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Increasing QCW streamer length

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BSVi

Mon Jul 14 2014, 06:52AM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

Hi guys!

I have build QCW coil. After some tweaking, It outputs 1.5m streamer, but I can't get any longer than that. My intention is to get 2m.

Coil data:
Secondary: d=10cm, L =15cm, d(wire)=0.28mm.
Primary: d(mean) = 13cm 9turns
Toroid: d=26cm, d(tube)=8cm.

Power supply is 240Vac which gives 330Vdc on the bus. Bus capacitance is 12000uF
ÐœÐœÐ¡ - 12.5nF

Ramp length = 20ms

With no streamer system resonates at 430kHz (upper pole) and with full load F drops down to 392kHz.

OCD traped 150A limit, so I'v increased limit to 200A :) I know that power stage I'm using (bridge of 2x paralleled IRGP50B60) can wishtand 300A, so I'd like to pump more Amps and get more streamer, but how can I do that?

Upd:
Finally, I have increased supply voltage to 370v (with BIG&HEAVY iron transformer) and got 2m of streamer, but I'm dont really like idea of building stepup converter.

Maybe, you have an advice on how to increase streamer without increasing supply voltage?

Mads Barnkob

Mon Jul 14 2014, 07:25AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I have not yet built a QCW coil, but you are making sparks that are over 13 times your secondary length, I do not even think this ratio have been achieved by anyone else.

Do you use a secondary mmc as Steve Ward did?

BSVi

Mon Jul 14 2014, 07:58AM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

I'ts easy to acheive high leader:secondary ratio in QCW coils by shrinking secondary, but it's much harder to increase streamer.

As for secondary MMC - I opted to use relatively big toriod. My theory is that toroid stabilizes and even "pushes" streamer up by it's E-field, so the bigger toriod the better results are.

I think Steve had to use small toroid and secondary MMC in his famous gun due to aesthetic reasons.

Steve Conner

Mon Jul 14 2014, 10:00AM

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

Well done, this is very impressive! :D I think it must be a new record for streamer length vs. coil length. Are you using high level modulation with a buck converter, or phase shift modulation?

You found that increasing the DC bus voltage made a longer streamer. So, reducing the impedance of the primary (fewer turns, larger tank capacitor) should have the same effect.

However, this will mean higher primary currents than if you increased streamer length by boosting the DC bus voltage.

One possibility would be to salvage a PFC front end from some big SMPS and use it to boost your DC bus to 400V.

BSVi

Mon Jul 14 2014, 11:01AM

Registered Member #1637 Joined: Sat Aug 16 2008, 04:47AM
Location: Kiev, Ukraine
Posts: 83

Are you using high level modulation with a buck converter, or phase shift modulation?

I'm using phase shift modulation

However, this will mean higher primary currents than if you increased streamer length by boosting the DC bus voltage.

Can you explain why is it so? As I undestand, Ohms law applys here, I=V/Z. And it's indifferent wthat to change - increase V or decrease Z.

I actually tried to decrease turns count by one and compensate with added capacitance. Current increased but streamer didnt changed much. Maybe I have to try to unwind more turns.

Oh, and have to mention - this coil has very high coupling - 0.39. Changing it to 0.2 decreases perfomance by about a half. Another idea to try is to increase coupling even more.

ne possibility would be to salvage a PFC front end from some big SMPS and use it to boost your DC bus to 400V.

I can actually build one myself with IR1155 or something like that, but I want to pack this thing in a box and dissapating another 200w from PFC could be troublesome. So, if possible I'd like to go without additional converter :)

Uspring

Mon Jul 14 2014, 11:34AM

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

However, this will mean higher primary currents than if you increased streamer length by boosting the DC bus voltage.
Can you explain why is it so? As I undestand, Ohms law applys here, I=V/Z. And it's indifferent wthat to change - increase V or decrease Z.

I believe Steve meant, that in order to increase streamer length you need more input power, either by increasing bus voltage or primary current or both.

Steve Conner

Mon Jul 14 2014, 12:28PM

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

wrote ...

Can you explain why is it so? As I undestand, Ohms law applys here, I=V/Z. And it's indifferent wthat to change - increase V or decrease Z.

Energy is equal to V*I*t. If you have 300V*200A*20ms that is 1200J of energy. Increase the DC bus voltage to 370V, and the current (assuming a resistive load) will increase somewhat due to Ohm's law. You have a current of 200*(370/300) = 246.7A, and an energy of 370*246.7*20m = 1825J.

If we want to achieve the same energy by leaving the DC bus voltage at 300V and decreasing the load resistance, then we must have a current of 1825/(20m*300) = 304A.

A QCW with IGBTs has huge switching losses and these increase at higher voltages, so it's not immediately obvious which of the above options is best. Are you using any sort of Class-DE arrangement with deadtime and snubber capacitors?

wrote ...

I actually tried to decrease turns count by one and compensate with added capacitance. Current increased but streamer didnt changed much. Maybe I have to try to unwind more turns.

Oh, and have to mention - this coil has very high coupling - 0.39. Changing it to 0.2 decreases perfomance by about a half. Another idea to try is to increase coupling even more.

I'm guessing that by unwinding turns you are also decreasing the coupling. These have equal and opposite effects. You need to decrease the turn count without decreasing the coupling. This might involve widening the spacing of the remaining turns or using several wires in parallel.

Uspring

Mon Jul 14 2014, 04:07PM

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

I'm guessing that by unwinding turns you are also decreasing the coupling. These have equal and opposite effects.

If we're talking about the effect on primary current at constant Vbus, I'd expect both reducng coupling and reducing primary inductance to increase current. The former would increase Qpri and the latter would imply more current at constant Qpri.

I'd guess the reason for better performance at larger coupling to be the OCD to kick in later. Also the copper resistance of the primary might make the coil less efficient wrt to the power transferred to the secondary.

teravolt

Mon Jul 14 2014, 05:29PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you want to increase your bus voltage can you use a buck transformer on your 220. it may not be very big. you may need a dual primary so maybe 220 or 110 if you have neutral to 24 and 24 at 10 amp is only a 500va transformer

Thomas W

Mon Jul 14 2014, 08:18PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

This stuff sounds really interesting... does anyone have any links to QCW design theory and how it works & so on?

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