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Odd looking primary current waveform

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Tom540

Tue Jan 27 2009, 07:00AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Hello peeps,

So the other day I made a quickie CT to measure my coils primary while running. I could not find the normal CT I use so I made this new one. Nothing special. The CT consists of two cores one with 33 turns the other with 31 turns cascaded for a total of 1023 turns and the output has a 10.2ohms across it. Ive done this many times but this time with a larger resistor so I can get a bigger voltage.

In the past I've gotten the normal looking envelope that peaks after a few cycles and then tapers down slowly but this time I get this waveform.

The tuning seems to be not perfect but close enough so I'm guessing this has something to do with coupling.
I am flat out of IGBT's at the moment so I can't do anymore testing or investigating. If I end up changing the IGBT"S two more times the board will be destroyed. I figured I'd see if anyone has any insight of why this waveform looks so crappy. The envelope is 120uS long and I think the scope is set to 10vpp per division

Any ideas?

-Tom

LithiumLord

Tue Jan 27 2009, 10:48AM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

The current has three distinct notches, therefore it's a sinc problem. Are you running a DRSSTC? It may be either a feedback issue or probably your coil is badly out of tune (if that's a DRSSTC).

GeordieBoy

Tue Jan 27 2009, 02:46PM

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

Hi Tom,

The current waveform looks pretty normal to me. If it's a DRSSTC you were scoping (you didn't actually say) then remember that a 2nd order resonant system always has two modes of oscillation. ie. Two distinct resonant frequencies, even if the primary and secondary are apparently "in tune with eachother".

The usual DRSSTC circuit with primary feedback will favour operation at one or other mode frequency, but the transient nature of each RF burst implies that some energy will alsways be coupled into both modes. If two nearby frequencies of the system are excited you can expect to see constructive/destructive interference effects in the time domain waveform. These give the ripples you are seeing in the amplitude envelope of the RF.

The driver might be primarily exciting the lower mode frequency but some energy will couple into the upper mode frequency. The frequency of the amplitude modulation you see in the current waveform, is the beat frequency between these two modes.

It's quite normal, so I wouldn't worry about it. In a conventional, spark gap powered Tesla Coil, both modes get excited equally, the AM envelope to the primary and secondary waveforms is much more intense. We call the instants where the primary current envelope falls to zero "notches" and try to make our spark gaps quench there!

-Richie,

Tom540

Tue Jan 27 2009, 05:01PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Hey Ritchie,

Its a DRSSTC. It seems as if the current doesn't want to taper off. It looks like if I increased the pulse width the signal would just sit at that same voltage for the entire duration. The secondary doesn't appear to be sucking up the primary energy very well. At least from this shot.

Also when I say in tune its a pretty vague statement when it comes to a DRSSTC. Ive been trying to figure out just how low I should be tuning the primary under the secondary. I half a turn on this primary didn't seem to make any notable difference.

Lithuim,

When you say a sync problem are you referring to the interrupter turn off synced with the resonant frequency? That should be fine. The driver works well.

-Tom

GeordieBoy

Wed Jan 28 2009, 12:31PM

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

> Its a DRSSTC. It seems as if the current doesn't want to taper off. It looks like if I increased the pulse width the signal would just sit at that same voltage for the entire duration. The secondary doesn't appear to be sucking up the primary energy very well. At least from this shot.

I think that in the steady state a situation will be reached where the power being supplied by the inverter equals the power being dissipated in the secondary discharge arc or streamers. Then the system is in equillibrium and will stay like that for the remainder of the RF burst. Primary voltage, primary current, secondary voltage and secondary current envelopes will all remain constant until something disturbs the equillibrium. In your case the ending of the RF burst from the inverter seemed to be that something.

I don't see why the secondary resonant circuit should necessarily "suck" energy out of the primary. I perceive one of the flaws of the DRSSTC for pulsed operation to be the relatively large amount of circulating energy that gets trapped in the primary resonant circuit. Although this doesn't transfer into the secondary spark, it does get partially re-cycled into the DC bus capacitor at the end of each burst so it is not all wasted. I guess what I am saying here is: If it wasn't necessary to pump up both the primary and secondary resonant circuits in order to get breakout, the peak power requirement of the inverter would be greatly reduced. Most of the energy put into pumping up the primary circuit just transfers back to the DC bus cap at the end of the burst via naturally commutated rectification.

-Richie,

Marko

Wed Jan 28 2009, 12:37PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Tom540 wrote ...

Hello peeps,

So the other day I made a quickie CT to measure my coils primary while running. I could not find the normal CT I use so I made this new one. Nothing special. The CT consists of two cores one with 33 turns the other with 31 turns cascaded for a total of 1023 turns and the output has a 10.2ohms across it. Ive done this many times but this time with a larger resistor so I can get a bigger voltage.

In the past I've gotten the normal looking envelope that peaks after a few cycles and then tapers down slowly but this time I get this waveform.

Perhaps if you posted full coil specifications, actual power level of the coil at the measurement, and spark length, maybe someone could help you? This way I think not.

If your shunt is indeed hitting over 8V as on your pic that equals 800A of primary current. No wonder your IGBT's are blowing up I would say, unless they are some heavy bricks.

Depending on size of your ferrites, the larger resistor might be causing saturation of the CT's and distorting the waveform (although it seems unlikely for 0.3V/turn even for really small ferrites and low frequencies).

Marko

Dr Hankenstein

Wed Jan 28 2009, 02:29PM

Registered Member #1642 Joined: Sun Aug 17 2008, 11:36PM
Location: Black Canyon City
Posts: 96

I tried 10 ohm burden resistor on my series-connected (33/33-1) CT and got pretty bad distortion of the wave form. CT's really like a much lower burden resistance. If you could keep it below 3 ohms the signal distortion due to saturation would be reduced dramatically.

Best regards,
Hank

Tom540

Wed Jan 28 2009, 05:54PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

Dr Hankenstein wrote ...

I tried 10 ohm burden resistor on my series-connected (33/33-1) CT and got pretty bad distortion of the wave form. CT's really like a much lower burden resistance. If you could keep it below 3 ohms the signal distortion due to saturation would be reduced dramatically.

Best regards,
Hank

Marko my burden resister is 10.2 ohms I seriously doubt my HGTG20N60's could push 800 amps haha. I'll post some morbid pics of the dead coil tonight.

Hank,

I'll try that. That makes perfect sense now that I think about it. Not enough current flowing out of the CT. I used the 10 ohms because my primary current was actually very low, under 200 I didn't actually realize it would be a problem. I though it would be subject to a lot of noise being such a low voltage and would be harder to measure. I'll give that a try as soon as I get more silicon.

Thanks guys,

Tom

Steve Ward

Thu Jan 29 2009, 12:03AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I doubt its a problem of burden resistance, ive made all sorts of CTs with resistance of less than an ohm, to 10 ohms with no real difference in performance. You must be careful to avoid additional "loops" in your wiring though, as this would impact the signal from the CT output.

If you arent generating long sparks, then this steady-state kind of primary current is typical. I only see the primary current drop down significantly when making long sparks. Also, the multiple notches at the start suggest your primary is tuned too high for best performance, and you may be running too many RF cycles.

So in short id tune it up and then turn it up

Tom540

Thu Jan 29 2009, 06:56AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

Steve Ward wrote ...

I doubt its a problem of burden resistance, ive made all sorts of CTs with resistance of less than an ohm, to 10 ohms with no real difference in performance. You must be careful to avoid additional "loops" in your wiring though, as this would impact the signal from the CT output.

If you arent generating long sparks, then this steady-state kind of primary current is typical. I only see the primary current drop down significantly when making long sparks. Also, the multiple notches at the start suggest your primary is tuned too high for best performance, and you may be running too many RF cycles.

So in short id tune it up and then turn it up

Steve, good call, I am a dipstick, In all my frantic tweaking of the coil. I changed my primary and didn't measure it. The primary originally was running at 185KHz and secondary was at 190. The new primary (copper tube) with a bigger cap only ran at 200KHz which is way above the secodnary Fo. I was slowly pulling windings off my secondary to try and tune it since I was maxed out on the primary. The secondary looked kinda dumb anyway being a little too long. Well anyway I never finished the tuning because my IGBT's blew, again and im all out now. Here's a video I made of it during some of this process.

-Tom

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