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Finn Hammer

Sun May 11 2008, 03:09PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,

We had a bit of progress this wekend, working on the rectifier and the transformer/rectifier oil tank.
The rectifier:

Is 4 strings of 16pcs MUR8100C Which are ultra fast recovery 12A 1000V diodes. And the cheapest diodes we could find in this range.

With a 2.5V forward drop, and an estimated 2A avg. we are looking at up to 80W loss per string, or 320W total rectifier loss.

That`s why we fitted generous heat sinks:

We are already considering to fill the tank with cooking oil, to cater for our needs for fried chips.

To keep things under thermal controll, and also for insulation purposes, we are building an alluminium tank, with heat finns,

Not welded yet, it will also hold the high voltage transformer.

The future progress will be featured in our next installment.

Cheers, Finn Hammer and Daniel Uhrenholt

Marko

Sun May 11 2008, 03:37PM

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

Cheers guys, that tank is looking amazingly bastard!

Make sure to put some cool looking bushings and a conservator tank atop of it

Were you thinking about putting the tank capacitor and inductor under oil too? How hot do they get?

Regarding rectifier dissipation:
Have you actually tried putting 2 amps through those diodes and measuring the forward voltage drop?
The 2.5 volt figure you stated seems to be specified for full 12A, and temp of 25 deg. C, at just 2 amps I'd expect it to be much lower, maybe 2x or 3x lower. Temperature rise will reduce the voltage drop too.

Some people are afraid of possible voltage disbalance caused by unequal junction capacitances, and use resistors in parallel with each seriesed diode. I'm not sure how big problem that actually is... but I'd hate to blow such a beautifully (and painfully) constructed diode string.

Marko

Dr. Dark Current

Sun May 11 2008, 03:38PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Finn, that looks awesome! 0_o

Finn Hammer

Sun May 11 2008, 04:44PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Marko wrote ...

Were you thinking about putting the tank capacitor and inductor under oil too? How hot do they get?

No, the cap is staying in the cabinet for now. We have not seen any heating yet, but have not gone to high amperage yet either. Perhaps a CELEM cap will be needed, we`l have to see about that.

Marko wrote ...

Regarding rectifier dissipation:
Have you actually tried putting 2 amps through those diodes and measuring the forward voltage drop?
The 2.5 volt figure you stated seems to be specified for full 12A, and temp of 25 deg. C, at just 2 amps I'd expect it to be much lower, maybe 2x or 3x lower. Temperature rise will reduce the voltage drop too.

You are right. I didn`t manage to find the 2.5V figure this time.
Curves suggest around 1.3 to 1.5 volts drop, which pleases us immensely.

Marko wrote ...

Some people are afraid of possible voltage disbalance caused by unequal junction capacitances, and use resistors in parallel with each seriesed diode. I'm not sure how big problem that actually is... but I'd hate to blow such a beautifully (and painfully) constructed diode string.

We have been considering this, and there is room for surface mount resistors, so we will perhaps do that.

Cheers, Finn

Steve Conner

Mon May 12 2008, 09:21AM

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

If the diodes are avalanche rated, there may be no need for balancing resistors. I'd be more worried about the small clearance between all those sharp-cornered heatsinks, even under oil.

GeordieBoy

Mon May 12 2008, 10:30AM

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

If all diodes are from the same manufacturer, (and ideally from the same LOT code) there should not be a problem. The SLR inverter provides a sinusoidal current to the rectifier bridge so it is less harsh on the secondary-side rectifiers than a hard-switched inverter.

One observation: If you turned the rectifier board through 90 degrees it would allow the cooling liquid to flow more easily between the heatsinks as it rises through convection. Heatsinks in a horizontal orientation across the flow of air (or cooling fluid) don't perform quite as well. Just an observation - surely not a problem.

Oh, and that is one awesome looking cooling tank!

-Richie,

Finn Hammer

Sun Jun 08 2008, 01:07PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,
Daniel welded the tank, and brought a nice thick slab of white POM for the lid.

After the 28 holes in the perimeter were drilled, and the corners rounded, the case now looks like this:

The rectifier and the transformer were positioned on the lid, which looks like this:

After a lot of headscratching, and catalog browsing, we decided to use a 37 pin circular connector between the primary coil and the switching IGBT`s.

Each pin is good for 13 ARMS so with 15 pins in use each way, there is a little headroom. I have used an identical connector for a disk shooter pulsing in the region of 20kA, and it still can come apart, so this ought to work.

To provide the holes for the connectors, the nice little Wohlhaupter boring head has just done it`s thing again.....

We are waiting for the parts now.....

Cheers, Finn Hammer & Daniel Uhrenholt

Dago

Sun Jun 08 2008, 01:24PM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

Looks AWESOME!

Marko

Sun Jun 08 2008, 03:49PM

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

Yikes Finn, why so many holes around the rim?

The upside-down design of everything with connectors on the ''floor'' is really clever.
The transformer looks really sexy in that second pic, but I keep wondering if it's secondary winding is a bit overkill?

I thought you were going to use some smaller ceramic bushings for the low voltage in, but the connector is a rather neat solution too.

What kind of bushing are you going to use for HV? And are you going to have some sort of expansion tank for the oil?

Marko

Finn Hammer

Sun Jun 08 2008, 06:22PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Dr. Arc flash wrote ...

Yikes Finn, why so many holes around the rim?

That`s to increase the chances of keeping the oil on the inside of the tank, by providing an even pressue on the seal. Of course, oil is likely to wick up trough the multipole connector.....

Dr. Arc flash wrote ...

The upside-down design of everything with connectors on the ''floor'' is really clever.
The transformer looks really sexy in that second pic, but I keep wondering if it's secondary winding is a bit overkill?

Actually, the "floor" is the lid, so the transformer and rectifier is suspended from the lid rather than standing on the floor.
Overkill? maby. I hope so, because that`s the name of the game

Dr. Arc flash wrote ...

What kind of bushing are you going to use for HV? And are you going to have some sort of expansion tank for the oil?

We are going to use standard N Clamp Plugs for RG213. The RG213 will probably be stripped of it`s outher shell, as well as the braid.

Cheers, Finn Hammer

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