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 s*it 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
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