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Back again with some questions...(long - yep, you know it)

Induction heater project. It's looking like I need maybe 4 x 50A 600V highspeed IGBTs, or a 100A single phase module. If someone has some low-cost suggestions, I'd be glad to hear them. I still haven't produced the formulas for peak current in terms of supply voltage, matching inductance and output power. It doesn't help that the last time I tried reading current waveforms, I got some nasty 27MHz parasitic shit that will absolutely not go away, despite a ceramic disc at the probe!!

I also need some chunky generic bits and pieces, like a switch and contactor. I have on hand a three-pole 30A switch, which is probably insufficient (I want to make full use of the 240V 50A circuit for my welder, if possible), and a contactor which is confusingly rated as:

-=-=- B25 (type I guess; BBC Petercem manufacturer) I(sub)th = I(sub)e AC1=45A, J(sub)i = 660xx (something scratched out) IEC 158-1 VDE 0660 ...(other ratings) U(sub)e | 220 | 380 | 500 | 660V~

--------+-----+-----+-----+------- AC3 | 6,5 | 11 | 11 | 11kW

-- -- -- -- -- -- -- -- -- -- -- -- (symbols, possibly international AC1: UL/CSA orginizations) 25A

-------------------------------------- (backwards-"R"U) 25A, 600V.a.c. ( symbol ) conductors: 60/75°C U | 120 | 240 | 480 | 600 | Va.c.

--+-----+-----+-----+-----+--------------- P | 3 | 7,5 | 10 | 10 |hp/3ph/3 poles

aux. cont. (if provided) 10A, 600 Va.c. A600

-=-=- (Hm, that was longer than I wanted to type..) So it's 45A, but 25A, *but*

10A? WTF?

If 25A in reality, can I effectively parallel two of the four poles for 50A switching capacity?

-- I'll add a timer circuit to sequence main power and control circuit power so the full 50A load (plus power supply capacitors!) is never switched on by any contacts (though it can be turned off at the main power switch).

Speaking of power supply, to get anywhere near 50A RMS, I need a good power factor. Instead of the several kilojoules of capacitance required for a smooth DC supply, I was thinking of using just 1000uF or so, which should give around 50% current duty cycle (for a power factor of say 0.75 or

0.707?). I could also use a PFC input, but that would be a whole other 10kW inverter in the system and would pretty much double my cost.

Back to the AC line, any thoughts on a breaker? I would like to have a 50A resettable double pole breaker, fast blow so it goes instead of the breaker back at the panel. I don't want to hassle with one-shot fuses, since with my odds they'll be going too often for that. ;-)

What else... about the circuit, I've settled on the same SG3524 PWM circuit, I'll use seperate drive transformers for each IGBT [pair] so I can generate a gate waveform of positive and negative voltage (unlike the current up-zero-down-zero cycle that works well with MOS), adequate to sink the current to get the devices turning on and off promptly. Since duty cycle will be near 50%, I can go with a straight square wave and get the requisite

+/-15V drive waveform.

About IGBTs and speed: devices with under 0.5uS total turn-off time are often spec'd for use well under 20kHz!? My MOSFETs commutate in 1-2uS in the circuit as-is, plenty fast for me, and they go all the way up to 100kHz! What am I missing here? Not to mention the only difference between related classes of e.g. IRG4PC5 - it's like, they're all almost the same speed, yet some are labeled "WARP", "High speed" and the rest regular. The data sheet shows different Ic(max) vs. F curves, but the parameters in the data tables are essentially identical!

About the circuit: for control, I'll drum up frequency, phase, voltage and current limiting feedback, so that if any of phase, voltage or current rise above a set, variable value, frequency is increased [further] above resonance so that all parameters decrease. Of course for frequency, that's just an open loop control and would set minimum frequency. Reason being, fixed frequency control is essentially useless, especially as resonance changes with loading and material. Constant tank voltage is useful for effectively constant temperature, and constant phase for constant power output (maximum power output being limited by supply voltage and matching inductor). I'm not sure what use limiting DC current will have (besides protecting the transistors), we shall see.

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams
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*Stands, tapping foot*

Too long again, or what?

Tim

-- Deep Fryer: a very philosophical monk. Website:

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welder,

50A

power

by

power

10kW
50A

breaker

circuit,

generate

requisite

100kHz!

related

yet

sheet

tables

rise

that's

Reply to
Tim Williams

Not yet, just been browsing the IR and whatnot from the usuals (Digikey, Mouser).

50A

And breaking at the same exact time isn't a problem or anything?

Okay, so a snubber across it (if I were switching inductive) would help a lot, besides the turn-on spike, no?

But since I'm not, then I can switch the full 45A, which is close enough to my 50A requirement to be okay? (Come to think of it, an electric current limit to 40-45A would be a good idea to keep the breaker from tripping. Heck, I could put an anti-SCR (..since I don't have a GTO on hand..) on the contactor so it latches when current exceeds 40 or 50A for a few cycles. That would be nice, though perhaps less fool-proof than a mechanical device.)

Which is why I'd rather not go with smoothed DC, cap input filter...

Interesting, seems rather cheezy but if it's good enough for a doctorate I suppose that'd work. Not replacable though, has to be opened.

By the way, how fast are fuses of the outdated "fuse panel" type? Cylinder and plug types are still quite available.

Hmm. I just realized that a). I have absolutely no idea where I would find such information, and b). I can't find anything with a few keywords online.

ZCS, ZVS at turn-on. Voltage and current go wild on switching (flyback towards the opposing diode). You know, half bridge switching mostly inductance.

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams

have you looked at any of the Semikron stuff?

yep

contactors are usually rated for opening inductive loads at rated current, umpteen times - its the splat that eventually snots them. If you sequence it & open/close at no current, they'll pretty much last forever.

its easier to get 50Arms with a ratshit power factor and humongous crest factor

try this:

5 litre plastic bucket. 2 ceramic standoffs, about 5" long protruding down into bucket, filled with de-ionised water. Use 5A fuse wire. The water keeps it cool, but at a critical current the water vapourises, the thermal impedance skyrockets and the fuse blows. IIRC 5A wire blows at about 300A or so - a buddy of mide used this trick for his PhD, as he had no budget for semiconductor fuses.

good call. -ve gate bias helps deal with Cmiller

AIUI you have either ZVS or ZCS (cant recall which, ZVS works best for FETs, ZCS for IGBTs) in which case the datasheet is less than helpful. suck it and see.

peak current limit & trip is always a good idea.

Cheers Terry

Reply to
Terry Given

Fair enough...

Oh, so it's the amps (which would need a series inductor instead of a parallel cap) that kills it? Gotcha.

Yeah, plus the growl will let you know it's in progress. Likewise for the control loop I'll give that a 100ms or so time constant, except current protection, which is going to want peak protection in the 1ms area I'm guessing? (Probably rhetorical Q, I can plug a datasheet for that...)

True... ;)

Good point.

I am open to any $50 donations you may have on hand. ;-)

Alrighty...

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams

I've seen plenty of contactors wired up this way.

in theory yes. In practice, not really. think current density, rough surface.

that measn of course you get 100Hz (120Hz) modulation on your load, but your load is an integrator so who cares.

but just the bees knees to get the system up and running, during which time you will blow a *lot* of fuses :) When its done, use HRC fuses.

slow, very slow or astonishingly slow.

even semiconductor fuses are slow. They are really there to prevent catastrophic damage, never to protect semiconductors (I^2t invariably ensures they cannot).

Ferraz-Shawmut (or whomever they morphed into) would be a good start.

so give it a whiz, and see which IGBTs run cooler. 2 basic types - PT and NPT. One has low Vcesat, high switching losses, the other vice-versa. Fred C. Lee et al at VPEC have written a whole bunch of papers on soft-switching IGBTs, they are on-line if you look hard enough. Sorry, I'm too lazy oopps I mean busy to dig up a link for you :)

Cheers Terry

Reply to
Terry Given

Oh yeah, forgot that! Lemme go toss in another quarter LM339 on the drawing...

A copper strip bolted to the C-E terminals is fine with me... as long as it's applied only when that transistor is "ON" and the bolts are removed within the ten microseconds before the other transistor turns on.

Since I'm going to use this variable up to a maximum (of dubious use, we'll see), I can just wire this to increase frequency - assuming something isn't wildly wrong with the output section (like I said, it's seperated from the outside world with a honkin' choke).

Yeah, like I said - SCR on the shutdown pin on the IC, or if not that then shut down the power control circuit (which controls the relays - a bit slow so a chipside electronic solution will still be required).

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams

10us for a desat. Do you have desat detection? if not, stick it in. A good gatedrive will allow you to bolt a flat copper strip directly across the C-E terminals, and *not* shit itself.

I've always used 3 protection circuits:

current limit - turn IGBT off for some minimum time: 50-100us or so, usually done digitally to ensure a minimum off time. that way, in hard current limit the switching frequency doesnt get out of control.

current trip - shut down, user-restart required

desat trip - ditto.

actually, whopping great rectifiers often have enough IIt that you can protect them if you are *very* careful

My fiance beat you to it :)

Cheers Terry

Reply to
Terry Given

sounds good. The desat trip will catch any serious faults - shorted choke, shorted IGBT

nice.

Reply to
Terry Given

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