DIY VFD Update

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Just in case anyone is intrested, here are some pix of my 100 amp DIY VFD being assembled. As soon as time permits i will post a video of the VFD in opperation. I still have to make a remote control pannel.
http://members.cox.net/azotic/inv1.jpg
http://members.cox.net/azotic/inv2.jpg
http://members.cox.net/azotic/inv3.jpg
http://members.cox.net/azotic/inv4.jpg
http://members.cox.net/azotic/inv5.jpg
http://members.cox.net/azotic/inv6.jpg
http://members.cox.net/azotic/inv7.jpg
Best Regards Tom.
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On Fri, 15 Aug 2008 03:19:17 -0700, with neither quill nor qualm,

Cool! It looks like the project is really coming together, Tom.
-- Challenges are gifts that force us to search for a new center of gravity. Don't fight them. Just find a different way to stand. -- Oprah Winfrey
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The big power modules mounted on the heatsink - what are they and what did they cost?
Thanks,
Joe Gwinn
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quickly quoth:

They are dual IGBT transistors also known as a GTR module. Basicly they are 2 NPN transistors conected in series to form a half bridge which means the are capable of switching both + and - DC power back and forth. You get AC output in the form of a square wave as a result. You can achive the same result by using 2 individual IGBTs.
The GTRs i am using have the capacity to switch 100 amps at 1200 volts, they have built in free wheeling diodes which cuts down on the component count. I bought these GTRs on ebay for $8.00 each, shipping was $7.55.
Best Regards Tom.
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So each module is a a half-bridge containing two IGBTs plus some frewheeling diodes. What make and model?
The reason I ask is I'm designing a homebrew impulse magnetizer, to be built around three 2900 microfarad 350 volt computer electrolytics I got surplus somewhere (no longer recall when or where). In this case, the current is all surge and no continuous, and the modules I'm considering are Diode-SCR half-bridges. (IGBTs don't seem suitable.) The circuit protects the electrolytics from reverse voltage by returning the inductive kick to the capacitor with the correct polarity to charge it.
The surge current will be something like 3000 amps for 10 milliseconds. I'm still working out the details, such as how big a surge current is needed. Even if the SCRs and diodes are happy, the capacitors have a limit as well.
I got the circuit idea from an expired US patent, 4,258,405 to Steingroever. Steingroever uses Mercury-pool ignitrons, as nothing else will withstand the currents necessary to use pulsed magnetic fields to form metal. <http://en.wikipedia.org/wiki/Ignitron
For more information on metalforming, see US patents 5,586,460; 5,684,341; 5,813,264; and 5,953,805. Also see the website of Dr Steingroever's company: <http://www.magnet-physik.de/ .
They use very high voltages and small inductances, to yield very short pulses, the better to create the eddy currents against which they push to apply force to the metal being formed.
For our purposes, and with some care, the ignitrons may be replaced with big SCRs. But even the biggest of SCRs is a wimp compared to a big ignitron.
Richardson Electronics still offers ignitrons and hydrogen thyatrons:
<http://industrial.rell.com/et_ignitrons.asp
<http://industrial.rell.com/et_Hthyratrons.asp
But they tend to be expensive.
Joe Gwinn
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I am using Toshiba MG100Q2Y51A GTR modules.
Intresting project your working on, there are IGBTs that might work for your application but there going to be pricey. I did see some 600 amp IGBTs on ebay this week that were reasonably priced. If your going to buy some on ebay make shure you ask the seller if you can return them should they be blown. Some sellers are selling units they claim are tested but they are not. I had to return some that had blown gates and would not fully turn off.
Best Regards Tom.
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I'll look these up.

My original instinct was that SCRs were better suited than IGBTs for this application, but I have not really dug into the issue. I'll get the datasheet for the Toshiba IGBT and think about it.

Good point. The price differential probably makes it worthwhile to eat the expense of a few bad devices, if it comes to that. But I would buy more units than strictly needed. And to cover lab accidents.
Joe Gwinn
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azotic wrote:

You have to keep them fully saturated, or the current hogs to the hottest part of the die, in a positive-feedback death spiral. The design of the FET to BJT drive scheme built into the transistor only allows so much base drive. SCRs are better at this, as increasing current increases drive to the internal transistor-like structures it is built from. You could also use a bank of parallel MOSFETs, although the 350 V supply requires using more of them. MOSFETs are so cheap, you might be able to buy a couple dozen new for what a used monster SCR or BJT would run you surplus. MOSFETs are quite tolerant of pulse overloads, too.
Jon
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Don't you want a 3 phase sine wave output? SE
azotic wrote:

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A sine is not nessesary to run a 3 phase motor.
Best Regards Tom.
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azotic wrote:

Does that mean that you just use a variable freq square wave, or is it like the commercial VFDs & use PWM on a carrier?
Bob
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Nota for President

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Any progress on drawing up a circuit diagram?
RWL
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quickly quoth:

Thanks, i plan on adding a jog and brake feature to the design which i think will be handy to have.
Best Regards Tom.
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http://members.cox.net/azotic/inv1.jpghttp://members.cox.net/azotic/inv2.jpghttp://members.cox.net/azotic/inv3.jpghttp://members.cox.net/azotic/inv4.jpghttp://members.cox.net/azotic/inv5.jpghttp://members.cox.net/azotic/inv6.jpghttp://members.cox.net/azotic/inv7.jpg
Just wondering, does the little bridge rectifier on the lower right of picture inv6 provide the power to the much larger switching elements on the heat sink. Is the large white cylinder in the last couple of pics the rectified DC filter cap? What do you calculate the ripple current in the DC filter cap to be?
Carl
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Are you planning on making and selling the PC board? Components?
Wolfgang
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The thought had crossed my mind. If i do decide to sell circuit boards for this project there will be a parts placement printed on patern on the top side. I am thinking that building a board will work like painting by the numbers, you would place the parts U-1, D-3 etc. into the board with the locations marked U-1, D3, etc. then just solder them in place.
Best Regards Tom.
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On Fri, 15 Aug 2008 19:07:53 -0700, with neither quill nor qualm,

Au contraire, mon ami! I strongly suggest using sockets as you did.
-- Challenges are gifts that force us to search for a new center of gravity. Don't fight them. Just find a different way to stand. -- Oprah Winfrey
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wrote in message

Oops, all the ICs will be socketed.
Best Regards Tom.
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wrote:

The output of the bridge rectifier is connected directley to the large white filter cap ( 2700uF @ 420Vdc ). I didnt do any ripple calculations for the HV dc buss since motors and transformers dont seem to have problem with ripple. However there are seperate regulated power supplies for the logic and gate drivers where ripple would be a huge problem. The large white filter capacitor at the end of the buss just happened to have the same center to center hole spacing as the GTRs which makes it possible to mount it right on the buss bars and eliminate the additional wiring that would be needed if i had to mount it elsewhere.
Best Regards Tom.
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Filter Caps do have a problem with ripple currents. You should also consider stored energy in the motor magnetics (1/2 * I * I * L) versus the capacitor (1/2 * V * V *C). Inductive kickback into the supply caps in a PWM system can cause some really spectacular capacitor failures. I highly recommend you have that cap in a stout box before you test at more than an amp or 2. If the cap blows you will be releasing 500+ joules explosively.
I think your rectifier bridge looks too small for the rest of the circuit.
Best of luck Carl
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