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induction heater project - feedback on inverter.

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Dinges

Tue Jan 12 2010, 06:32PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

As I was at my favourite hardware store the other day (which also happens to have a welding machine repair dept. in-house) I tried my luck.... explained what I did (hobby electronics, needing parts (in my case, large ferrite cores) for a DIY induction heater) and that these were usually inside welding inverters. Asked if they were throwing an unrepairable machine away that I might gut for parts.

5 minutes later, Peter walked out a happy man.... with 4 large welding inverters under his arm.

(also note the RF HV arcstarter in the middle/front left)

(another arcstarter in there too)

And I'm particularly happy with this cute little Kemppi MasterTIG 1500 inverter (230Vac, not three-phase 400Vac like the other machines):

1263320981 2511 FT81707 Kemppi Mastertig

1263321054 2511 FT81707 Kemppi Mastertig 2

Had a bit of a chat with the guy who runs the repair department, which was interesting. As was to be expected, they don't repair at component level but simply replace modules.

The loot: large 3-phase rectifiers (ISOTOP case), large cooling fins, big MOSFETs, big thyristors (SCRs), gate-drive transformers for same, a few huge caps, gas solenoids (another thing the doctor ordered for the microTIG), large rectifier diodes screwed into even larger heatsinks, etc.etc.

There are also two RF-HV generators in there, with Tungsten spark gap... basically meaning my original experiments (using a ferrite rod and DIY HV generator and make-shift spark gap) were needless, as I've now got the professional thing right here in my hands, including the large ferrite cores to couple it into.

I couldn't help but smile as I saw 1890s-technology (tungsten spark gap) in a machine from 1999, which otherwise consisted of fancy power semiconductors and a microprocessor to control it all....

Those inverters are veritable gold mines of parts.

Peter.

Steve Conner

Tue Jan 12 2010, 09:44PM

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

Indeed they are! Or "OMG inverter pr0n" as our younger members might say.

I'm also surprised to see Tesla's old spark gap in there. It should be possible to make a solid-state arcstarter using similar principles to the DRSSTC, OLTC, SISG or what have you, and I half expected that welder manufacturers would have done that. But maybe they decided it wasn't worth the extra cost and complexity.

GeordieBoy

Wed Jan 13 2010, 10:17AM

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

> I'm also surprised to see Tesla's old spark gap in there...

I too am very surprised! That old Tesla circuit with the multiple series spark gaps generates a horrendous amount of EMI, that can (and probably DOES) leave the welding box as radiated and conducted emissions. So from an approvals point of view I would have thought that any nice clean solid-state solution would make things a lot easier!

Are the inverters UL or CE marked?

-Richie,

Steve Conner

Wed Jan 13 2010, 11:01AM

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

GeordieBoy wrote ...

That old Tesla circuit with the multiple series spark gaps generates a horrendous amount of EMI, that can (and probably DOES) leave the welding box as radiated and conducted emissions.

Those "conducted emissions" are the very thing needed to start the welding arc

I imagine there must be an EMI exemption for welders. Also, the coupling core is hopefully a power ferrite grade that will be very lossy at VHF, so that should help.

Dinges

Wed Jan 13 2010, 11:34PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

The Kemppi uses a RF arcstarter circuit without sparkgap - haven't figured out exactly how it works. I'm trying to repair the Kemppi for personal use.

As to welders being exempt from EMI requirements.... could very well be. What I do know is that every manual of inverter welders that I'm familiar with clearly states to *not* use it in 'sensitive' environments, such as workshops with CNC machines or in hospitals, but to use 'lift arc' starting in those situations.

Normally, the RF arcstarting is only used for a short time, just before the 'start weld' button is pushed and the arc is struck. All machines I'm familiar with turn off the RF arc start if an arc hasn't been struck within 1-4 seconds, limiting on-time of the RF HV generator.

Of course, there are also welding machines where you have the option to have RF HV permanently on....

Peter.

Dinges

Thu Jan 14 2010, 12:04AM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

Work on the 1kW induction heater has been progressing steadily. Below an image of the inverter, with the driver board.

Close-up of the driver board:

The schematic of the inverter:

Schematic of the driver:

(final changes to the driver, not yet corrected in the schematic: the 100 ohm resistor has been exchanged for a 220 ohm one, the 1k resistors to the base of the BC547/557 have been increased to 6k8, and the 4.7 ohm gate resistors have been removed)

Did a first test, with a work coil and tank capacitor array I still had lying about, resonant at about 200 kHz. Used the function generator to provide a signal to the drive board.

Cranked power up (turning up the variac) till 130W. The workpiece, an M8 bolt, became quite hot (estimated 350 deg.C). Unfortunately, and as was to be expected, the powdered-iron toroid I was still using got quite hot too, so I didn't dare to go higher than 130W input power. Next task is making a proper tank circuit with decent coupling transformer.

The inverter current sampling circuit works fine, but had an insanely large time constant, so the 470k resistor was replaced by a 4k7 one, making it respond much less sluggishly to changes in inverter current.

Below are images of the scoped waveshape at the gate of the FETs. It's much slower than I'd hoped for, about 800ns rise time; I think this is excessive for a 100kHz induction heater?

Removing the 4.7 ohm gate resistors decreased the risetime a little, by about 100ns (it was originally 900ns). Removing the resistor in the primary of the gatedrive transformer didn't speed the circuit up, but greatly distorted the waveshape, so that resistor was left in place.

Inductance of the GDT is about 600uH, the leakage inductance 0.3uH.

Now putting the last hand on the PLL controller board, mostly based on the work by HernÃ¡n GonzÃ¡lez with some small modifications.

All comments, advice, corrections, etc. welcome.

More pictures and information here: http://picasaweb.google.com/motorconversion/Induction_heater#

Peter.

GeordieBoy

Thu Jan 14 2010, 11:10AM

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

The gate drive slopes do look a bit slow, and there is no hint of overshoot or ringing, so you should be able to speed them up considerably if you wanted to.

However, the inverter in a typical induction heater operates in a zero-voltage switching mode anyway. You may find that increasing the slew-rate of the gate-drive signals doesn't give much reduction in device temperatures. Even with slow gate-drive slopes the switching losses when operated in ZVS can be suprisingly low.

If you want to speed up the gate-drive slopes I would recommend looking at dedicated MOS gate-drive IC's like Microchip/Temic TC4421/TC4422 or the UCC things that everyone on here seems to use. I'd also loose the ferrite-beads on the gates unless they were put there to solve a particular EMI problem. (Often EMI requirements and efficiency requirements are totally contradictory. i.e. Slug the gate-drive and slow down switching edges for reduced EMI, _OR_ speed up gate-drive slope and switch in the minimum time for maximum efficiency. Somewhere in the middle there is usually an acceptable solution!)

-Richie,

Dinges

Wed Jan 20 2010, 11:52AM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

Just had a chance to have a quick look at that Kemppi welding machine to see what might be wrong with it. Had already taken out the control boards and tested all the electrolytics and power supply voltages - all were fine. Wound a single-turn test winding over the gate-drive transformer for the thyristor to connect a scope to; re-installed the board and turned the machine on.

1263988281 2511 FT81707 Kemppi Scoping Gate Drive Transformer

1263988233 2511 FT81707 Kemppi Drive Signal

Perfect pulses on the scope, so it appears the thyristor is getting a drive signal. That rules out just about all of the control electronics as possible fault factors (note pulse is maximum width, but that's to be expected as no output current flows, so no current feedback to close the loop and regulate pulse width back to the set value)

Then I tried to figure out where the various power components were. But before doing so, just to be sure, decided to short the outputs of the transistor with a large screwdriver (= shorting the electrolytic caps). Shorting the caps gave major sparks.... So looks like DC input is there as well.

That seems to strongly indicate a defect thyristor block. Trouble is, to get at it (or any other part), the machine needs to be taken nearly completely apart (the downside of a small, compact machine....). And very hard to test-run it when taken apart. But everything so far seems to strongly indicate a defect thyristor brick, which means it's probably a simple repair. Strange though, I didn't expect thyristors to fail open (i.e. infinite resistance). Not sure I have a suitable replacement - only spare thyristor brick I have is 55A/800V, and is physically smaller than the original one. The thyristor bricks in the other 3-phase machines were all much larger.

Funny though - it's basically an overgrown PC PSU, at 1500W. Half-bridge design, using two large thyristors (in one brick). 2 electrolytic capacitors, the usual series capacitors to the transformer, and 2 flywheel diodes (will have to check those too).

If the control board would've been dead and unrepairable, I think I'd have replaced it with a TL494 hack.

May take a while before I get some more time & chance to have a closer look at it.

Peter.

IamSmooth

Wed Jan 20 2010, 01:43PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Peter,

What are you using to control the power into the inverter? Is it a variac? If so, what are its specifications?

Dinges

Wed Jan 20 2010, 05:25PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

With the induction heater, I'm using a variac plus an insulation transformer. The insulation transformer is only good for 250W, the variac is a 220V input, 0-260V output (@3.15A), also undersized for this application.

Have just scrounged a case to build the induction heater into, an old Compaq mini-tower. After removing some internal metal parts, it's now an empty box. All that needs to be done is to make a new front cover out of aluminium.

My previous reply is not an induction heater though, but a welding machine I was given and that I'm trying to repair, instead of gutting it for parts. And a correction is in order - there are no thyristors in it, it's controlled by a 75A/600V/350W IGBT brick which is defect, unfortunately.

Peter.

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