Measured INDUCTANCE of my welding reactor

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hence the "splat" test, which has the added advantage of measuring Isat, too :)

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No, the interphase transformer is not part of the inductor, although it and the main transformer have some leakage inductance. The interphase transformer is there to help balance the load between the two Y secondaries of the 6-phase transformer. The magnetic field in the interphase transformer will approximately cancel to 0 when current is exactly balanced between the 2 Ys, so if you measured the inductance from the center tap to the both sides, as the cutrrent flows in use, the inductance should be low. Inductance to one side would be much higher but not directly relevant, and it might saturate at not too much above the maximum design current imbalance, way below the 400 A the reactor was designed for. But you know all this by now because you found ABSE :-).

Measure only the reactor.

More later if I have time.

If your inductor has an iron core (likely), it will behave quite differently at 5KHz than at 60 Hz due to differences in eddy current and hysteresis losses. It will also have different values at higher levels of excitation.

A better way to measure it might be to use a small current sense resistor and note phase shift with excitation similar in magnitude and frequency where it would be used.

Correct

I was going to wire a space heater in series with the inductor, and measure the voltage across the inductor.

That's cute, a space heater, all warmed up and glowing. :>)

So let's see, an inductor's stored energy is E = 0.5 L I^2, which is = 37J for you at 200A. That's nothing to sneeze at! If a node capacitance is say 2nF (a MG200Q2YS40 has less than 1000pF of capacitance above 40V), the flyback spike voltage will be V = I sqrt (L/C) = 200 sqrt (1.85mH/2nF) = 192kV. Ouch!! Obviously this means the open-circuit flyback spike will be high enough to cause breakdown someplace, hopefully at the welding tip, etc., but if not, where will it be???

I assume it won't be at the IGBT -- it had better not be!! Just a quick analysis:

OK, let's say the IGBT breaks down at 1200V. This would mean we're expecting the IGBT to handle a peak power of 1.2kV times 200A = 240kW. Ahem... The inductor's current decay rate with a 1.2kV drop will be dI/dt = V/L = 0.65A/us, which means it'll take about 310us to ramp down to zero. So, we have a 240kW peak-power pulse lasting 300us. Looking at the MG200Q2YS40's Transient Thermal Resistance curve (page 4), we see the thermal resistance is 0.01 C/W at 1ms. We can extrapolate, using the square-root rule, and estimate the 30us region has a thermal resistance of about 0.0018 C/W. Doing the crude back-of-the-envelope calculation, this corresponds to a junction temperature rise of 444C for 240kW. A full calculation would show a higher value. We're in the order-of-magnitude "ballpark" of safe breakdown, but sadly, without some other protection to prevent breakdown, the IGBT will probably be toast. Perhaps a 5-10x larger part could safely handle such a large breakdown...

I've been trying to make sense out of this thread. There was one tiny mention of "saturable reactor". The only reason one would care, since most cored inductors are saturable to some extent or other, is if one intended to operate in saturation.

One way to describe saturation is that the inductance is VERY dependent on the current. A low current measurement may be interesting, but does not accurately describe the small signal operating condition for a higher fixed current. We haven't even started talking about the time variant (large signal) behavior. Or the effects of hysteresis.

So, even if you could determine the inductance to 15 decimal places, what could you do with the number?

In a welder, don't you care most about what happens at welding current? mike

Glen, I made another attempt to measure inductance. It now came to be

I wired a space heater in series with the inductor, and measured relevant values.

Another attempt at inductance

Vac 124.4 Voltage across inductor 8.69 Current 12.5 Formula: V=I*2*pi*f*L or L=V/(I*2*pi*f) Inductance= 0.0018450 Henry 1.8450 mH

(Sorry to jump in a thread but..) A good reason to use a power supply-voltage-limited topology like half or full bridge so the flyback is clamped by diodes. Hence what I'm using in my similarly high powered induction heater.

Tim

P.S. Heh, interesting coincidence that I'm replying here as I flip through The Art Of Electronics. Hey Win, was it intentional that the figure on page

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

Makes sense, assuming strict linearity of this inductor.

Winfield, is 37J enough to raise the temperature of the junction so much?

In any case... The power to be handled at full shutdown (not temporary shutdown lasting a fraction of a microsecond), is obviously large. In a welding situation, it amounts to turning the bridge off, hard, at full welding current. Imagine what would need to happen: I am welding at high current, and someone or something would switch off the bridge.

Could that happen? Sure. If the power supply of my circuit dies, or the power goes out, or my wife does something she'd never do (touch the welding machine), all that could happen this.

But this would not be an event that would happen repeatedly. On the repeated basis, we have fast switching events occurring very quickly happen several dozen or hundreds of times per second. But these do not last long.

Surely, I want to have a snubber circuit. I decided to have an RCD snubber circuit, based on Terry's suggestion. In addition, I would add a bunch of varistors to handle these unusual shutdowns.

OK, I will buy approx 2 mH as your inductor value. There will be some additional leakage inductance from the transformer contributing to stored energy, the functional equivalent of a (hopefully only slightly) larger inductor.

Now you have enough info to simulate your circuit with LTSpice, which I think should be your next step.

Not much time to think about this right now as I am headed out to a place with no electricity or phone service within miles in a few minutes, but I will be back on line Mon or Tues next week.

Glen

That's nice that these numbers make sense to you. I rather like this test better than the 5 kHz wavetek test.

Thanks Glen. With these numbers, the calculation for capacitance in the snubber circuit gives large, but not insane values.

Can I use an electrolytic cap in the snubber? Would it be safe to oversize the cap a little bit?

You've posted a lot of times with various details, and some of your plans have changed as a result of comments from people. Could you give an overall description of what hardware you have and how you plan to use it all? For example, I think you bought a welder at auction for $9.99. Will that be the DC source feeding your inverter? Will you leave the inductor you've been talking about lately between the DC welder and your inverter? Is the inverter going to be a full bridge? Will the inverter frequency be variable, and if so, what range of frequencies will it cover? Etc., etc.

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So if I understand correctly, in your added-on AC mode, it goes like this: DC comes from the welder's power supply, through the inductor, through the inverter, to the commutator, thence to the work piece. Is this right?

What is the unloaded DC voltage from the welder?

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Sorry, I think that at times I assume that people follow every post of mine, which, of course, is wrong and presumptuous.

Yes, I bought a tig welder for $9.99, here are the pictures, PDF manuals and the story:

It has a 100% duty cycle and I would like, eventually, to make it do a bit of automatic welding. It is a 3 phase welder that I am running from my homemade phase converter:

I want to build a full bridge inverter that would make square wave AC from DC.

I want to place it between the reactor/inductor and the commutator that switches welding terminals from DC electrode negative (DCEN) to DC electrode positive (DCEP).

The middle position of the commutator, which now simply disconnects terminals, would become a Squarewave AC position and would start the inverter. The inverter circuit would consist of the timing circuit, possibly an extra control for lift-arc, generator of two logically opposite ON and OFF signals for upper and lower gates, gate drivers, and four Toshiba 200A IGBTs. There will also be a snubber circuit to protect IGBTs in case of an instant turn off (which I hope should not happen).

The inverter will have two variable parameters, frequency (tentatively from 20-40 Hz to 300-400 Hz) and duty cycle (relative percentage of EP and EN in every period). Duty cycle would vary from, say, 10% to 90%.

In the next few days, I am going to start messing with IC chips on breadboard.

I have a few test equipment pieces, like two oscilloscopes, numerous voltmeters, signal generators etc.

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note also that there is a high voltage, high frequency arc starter that goes between the commutator and the tig terminal.

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not exactly, it would bypass the commutator and flow from one (source) pair of commutator contacts and to other (exit) pair of commutator contacts.

then through HF HV arc starter

then to the workpiece.

Basically, yes.

Loaded, it is more like 28V while welding and much, much less when electrode sticks to the work piece.

I welcome your comments...

Most if not all electrolytics have too much Equivalent Series Resistance (ESR) for use in snubbers. Usually low ESR film caps are used. Suggest you find out how much ESR your caps have, and simulate. If you can't find the ESR rating for your caps, forget about using them in a snubber. Low Equivalent Series Inductance (ESL) is also a very good thing for snubber caps. ESR is a problem because it increases the time it takes to get energy into the cap, and it dissipates heat inside the cap, possibly causing the electrolyte to boil.

Also, think about using a fairly hefty safety factor, remembering that you have not measured the transformer leakage inductance, which will also contribute to the energy the snubber must handle.

The "LTSpice" simulator I have been suggesting you try is also called SwitcherCAD III. This will allow you to see for yourself some of the effects of snubber ESR, althoug it will not calculate cap temp rise for you.

I got it.

Glen, to be honest, I am not very optimistic about doing simulation. Too many unknowns and fudge factors. Plus, it is too easy to make a mistake and simulate something that's not related to what my circuit will be doing.

What I would like to do is find some way to actually measure the peak voltages ad dV/dt and such. Any suggestions for doing so, given that I own a Tek 2445 scope?

Also, did you see my posts about getting the timing/duty cycle circuit to work. It works now...