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Supercapacitor coilgun

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pisoiu

Tue Jun 26 2018, 07:56PM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

I plan to move further with the project and make some tests with higher numbers. But I need a bit of help. In the next design I plan to use some maxwell supercapacitors, a number of them, around 5-6. The current they will be able to supply will be around 10kA. In order to properly manage such currents, the main connection will be made with copper busbars, for now I have 20x3mm available. It is easier this way, as I can solder switching mosfets directly on them using some pcbs as support and connection for driving circuits. However, the problem is related to connecting copper busbars to capacitor ends, which are threaded with M10 and made from aluminium. I do not have experience in aluminium-copper joints which must withstand such currents (around 1500A per connection). Does anyone have any advice? Thank you.

DerAlbi

Wed Jun 27 2018, 12:28PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

The way you calculate you current hints that you plan to use the capacitors in parallel. Thats a big problem since the low voltage is your biggest issue.
Put them in series: it will give you at least 15V to work with which leads to much more manageable currents.
I feel like you have the misconception that coilguns are magic beast that operate at ridicules high currents the higher the better. Thats not true. See it as a motor. Depending on how you wind your coils you can build equivalent motors with the same power output with 1V and 10A or 10V and 1A.

pisoiu

Wed Jun 27 2018, 02:26PM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

Yes, I do want to use them in parallel, just forgot to say that :)
I usually avoid misconceptions, just want to make tests and see the results. Putting them in series is also a good idea but for now I just wanted to avoid cell balancing issues. Series connection also does help from a mechanical point of view.

DerAlbi

Wed Jun 27 2018, 07:16PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Wrong focus. Solve the balancing issue, its the way easier problem compared to all the effects that the low voltage does.

At 2.5V @ 10kA, your setup only allows for 250ÂµOhm total resistance. That is the sum of Cap-ESR, Mosfet-Rds_on and Coil-ESR and every solder joint between the components. Add the condition that the Coil-ESR should at least pose half (better 3/4) the overall resistance your setup is simply impossible to manage. All that because you dont want to balance the capacitors? Bad trade.

klugesmith

Wed Jun 27 2018, 07:57PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Yes, all that Der Albi said.
Let's focus on the part about rewinding coils (as in motors) to match different supply voltages and currents, with no difference in coil size, weight, or efficiency.

Suppose you are trying to set a record for lowest voltage, in a coilgun with respectable output energy.

You could start with a coil size, shape, and weight that performs well in somebody's 40 V or 400 V system. To adapt it for 2 V, the winding area needs to be partitioned much more coarsely into series-connected turns. Current density (A/mm^2) and volts per turn are invariant.

If the reference design had more than 2 volts per turn, you are out of luck.
Suppose the reference design had 1/2 volt per turn. Then the 2 V version needs 4 turns, each filling 1/4 of the winding area. (Let's ignore the interconnection and switch requirement.)

It might be practical to take a heavy wire with many strands, replace the factory-applied insulation with something much thinner, and wind it around a form. Or you could saw a spiral slot in a thick metal tube, like a flexible shaft coupling:

If you configure six capacitors in series instead of parallel, the optimized coil will have six times more turns and 36x as much resistance and inductance. For the same energy, and same stress on each capacitor. The losses due to switching and interconnecting wires can be much smaller.

pisoiu

Wed Jun 27 2018, 08:54PM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
Location:
Posts: 14

Thanks for the tips. I will give more thought to series connection, I have to make some calculations.
At an earlier stage I made some calculations in which the coil resistance was around 1/2 of total resistance and indeed the losses looked quite high. The coil I considered was made with a 6mm copper rod.

hen918

Thu Jun 28 2018, 01:50PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The issue with very low voltage isn't the different coil specifications (yes the coil will still obviously need fewer, larger turns), but the parasitics, most importantly the parasitic inductance and resistance. The time constant of your system is approximately L/R where R is the total resistance and L is the total inductance. When you have to wind your coils with so little inductance to get the greatest current and an appropriately short rise time, the inductance of the capacitor connecting wires, and the MOSFETs start becoming an issue. (I would expect the ESL of the capacitor is similar to the inductance of your coil, but the datasheets don't mention this unfortunately). A multistage coilgun is going to need a very long barrel to accommodate the long pulse width.

pisoiu

Fri Jun 29 2018, 07:09AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
Location:
Posts: 14

Well, the barrel will be long. All 6 capacitors stacked in series have around 1 meter.

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