DRSSTC tuning

[kubajed]

Hi,
my tesla coil has the following properties:

1. Secondary:
Resonant frequency: without torus: 80.8 kHz, with torus: 61.8 kHz
Wire: 0,6 mm
Resistance: 145 R
Height: 1170 mm
Diameter: 250 mm

2. Primary
Inductance: 25,3 uH
Cone: fi bottom: 310 mm, fi top: 520 mm, height: 150 mm
Pipe fi 15 mm
Turns: 8

3. Torus
R = 225 mm
r = 85 mm

4. MMC
564 nF, Vdc: 16.5 kV, irms: 156 A PMB2253470KSP (6 in parallel, 5 in series)

5. Full bridge
2x ff600r12kl4c
6800u filtering
5u snubbers
8x 100 V TVS

6. Driver
Modified Predicter, additional 80 VDC minibridge. GDTs, 1:200 CTs 0.5R shunt resistor





Current and voltage without secondary (360 VDC, 1200 Apk):






The problem occurs when I add the secondary:







There is a spike on the current waveform, and this weird voltage tail. Do you know what caused it ad if this is dangerous? I'm measuring the current using additional, separated CT, the oscilloscope has grounding disconnected. With 100 VDC on the bus I am able to get about 25 cm sparks (tuned the coil more or less). I'm grounding the secondary, heat sinks, GNDs of driver and minibridge to the same point. The socket's earthing is also connected to this point.

The phase leading works correctly, I'm able to set the ZCS.

Gate waveform:

[AstRii]

The spike on the current waveform is probably just noise, try scoping the primary current with common mode filter. The voltage tail is due to recuperation of energy back to DC capacitors through IGBT diodes, it's perfectly normal. I would say your waveforms are actually quite perfect.

Very nice clean build, but get rid of those TVS diodes on the bridge before they cause troubles

[kubajed]

Thank you for your reply.
Why can those diodes cause problems? I used them to suppress any voltage spikes.

I'm glad that these waveforms are ok. But I'm a bit concerned about the spark length. At 100 VAC it is about 20-30 cm to a grounded rod, no corona discharges (at least I haven't seen any during the day).

Is it the energy coming back from secondary causing the tail shape?

Also, do you think that connecting the coil's own grounding to socket's ground is a good idea?

[AstRii]

Why can those diodes cause problems? I used them to suppress any voltage spikes.

It's fine to use them to suppress voltage transients between G-E of your IGBTs because in case the voltage spike causes them to conduct, your GDT cannot provide much current to overheat them. But the current flowing in the bridge is in orders of 1000Amps! If the TVS starts to conduct, it will overheat quickly and fail short circuit which can and will destroy your IGBTs. Those transistors can take much more avalanche energy than the TVS diode. You would need many of them in parallel. (It also adds unnecessary capacitances and inductances to the bridge).

Is it the energy coming back from secondary causing the tail shape?

Not all the energy from the primary gets transferred to the secondary, the tail is caused by the leftover energy in the primary getting back to DC bus capacitors.

Also, do you think that connecting the coil's own grounding to socket's ground is a good idea?

Well it's better than nothing. I always ground the secondary to the mains socket but every time I run my coils, I connect some earth connection in parallel (Any nearby grounded object, usually central heating pipes or threaded rod stuck into ground)

[kubajed]

Than you for explanation. If that is the energy from the primary circuit, why the tail differs that much in shape with and without the secondary?

Do you have any idea what could be a cause of small arcing? I think my ground path is quite good, I put about 0.7 m of a rod into the soil nearby the coil.

[AstRii]

If that is the energy from the primary circuit, why the tail differs that much in shape with and without the secondary?

I'm not sure, but that might be because with secondary in place, some energy still gets transferred to the secondary, which might change the shape? Not sure about this one.

Do you have any idea what could be a cause of small arcing? I think my ground path is quite good, I put about 0.7 m of a rod into the soil nearby the coil.

That is probably the best way to ground the secondary coil. But again, I'm not sure what explains the small arcs.
Maybe you just run too short on-times (unlikely). Maybe your coil still needs some tuning (more likely). 

[kubajed]

The on-time was in the range of 100-200 us and about 30 BPS.

Ok, I managed to get 1 m long sparks at 100 VAC input, however they only occur when I put a grounded object as a target, no corona discharge. When I was trying to achieve longer sparks and moved the target a bit, some streamers hit the secondary (from toroid to secondary) slightly above the strike rail.

[AstRii]

Are you using some breakout rod from the toroid?

Also, if your secondary got hit, it's an indication that you need bigger toroid to shape the electric field better for protection.

[kubajed]

Yes, I’m using a sharp rod on the top of the toroid.

Even visually the toroid is a bit small, so it’s size actually could be a problem.

[AstRii]

Well I don't know what's wrong with the arc size but the toroid definitely needs an upgrade. The toroid must distribute the electric field well enough to protect from any primary/secondary strikes.
I recommend you to stop with your experiments until you upgrade the topload to prevent any secondary or primary strikes which can be potentially destructive.

My rule of thumb is that the topload major diameter should be just slightly smaller than the secondary height and it's minor diameter should be just slightly smaller than the width of your secondary.
In your case I suggest R = 1000mm and r = 200mm toroid.

[Duane B]

Looking at the gate waveform, is the gate voltage the yellow waveform and the primary current the blue waveform? If the primary current is the blue waveform then I see a beat pattern of approximately 13 kHz (the time between the two humps). This indicates to me that the primary and secondary may be off-tune by 13 kHz.

If corona sparks are not coming off the secondary then all the stored energy has to ring down through the secondary and the primary after gate drives stop. The primary and secondary are a coupled circuit and they will ring down their energy together. I would say that most of the stored energy will be in the secondary on the ring down.

[davekni]

Great details and scope plots to start this thread.  Thank you.
BTW, is that first primary-only scope plot really at 5V/div for current?  That would correspond to +-13V, or +-5200A.  Or, is your CT burden resistor 2 ohms for this scope trace (for 100A/V with 200:1 CT)?

The spike on the current waveform is probably just noise

Yes, probably a spike coupled from a secondary arc through parasitic capacitances and inductances.  Not necessary to fix.

    If that is the energy from the primary circuit, why the tail differs that much in shape with and without the secondary?

I'm not sure, but that might be because with secondary in place, some energy still gets transferred to the secondary, which might change the shape? Not sure about this one.

Yes, I think that is accurate.  With primary only, the high-frequency voltage ring-down tail is the LC resonance of primary inductance with IGBT capacitance.  Secondary ring-down adds to that high-frequency ring-down, with clipping due to IGBT diodes.

Concerning grounding, I use both line ground and large counterpoise (conductive sheet under entire DRSSTC assembly against concrete or whatever surface your DRSSTC is standing on).  Can be aluminum foil or screen or whatever.  A counterpoise can be more effective (lower impedance) than a rod at DRSSTC frequencies.

Well I don't know what's wrong with the arc size but the toroid definitely needs an upgrade. The toroid must distribute the electric field well enough to protect from any primary/secondary strikes.
I recommend you to stop with your experiments until you upgrade the topload to prevent any secondary or primary strikes which can be potentially destructive.

Yes, I agree here.

One final suggestion:  If at all possible, I'd test after dark.  Corona is almost impossible to see in daylight.  When dark, you can spot corona and the beginning of arcs to problematic places (such as secondary coil) before they cause too much damage.

[kubajed]

Well I don't know what's wrong with the arc size but the toroid definitely needs an upgrade. The toroid must distribute the electric field well enough to protect from any primary/secondary strikes.
I recommend you to stop with your experiments until you upgrade the topload to prevent any secondary or primary strikes which can be potentially destructive.

My rule of thumb is that the topload major diameter should be just slightly smaller than the secondary height and it's minor diameter should be just slightly smaller than the width of your secondary.
In your case I suggest R = 1000mm and r = 200mm toroid.

Ok, thank you, I was thinking about increasing the size of the toroid, but now I see it is inevitable. Did you mean diameters instead of radiuses, so that R = 500 mm and r = 100 mm?

Looking at the gate waveform, is the gate voltage the yellow waveform and the primary current the blue waveform?

Yes.

Great details and scope plots to start this thread.  Thank you.
BTW, is that first primary-only scope plot really at 5V/div for current?  That would correspond to +-13V, or +-5200A.  Or, is your CT burden resistor 2 ohms for this scope trace (for 100A/V with 200:1 CT)?

This CT has a ratio of 1:85 and the resistor was 1R.

Thank you for all your suggestions. What do you think about the primary though? Does its construction result in too high coupling? Should I modify it?

[davekni]

What do you think about the primary though? Does its construction result in too high coupling? Should I modify it?

Can't tell just by looking.  Enter your design information into JavaTC.  It will give great estimates of parameter values including coupling.  (Pay attention to diameter and radius entries.  JavaTC is a mix, so easy to make mistakes.)

[kubajed]

I used JavaTC to calculate the coupling factor, and with this primary it is 0.235. If I lower the primary by 10 cm it will be 0.145.

[davekni]

I used JavaTC to calculate the coupling factor, and with this primary it is 0.235. If I lower the primary by 10 cm it will be 0.145.

Yes, 0.145 is more typical for DRSSTC.  Usual issue with high coupling is "racing sparks", small arcs up the lower part of the secondary and sometimes to the primary too.  It is possible that you sere experiencing racing sparks.  I doubt they would be visible in daylight.  Even the normal intended arcs from the break-out point can be difficult to see in daylight.

[kubajed]

I think I'll go for this type of a topload. What do you think?

[Mads Barnkob]

I think I'll go for this type of a topload. What do you think?

It will work just as fine as rings or full blown toroid. BUT. There is one but You need to ensure that the distance between spiral turns is no bigger than what you see in the picture, else it will start having "sharp corners", where you will get random break outs from.

[kubajed]

Hi,
as mentioned previously, the secondary got hit, as shown below:



I repaired it by soldering two or three turns together, shorting the bottom and top part of the broken secondary, but creating one or two closed loops. Is it good enough, or the closed loops will overheat?

[davekni]

I repaired it by soldering two or three turns together, shorting the bottom and top part of the broken secondary, but creating one or two closed loops. Is it good enough, or the closed loops will overheat?

Yes, will overheat.