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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
They might be able to handle 200A peak, but certainly not RMS which often is the overseen factor in terms of MMC overheating.
If you open the table with good capacitors, there is a aerovox snubber capacitor, which is more sturdy than yours, try to use those values for comparison.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
jpsmith123 wrote ...
Here's my proposed primary circuit: It's a Marx circuit, with the capacitors mechanically (and of course electrically) integrated into the single turn primary. I still have to figure out the hardware.
This circuit looks quite strange. The resistors will dissipate a huge power, the voltage source is too high, and the primary inductance too small.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I should've mentioned that those values are basically just "place holders"; although I do plan to use a 50 kv supply to charge the caps. Basically what I want to do is something similar to a "Rohwein" transformer, i.e., a single turn, low-inductance primary winding pulsed at 100 kv, with maybe 30 to 35 turns in the secondary, hoping to get an output of 2 to 3 MV.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Mads Barnkob wrote ...
They might be able to handle 200A peak, but certainly not RMS which often is the overseen factor in terms of MMC overheating.
If you open the table with good capacitors, there is a aerovox snubber capacitor, which is more sturdy than yours, try to use those values for comparison.
I cant find the datasheet for these caps, but they are quite large and the supplier said they should take high transient currents. I think the cheapest option for me will be to get more and arrange 3 or more banks in parallel, im not too sure what configuration will be the easiest, but its so cost prohibitive for me to order those aerovox snubbers from overseas.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
nzoomed wrote ...
Mads Barnkob wrote ...
They might be able to handle 200A peak, but certainly not RMS which often is the overseen factor in terms of MMC overheating.
If you open the table with good capacitors, there is a aerovox snubber capacitor, which is more sturdy than yours, try to use those values for comparison.
I cant find the datasheet for these caps, but they are quite large and the supplier said they should take high transient currents. I think the cheapest option for me will be to get more and arrange 3 or more banks in parallel, im not too sure what configuration will be the easiest, but its so cost prohibitive for me to order those aerovox snubbers from overseas.
What is their physical dimensions? From your picture they seem to fit in a closed hand.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Mads Barnkob wrote ...
nzoomed wrote ...
Mads Barnkob wrote ...
They might be able to handle 200A peak, but certainly not RMS which often is the overseen factor in terms of MMC overheating.
If you open the table with good capacitors, there is a aerovox snubber capacitor, which is more sturdy than yours, try to use those values for comparison.
I cant find the datasheet for these caps, but they are quite large and the supplier said they should take high transient currents. I think the cheapest option for me will be to get more and arrange 3 or more banks in parallel, im not too sure what configuration will be the easiest, but its so cost prohibitive for me to order those aerovox snubbers from overseas.
What is their physical dimensions? From your picture they seem to fit in a closed hand.
They are about 60mm wide, to me that was big because i thought they were going to be alot smaller after looking at the photos.
I know they are a self healing type, so thats always good.
The supplier told me each one would be rated at around 25 amps each.
I thought peak current was the thing to worry about not RMS, since the RMS current is always lower than the peak.
Infact, the calculators ive used tell me i should expect 800A peak, but 80A RMS.
Bus voltage is only about 680v max so is alot lower than the 16KV rating the bank will be.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Bus voltage has nothing to do with MMC voltage rating. You need to look at peak current, frequency and MMC capacitance to work out peak MMC voltage - I think the 4hv wiki might have the info, but it's easy to work out from the impedance of the caps at your operating frequency
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Hydron wrote ...
Bus voltage has nothing to do with MMC voltage rating. You need to look at peak current, frequency and MMC capacitance to work out peak MMC voltage - I think the 4hv wiki might have the info, but it's easy to work out from the impedance of the caps at your operating frequency
I used the calculator that mads has on his site and my MMC voltage works out to 9600v, im not sure if thats peak voltage or rms, but as long as my bank can take 10KV, that should be OK.
Does anyone use microwave oven capacitors? I can get these for free and they are rated at 1uf, and 2KV i believe, 5 in series would give me 0.2uf at 10KV.
TBH, im not sure how the MMC voltage gets so high with so low bus voltage, with a SPTC, the NST can put put 20KV, so i was assuming that SSTC's can put out the same power because there is no voltage drop across a spark gap.
From what i read, the capacitor will charge up to a higher voltage with each cycle, but i didnt think it could charge up any higher than the applied (bus) voltage?
Anyway, i will need some more caps, i might risk these ones i have for the time being, but im going to need something more capable of higher RMS current. For testing i should be OK on low power, but if im to drive this coil to its max, i can theoretically run the coil up to 1000A i believe.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Remember that the MMC is part of a resonant circuit - a series LC circuit presents a very low impedance at resonance, which lets you drive large currents into it and consequently generate large voltages across the inductor and capacitor.
I wouldn't run too close to the max MMC voltage, but the calculator probably gives peak voltage rather than RMS, so you've actually got more leeway than you thought. 440V * 1.414 * 21 = ~13kV.
I'd recommend you try and do the calculations of this stuff yourself - the MMC calculator is very useful but it's important to understand why it gives the results it does. The understanding will help a lot when you're later trying to get the coil to run, or deal with a problem. Also it could be worth trying to simulate a coil - LTSpice is free, and though it's a bit of work learning how to use it the skill will be very useful if you keep up with electronics as a hobby/profession.
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