Author Topic: Drsstc 3  (Read 1371 times)

Offline Hydron

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Re: Drsstc 3
« Reply #20 on: June 18, 2018, 12:31:42 PM »
I agree with Mads about your MMC - 3 in series is only 6kV, which could significantly limit the peak current for a coil of this size, especially if you find that you need to reduce the number in parallel to 2 to get a lower total capacitance (0.47uF seems on the high side compared with other 160mm coils). If you already have the caps then make sure you the MMC construction allows for you to configure them in 4s2p for an 8kV/0.235uF option as well. To do it properly I'd suggest getting a few more to allow for a few more configurations up to 6s/3p (as per snubber suggestion below).

As for the snubber question, for both MMC and DC bus snubber use I have used the similar Aerovox snubber caps in a DRSSTC of a similar size (CM300s, 160mm*730mm secondary).

Mine were 2uF/1000V though, so the MMC ended up with a couple of fairly long strings to get a sensible capacitance/voltage rating. This was OK though, as I had over 50 of them available - my MMC allowed for up to 24 in series in each of 2 strings (with capacitance selected by choosing where to attach a shorting jumper on each string). This MMC works very well, and didn't seem to get warm even after a continuous 10-min run at ~1.5kW heating up a dummy load (cast iron skillet with bacon and eggs).

If you were to use the snubbers for a MMC I'd say that you'd need at least 5 in series and 2-3 strings in parallel, e.g. 6s3p for 18 caps total. Have a play with Mads' MMC calculator on his website and your expected coil parameters.

Offline oneKone

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Re: Drsstc 3
« Reply #21 on: July 03, 2018, 12:10:13 PM »
cheers hydron, i'll take your advice about the mcc. Originally i think i was trying to be too conservative about how much money to spend on it.

In other news i've found these capacitors:
 https://www.ebay.com.au/i/401383408371?chn=ps

If i'm reading that correctly that's a bargain! especially with the high ripple current described in the data sheet.

I'll hopefully be returning to this project very soon. i think i'll order the capacitors (bus) and the heatsink this friday, then the mmc capacitors in another week or so.

Edit:
The only downside I see with the capacitors I linked is the diameter. I can mount the capacitors on-top of the igbts to get the lowest possible inductance but it's more than I would like, in saying that I've seen voltage multipliers on drsstc inverters that also look high inductance that use bigger snubbers...
« Last Edit: July 04, 2018, 05:22:32 AM by oneKone »

Offline Mads Barnkob

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Re: Drsstc 3
« Reply #22 on: July 04, 2018, 10:16:42 AM »
It is a fair price for a brand new capacitor of that size, 2nd hand with long time on shelf or so would properly be around 25-50% of that price, if you found it on ebay.

You can actually quite precise judge a electrolytic capacitor on its physical dimensions if its made for high ripple current. The large terminals is a dead give away, but also a large diameter and short length is a clear indicator of that. Long capacitors are not good for large ripple currents as they would heat up much more near the terminals than at the bottom.

Just use some good big copper bus bars to mount them on the side of the IGBTs with a 90 degree bend or something like that. Remember that electrolytic capacitors should not be mounted upside down due to the vent hole, and you mount it on the side, see if you can possible have the vent plug/hole as much upwards as possible.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Hydron

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Re: Drsstc 3
« Reply #23 on: July 04, 2018, 12:00:30 PM »
I agree that it looks like a fair price for a new one. I assume you're planning on using a couple in series for voltage doubled 240V mains? This would give a similar capacitance to what I'm using on my similar sized coil, which seems to work ok (though is probably on the low side of acceptable).

As for mounting, I'd check the manufacturer recommendations - my RIFA (Kemet) caps are ok in any orientation but the manufacturer warns that if they vent then the mess will be worse if they are not facing upwards. Was not an option for me so mine are mounted downwards, and I'll just have to deal with it if they vent.

Offline oneKone

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Re: Drsstc 3
« Reply #24 on: July 05, 2018, 01:28:31 AM »
Cheers guys,

I am planning to mount them vertically. I was originally going to try aluminium for my bus bars but I think I'll start looking for copper plate. 

@Hydron. You're correct. Originally I was just going to run rectified mains but I thought while I'm building it I might as well go with a voltage doubler config.

Offline Usernametaken

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Re: Drsstc 3
« Reply #25 on: July 12, 2018, 10:14:41 AM »
If you find copper please let me know :)

Offline oneKone

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Re: Drsstc 3
« Reply #26 on: July 14, 2018, 04:00:09 AM »
@Usernametaken,

i've found a couple of sellers on ebay that sell copper sheet/plate. if you search for lets say "5mm t2 copper sheet" you'll get a few hits. I'll more than likely order from ebay, just waiting on getting the capacitors first. 

High Voltage Forum

Re: Drsstc 3
« Reply #26 on: July 14, 2018, 04:00:09 AM »

 


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