put a square wave into the feedback input when tuning a gdt and gate resistors without a secondary coil placed in (on iron load). Do not know whether it is reasonable to put the square wave in with the secondary placed in, too. Is it? I can try.

Offline Radek

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Re: DRSSTC no sparks
« Reply #12 on: April 26, 2019, 05:25:34 PM »
I think, you are on the right track here. The bridges power capabilities should be matched to your primary tank impedance. A ballpark estimate for the best primary impedance is given by:

Vmax/Imax ~ 0.5 * k  / (2*pi*f*C)

where Vmax and Imax are the bridges peak output voltages and currents, f the primary resonance frequency and C the MMC value. A bit more on this can be found at https://highvoltageforum.net/index.php?topic=113.msg657#msg657

For your primary, we get for the right side (k=0.14, f=80kHz, C=0.35uF) value of about 0.4 ohms. The left side comes out to 1.25 ohms using Vmax=250V and Imax=200A. There is a big discrepancy there, which implies, that the bridge will be overloaded unless you shorten the burst. But that will reduce the possible energy input to the primary. You need to reduce the capacitance of the MMC and increase the number of primary turns to keep the resonance frequency.

The drawback of a higher impedance primary is, that the slower primary current ramp-up will require a longer burst period.


It is never too late to fix it. :)

Anyway, if I understand it properly, there is a purpose to get an equation of both sides. Considering by design, the best way is to decrease the MMC capacitance to 157 nF (2 strings with 6 caps). I should have enough spare primary inductance to be able to keep the resonance frequency around 80 kHz (considering the secondary f 83-84 kHz). After this adjustment, the right side of the formula should be equal to 0,82 Ohms. To get as close as possible to this value, I should get Vbus about 170 VDC and Imax 200 Apk, which makes 0,85 Ohms. However, as you said, to get this current at quite low voltage (170 V), I'm gonna need longer ontime. I calculate the number of needed halfcycles about 15 and that makes max. ontime about 94 us. Perhaps, you try to check those numbers if it does not bother you. I'd be certain I do it the right way, at least.

Cprim = 157 nF
Lprim = 25 uH
fprim = 80 kHz -> T/2 = 6.25 us
Zprim = 12.6 Ohms
Imax = 200 Apk
Vmax = 170 VDC


The other point is, that in your bridge voltage scope shot, the voltage does not look like a square wave but drops considerably at the top. A likely cause is too thin wiring between the bus caps and the bridge.


As for the voltage drops that I've also noticed, I think it might be because of the low capacitance of the voltage divider (Mads has disproved it, however). I personally doubt it is caused by the too thin bridge connections since it is made from 3 stacked copper plates with overall thickness about 3mm. The picture is part of the attachment.

Offline Uspring

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Re: DRSSTC no sparks
« Reply #13 on: April 27, 2019, 06:26:24 PM »
Your calculation looks ok. Somewhat better would be an even a bit smaller MMC capacitance, since it would allow for a higher input voltage, increasing power input. A single string of your caps could do that, but then there are chances of overloading the caps.

The voltage drops could also be caused in the connection between the bus caps and the primary. I'm a bit at a loss to explain that. Cap discharge of this amount is unlikely, but the voltage drop has to come from somewhere.

Offline Mads Barnkob

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Re: DRSSTC no sparks
« Reply #14 on: April 27, 2019, 08:44:59 PM »
As for the voltage drops that I've also noticed, I think it might be because of the low capacitance of the voltage divider (Mads has disproved it, however). I personally doubt it is caused by the too thin bridge connections since it is made from 3 stacked copper plates with overall thickness about 3mm. The picture is part of the attachment.

I would like to clarify that it was on a SSTC that I used low capacitance power film capacitors for the voltage divider, in my half-bridge DRSSTC I used 470uF electrolytic capacitors.

However, I did also only 10uF on my DRSSTC4, which however was an entirely bad idea :) http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iv/
« Last Edit: April 27, 2019, 08:57:51 PM by Mads Barnkob »
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Offline Uspring

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Re: DRSSTC no sparks
« Reply #15 on: April 28, 2019, 07:37:40 PM »
I hadn't looked in into the voltage divider issue. Mads post pointed me there. The 2 2.5 uF caps will look like a 5uF cap in series with the primary tank. A 200A current at 80 kHz will cause a peak voltage drop of 80V. That explains the drop measured in your scope shots. You see a sine wave superimposed on the bridge output square wave.
In terms of performance, that's probably ok. It will just make the measurement look strange. As Mads pointed out, one has to be careful using low capacitance electrolytics there, since they have a larger ESR than higher capacitance types and might explode under the current load.

The Vmax in the equation quoted is an amplitude i.e. half of the peak to peak voltage. If I've understood correctly, you probably mean the voltage of the bus caps, when you talk about 200V voltage. With a voltage divider that implies an amplitude of only 100V.

You could double the input voltage by using a voltage doubler circuit as in here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/rectifiers/ . That will double the input power. You wouldn't need any new components for this but just change the wiring. Your 1200V IGBTs should be able to handle this. Your line voltage rectifier should have the sufficient voltage rating for the doubler.

Offline Radek

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Re: DRSSTC no sparks
« Reply #16 on: April 28, 2019, 07:41:42 PM »
Your calculation looks ok. Somewhat better would be an even a bit smaller MMC capacitance, since it would allow for a higher input voltage, increasing power input. A single string of your caps could do that, but then there are chances of overloading the caps.

Yea, that is no problem. However, I do not think this, in particular, could affect the spark length at this moment. I also intend to buy bigger IGBT later on, so Imax would be probably higher then. There will be also more changes because of that. For now, I just need to figure out the spark length at this Imax.

As for the voltage drops that I've also noticed, I think it might be because of the low capacitance of the voltage divider (Mads has disproved it, however). I personally doubt it is caused by the too thin bridge connections since it is made from 3 stacked copper plates with overall thickness about 3mm. The picture is part of the attachment.

I would like to clarify that it was on a SSTC that I used low capacitance power film capacitors for the voltage divider, in my half-bridge DRSSTC I used 470uF electrolytic capacitors.

However, I did also only 10uF on my DRSSTC4, which however was an entirely bad idea :) http://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-iv/

Well, I can try to put there two electrolytic capacitors 220 uF 400 VDC to find out whether it helps or not. What is more important, however, it to figure out the spark length.

Anyway, are you sure I should not try to move with the primary to increase the coupling? I mean, could the primary winding be too down compare to the start of the secondary winding despite what coupling coefficient I've got from javaTC.
« Last Edit: April 28, 2019, 07:43:25 PM by Radek »

Offline Uspring

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Re: DRSSTC no sparks
« Reply #17 on: April 28, 2019, 10:06:22 PM »
Mads wrote:
Quote
My DRSSTC1, which is similar in size to yours, ...
That's a generous statement. The full bridge version puts out up to 500A into the primary with an amplitude of about 600V. That's 2.5 times more currentwise than Radeks design and 6 times more voltagewise.  Together 15 times more power. Since it produces about 1.5m arcs and arc length goes with about the sqrt of power, that would imply about 40cm for Radeks current version. The difference in total energy is still a bit bigger, since Mads uses longer bursts.

Mostly a larger coupling works better. Tuning becomes critical for low k and is perhaps the reason for the present performance. AFAIK the only reason for smaller couplings is the tendency for flashovers from the secondary to the primary when too closely spaced. So go ahead and move up the primary. But once you got your coil flashing, you might want to move it down again. :)


Offline Mads Barnkob

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Re: DRSSTC no sparks
« Reply #18 on: April 29, 2019, 08:18:09 AM »
Mads wrote:
Quote
My DRSSTC1, which is similar in size to yours, ...
That's a generous statement. The full bridge version puts out up to 500A into the primary with an amplitude of about 600V. That's 2.5 times more currentwise than Radeks design and 6 times more voltagewise.  Together 15 times more power. Since it produces about 1.5m arcs and arc length goes with about the sqrt of power, that would imply about 40cm for Radeks current version. The difference in total energy is still a bit bigger, since Mads uses longer bursts.

You are absolutely right, I should have been a bit more precise in what parts I was comparing, as it was mostly based on the resonant frequency and MMC size :) I do however only run it at single phase, so 320VDC on the bus. But half-bridge vs. full-bridge, peak current and on-time wise you are spot on
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Offline VNTC

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Re: DRSSTC no sparks
« Reply #19 on: May 05, 2019, 11:53:45 AM »
Hi,Radeck.
First, the burst length should be  can extends as long as 100us
Second let take a look at 2 images in file attached if your case correspondent with upper image, your coils was detune, include in this case the coil still give few cm coron
Goood luck.

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Re: DRSSTC no sparks
« Reply #19 on: May 05, 2019, 11:53:45 AM »

 


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[Dual Resonant Solid State Tesla coils]
erdal
August 14, 2019, 12:53:10 PM
post Van de Graaf is leaking charge
[Static electricity]
Patric
August 14, 2019, 09:22:21 AM
post Re: Drsstc 3
[Dual Resonant Solid State Tesla coils]
oneKone
August 14, 2019, 04:39:11 AM
post Re: MMC orientation and variac
[Spark gap Tesla coils]
thedoc298
August 13, 2019, 07:10:31 PM
post Re: MMC orientation and variac
[Spark gap Tesla coils]
MRMILSTAR
August 13, 2019, 05:17:23 AM
post MMC orientation and variac
[Spark gap Tesla coils]
thedoc298
August 12, 2019, 09:54:35 PM
post Re: My second DRSSTC
[Dual Resonant Solid State Tesla coils]
erdal
August 12, 2019, 08:40:16 PM
post Re: Small Phase change build R600a
[General chatting]
Hydron
August 12, 2019, 07:27:23 PM
post Re: Teardown of Danfoss VLT6000 4kW Variable Frequency Drive
[Electronic circuits]
klugesmith
August 12, 2019, 09:18:15 AM
post Re: Teardown of Danfoss VLT6000 4kW Variable Frequency Drive
[Electronic circuits]
shrad
August 12, 2019, 09:06:34 AM
post Re: Teardown of Danfoss VLT6000 4kW Variable Frequency Drive
[Electronic circuits]
klugesmith
August 12, 2019, 09:02:38 AM
post Re: A problem solved with hand tools
[General chatting]
klugesmith
August 12, 2019, 08:29:20 AM
post Re: Ebay [US]: MITSUBISHI CM300 1200V-300A IGBT (30$ a piece)
[Sell / Buy / Trade]
oneKone
August 12, 2019, 05:25:24 AM
post Re: Ebay [US]: MITSUBISHI CM300 1200V-300A IGBT (30$ a piece)
[Sell / Buy / Trade]
acobaugh
August 10, 2019, 08:14:35 PM

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