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Effects of water networks

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Ben Solon

Sun Jun 24 2012, 01:15AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Yea, even 100k/cm would be acceptable. Their will be at least 100cm between each heatsink. And yes, it will be closed loop. I will try to keep it as airtight as possible, but it's not a priority. Neither is distilled water. Do you know of anything that can be run through piping to put a coating on it? Just about every section of tube will be non-metal except for the water blocks and coil, so the dc component will only apply to the potential between blocks which will all be grounded, the tank coil, and the unexpected random component I haven't thought of yet.

But as a side note, I will be using a toshiba brick as mentioned in another thread. What about ways to directly cool the brick? I saw that post a long time ago about diable tonorre in which Steve gave us insight as to how they cool windmill igbts. I can Cnc a water block like so, but how to hold the water inside? Do I just find a nice sheet of rubber and cut out the shape of the water channels?

radiotech

Sun Jun 24 2012, 05:07AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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If the coil is heating the water, the water is lowering the Q, just like the workpiece is.

Steve Conner

Sun Jun 24 2012, 09:10AM

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

No, it's the I2R losses in the coil that both reduce the Q and heat the water. The water just stops the coil from melting.

You can even argue that water cooling increases the Q, because the resistance of copper increases with temperature.

Ash Small

Sun Jun 24 2012, 09:46AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Other options are oil cooling, or a refrigerated system using 'fluoro-inerts', I'm only mentioning these as 'possible' alternatives, every one else seems to be ok just using water:

Steve Conner

Sun Jun 24 2012, 10:03AM

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

I've seen an extreme DRSSTC that used an oil-cooled primary and water-cooled IGBTs. The oil circuit dumped heat to the water circuit through an oil-to-water heat exchanger. The water circuit consisted of garden hoses running from the public water supply to a drain.

If you use an isolating transformer between your inverter and the work circuit (a good idea for safety in any case) the work coil can be grounded. The baseplates of IGBT bricks are isolated and can also be grounded, so there will be no problem using a single water circuit for everything.

Without the isolating transformer, the work coil will have (I think) an average DC component of half the DC bus voltage relative to ground.

Ben Solon

Sun Jun 24 2012, 01:42PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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ah, ok. i dont like impedence matching inductors anyways. isolation is key. what if i where to leave it floating though? there would be no coupling to any other part or ground, other than capacitive effects of the hf. at least thats my theory. prove me wromg please XD

but ash: i would use oil or another other notnconductive fluid, but this is a big system, and i want to melt steel. for the long runs and/or max power runs i want to be able to hook a garden hosse up to it and run open loop with fresh cool water always being "circulated"

Ash Small

Sun Jun 24 2012, 03:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ben123324 wrote ...

but ash: i would use oil or another other notnconductive fluid, but this is a big system, and i want to melt steel. for the long runs and/or max power runs i want to be able to hook a garden hosse up to it and run open loop with fresh cool water always being "circulated"

I agree. I was just pointing out some alternatives to using water, Fluoroinerts would require a suitable refrigeration system as well. Oil would need to be a 'closed loop' system and isn't really justified for this application, as others have achieved perfectly adequate results using water, even for IH's used commercially by blacksmiths, etc.

It's just that this thread is titled 'Effects of Water Networks', I was just adding some (relevant?) additional information.

Ben Solon

Sun Jun 24 2012, 10:32PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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It certainly adds info. And if I do anything a bit smaller like a closed loop drsstc of a high enough power, now I will know where to turn!

But as for my previous question, does anyone know of some sort of sealant I can flow through the pipes to reduce corrosion? It would have to be somewhat plexible so it doesn't crack off due to thermal expansion. Maybe some sort of liquid rubber?

And completely off topic- Steve: you mentioned to me a while ago using pulse skipping to adjust power. That's what I am choosing to use at this point. Have you done this before? My main question right now is whether to use a micro or just logic. A micro would provide high resolution solution, but a high speed divide by ten ic would allow me to adjust power easily by selecting power by increments of ten percent through selecting how many outputs of the ic pass.

Steve Conner

Mon Jun 25 2012, 10:06AM

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

Yes, all of my DRSSTCs and my induction heater used pulse skipping for current limiting. The system I use is very simple, if the inverter output current goes above the limit, then the gate drive is turned off until it drops back below the limit. A flip-flop is used to quantise the enabling and disabling of the gate drive to the nearest complete cycle. No half-cycles are allowed as that would cause DC offset problems.

The peak value of the output current is used, sensed on a cycle-by-cycle basis.

I use a PLL in my driver, and while the gate drive is disabled the phase detector is also disabled, so the PLL holds on the frequency it was at. (Actually it slowly slumps back to the starting frequency, but that isn't by design, it's just a weakness of the circuit.) When the gate drive comes back on, it resumes tracking and picks up where it left off. I'm not sure how it would all work out with a non-PLL feedback scheme.

Pulse skipping makes the antiparallel diodes work hard. Every time a cycle is skipped, they have to pass the full load current. So it's probably best suited to IGBTs as they have good diodes built in. MOSFETs might well be happier with some other control scheme.

Ben Solon

Mon Jun 25 2012, 11:55AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Ok, sounds good. I thought you meant just counting off pulses and skipping one. This makes it much easier though, all you do is set a power level and let current ring up untill it trips at a preset level. Thanks.

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