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> High Current Snubbing, Selecting capacitors
Tank Crusher 210
Posted: August 20, 2015 07:09 am
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I was playing around with a few ideas in my head again lately, and I was wondering if someone would care to help me with some of the more involved math surrounding inductive voltage spikes.

I have a hypothetical situation in which a current of 400 amperes RMS will be stopped in a period of around 500 nanoseconds, with inductance of a line transformer and 50ft or less of 208/240V wiring, best guess around 10~80uH.

My calculation of:

Yields a result of:

Now, it doesn't take the smartest person on earth to realize that something doesn't quite seem right here. blush.gif

I know this formula does not account for the presence of any snubber capacitors, but I would probably select a capacitance somewhere in the 10~100uF range based on a complete guess.

The 400 amp RMS value is a momentary maximum in this application, to permit starting of motors, transformers, and other <5 second surge loads.

The transistors in this application will be these: http://www.datasheetarchive.com/dl/Datashe...DSA00417947.pdf

I'd like to figure out what sort of *realistic* voltage rises I should expect with *realistic* capacitor choices in this scenario.

I don't suppose someone would be willing to correct me on my math? unsure.gif

Is direct line switching even a feasible goal, or will I have to rectify, filter, and invert in order to switch this sort of power? The capacitors to filter 400 amps single phase FWR at line voltage for inverter duty can't possibly be cheap.
Posted: August 20, 2015 08:31 am
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This was that electronic variac thing, right?

Do what I suggested earlier (I think): build it just like a buck converter, with a supply bypass (has to be a nonpolar film cap, as large as you can manage, but ultimately limited by current draw/PF from the mains side), series filter inductance, and output filter cap.

This way, the half bridge always clamps the voltage to its supply. The anti-series transistors need to be driven in a synchronous buck configuration, since you obviously can't use diodes on AC.

Transient protection should be provided from a beefy MOV on the primary side, which will clamp (under fault conditions like heavy surge and semi-nearby lightning) to about double the supply, so it might be desirable to use >=600V devices on 240VAC.

You may want a soft start kind of circuit to deal with the input filter capacitor. Plugging, switching or contactor-ing that would be kind of annoying for inrush...

The ~10uH (which seems reasonable for wiring, not to mention if there's a motor or transformer out there) will never even be seen by your circuit, and the actual on/off risetime seen by the load will be in the milliseconds range.

You'll be looking at $1000 for transistors (if you bought new), under $100 for capacitors, $unknown for the inductor -- it would probably have to be custom; salvage or buy some really big powdered iron or gapped ferrite cores? -- should be $200 max for build-your-own, and $50 should be reasonable.

Don't forget current limiting! An active current limit would be interesting, but even if it's just a fault detector (for that matter, desat detection on the IGBTs is an excellent idea, too), that'll save you the cost of transistors every couple of tests.


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Posted: September 11, 2015 03:57 am
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QUOTE (Tank Crusher 210 @ August 20, 2015 07:09 am)
Is direct line switching even a feasible goal, or will I have to rectify, filter, and invert in order to switch this sort of power?  The capacitors to filter 400 amps single phase FWR at line voltage for inverter duty can't possibly be cheap.

You might want to look at something like this. A size 6 will go over 400 amps three phase.


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