Author Topic: 8x MOT transformer, weird power draw  (Read 827 times)

Offline AstRii

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8x MOT transformer, weird power draw
« on: January 09, 2022, 04:22:11 PM »
Hello,
So recently while I should be learning for exams I rebuilt my transformer with 8MOTs from 2s4p configuration to 4s2p for 8kV output.


The transformer has been running quite well (haven't yet tried more than 180VAC in though). The transformer will run at 400VAC eventually.

(180VAC, 20A).

But there is one weird thing that just cannot let me sleep. I have measured the output inductance to be 44H, at 50Hz this means that for resonant MMC I need 230nF capacitance.
(I have measured the secondary inductance with an LCR meter DER-EE DE-5000 at 100Hz)
I have added 4 MOCs in series for total of 280nF which I expected to be close enough, yet the arcs were pretty much non existent. Instead of making the capacitance lower, I tried to make it higher by adding another 4 MOCs in series in parallel for total of
~560nF. Now the arcs started to really kick in, at 180V input it draws over 10A. I added yet another 4 MOCs in series for total of 840nF now the arcs are just slightly larger than before with 560nF, but the current draw is almost like double.
Isn't this weird? I would expect that the arcs would be the biggest at secondary resonant frequency, yet it seems that simply higher capacitance = higher power draw (and slightly bigger arcs).

This is also confirmed by Danyk's (DiodeGoneWild) video where he made almost the same transformer.

At 11:33 you can hear him saying that more capacitance doesn't improve the arc length anymore, but it does draw more current from the outlet.

So what is going on?

Thank you for any response!
- Mark

Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline davekni

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Re: 8x MOT transformer, weird power draw
« Reply #1 on: January 10, 2022, 03:54:59 AM »
It is more fun learning by experiment than by studying, though both are necessary.

First, is 44H output leakage inductance (shorted primary) or open primary inductance?  An ideal transformer has infinite open-primary inductance and would make arcs.  Maximum current draw requires matching caps to leakage inductance.  That includes variac inductance, so would be measured with input to variac shorted and variac set to your nominal 180V dial position.

Second, steel (lamination) permeability is quite non-linear with field strength.  Especially non-linear at both low-fields (low voltage and current where the meter operates) and at high fields (near saturation).  Meter measurements may be off by 2x or more compared to typical transformer use.  High-field non-linearity results in complex behavior at high arc current:  arc current changes field strength which changes inductance which changes resonance and therefore changes arc current.  If it weren't for this non-linearity, I'd expect max arc performance to match max line current draw.  BTW, the optimum capacitance may change for 400V operation relative to 180V due to non-linearity.

Have you taken any steps to improve MOT insulation between windings and laminations?  I needed to add epoxy to get even my 2s configuration to not arc (with one secondary end grounded and resonant MMC charging to 9kV peak).  With floating secondaries the voltage may roughly split evenly, but peak will not be far below +-9kV.  You may find an issue when extending to 400Vac input.
David Knierim

Offline AstRii

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Re: 8x MOT transformer, weird power draw
« Reply #2 on: January 11, 2022, 03:21:37 PM »
Thanks for the answer Dave!

The 44H inductance was with open primary.
I also talked with my electronics teacher about this, he said that any inductance measurement with these LCR meters for iron cores is virtually useless as you only cover very small portion of the hystheresis curve with those low voltages and low currents.
That's basically almost the same what you wrote.

This is all more complex than what I expected but yes it does make sense that the arc changes the inductance now when I think about it more.

Have you taken any steps to improve MOT insulation between windings and laminations? 

I disconnected the secondary windings from the cores and used some kapton tape and 3d printed plastics between the cores and the windings. I have now tried to run the transformer at 250VAC and no insulation breakdown yet, but that may change soon, to be honest I'm not that confident about the insulation.
Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline AstRii

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Re: 8x MOT transformer, weird power draw
« Reply #3 on: January 11, 2022, 06:19:49 PM »
So unfortunately, at 300VAC input this happened:

The insulation broke down on one of the transformers. Although it's probably expected, I have measured the arc current with a clamp-meter and it showed up 2A. 2A at 50Hz through ~550nF that's almost 12kVrms => >16kV peak.
I think I will replace the broken MOT and go back to 2s4p configuration.
Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

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Re: 8x MOT transformer, weird power draw
« Reply #3 on: January 11, 2022, 06:19:49 PM »

 


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