Chamber Grounding Through Turbopump

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Maxwell_Epstein
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Chamber Grounding Through Turbopump

Post by Maxwell_Epstein »

Hello all!

I am sorry to be asking so many questions back to back, I am simply making lots of progress rather quickly.

I was about to do my first plasma test with my new 30kV transformer earlier today. Before doing the test, I checked the resistance over the 10 ohm resistor I'm using to measure current. The chamber wall is connected to ground through that resistor and I am measuring voltage over it.

When I measured the resistance of that resistor with a multimeter, I found that it was just a little over 4 ohms, not 10. When I disconnected the resistor from the chamber, the resistance jumped back up to 10 ohms.

I then reconnected the resistor and, just in case, unplugged my Turbopump from it's power supply and the resistance went back to 10. My guess is that the turbopump case must be by design connected to ground (through a resistor or other electronics) through that cable, and that is what's causing the drop in resistance.

I've seen lots of people using turbopumps in their fusors, but I haven't been able to find anything on whether or not the turbopump being grounded by itself affects fusion or could put my turbo at risk of damage. This may be a non-issue, but I wanted to be 100% sure before potentially damaging anything.

Please let me know what my next steps should be!

Thank you!
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Liam David
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Re: Chamber Grounding Through Turbopump

Post by Liam David »

As you note, if you want to accurately measure the current between the chamber and ground, you need to make sure that the chamber is floated using only your measurement resistor. Most turbo pump housings are grounded for safety reasons and unless you were to add e.g. a ceramic break (not necessarily recommended), you have two options. One is to measure the total resistance to ground and use that as your conversion value. A much better idea would be to use the low-voltage side of your power supply since all the current must pass through there regardless of what's connected to your chamber.
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Maxwell_Epstein
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Re: Chamber Grounding Through Turbopump

Post by Maxwell_Epstein »

Thank you for the quick response and good advice! I'd thought about measuring the amperage into the transformer, but was worried that that would be inaccurate in some way (the transformer drawing slightly more amperage than it outputs to the grid). If that isn't as much of an issue as I assumed (i can test that for my specific transformer just to be sure too), I'll definitely do it that way so my chamber isn't floating at all and I can very easily measure current on the lower voltage input side.

Thanks!
Max E.
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Liam David
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Re: Chamber Grounding Through Turbopump

Post by Liam David »

Take a step back for just a moment and consider your sentence: "the transformer drawing slightly more amperage than it outputs to the grid" could only be true if there is some alternate path to ground than the plasma. That could be another physical connection (which you don't have) or leakage current (if enough to modify your measurements, you'd have bigger problems). In the absence of other connections, Kirchhoff's current law (a statement of charge conservation) requires that the current is the same through the transformer as through the plasma as through the chamber to ground.
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Maxwell_Epstein
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Re: Chamber Grounding Through Turbopump

Post by Maxwell_Epstein »

You're absolutely right. I was certainly not implying that current was disappearing into thin air, nor that a significant amount was being dissipated via heat within the transformer itself. I was just thinking that although a simple transformer does not have other connections to ground, I do not know what the internals of my hipotronics supply looks like and there might have been another connection to ground within the transformer. As that doesn't seem likely, I'll definitely move ahead with setting up metering! Thanks!
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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Rich Gorski
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Re: Chamber Grounding Through Turbopump

Post by Rich Gorski »

Based on your resistance measurements it seems like your turbo power supply creates a parallel path across your 10 ohm measurement resistor. Your Hipotronics supply has a ground stud on top. I assume you have the 10 ohm connected there and back to the chamber. As the above post says you need to use the total resistance (4 ohms I think) when you do your current calculation from the voltage drop. One test you might consider is to run your supply with say a 10M ohm resistor (10 watt) as a load on the power supply to test the current measurement. Connect the 10M between your HV lead and the chamber (simulate the plasma). If you set the supply to 10kV then the current should be I = 10000/1e^7 = 0.001A through your 4 ohm combined resistance and the drop should be 0.004V. The 10Mohm R will dissipate 10 watts of power so it will get warm.

Rich G.
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