Author Topic: Drsstc 3  (Read 1822 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.
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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. 

Offline oneKone

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Re: Drsstc 3
« Reply #27 on: July 21, 2018, 10:44:21 AM »
The capacitors have come! For me it honestly makes a difference to hold the item and see how it's going to sit in relation to the igbts. At the moment I'm thinking about getting 5mm thick 30mm wide copper plate, I'm just trying to figure out a way that will have the least wasted material.


Offline futurist

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Re: Drsstc 3
« Reply #28 on: July 21, 2018, 05:51:07 PM »
I use the same capacitors for my DRSSTC. They seem pretty robust, only downside is that they don't come with screw stud so you need to make some sort of a mount
« Last Edit: July 21, 2018, 05:53:41 PM by futurist »

Offline Mads Barnkob

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Re: Drsstc 3
« Reply #29 on: July 22, 2018, 08:31:11 PM »
5mm copper sheet is way overkill for the required current/low inductance, but as the capacitor does not have a mounting stud, you could construct the bridge bus bar in such a way that its the mechanical structure that holds all the weight of the capacitors.

Nice little stubby electrolytics by the way, for sure made for high ripple current! ...or low clearance designs :)
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Offline oneKone

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Re: Drsstc 3
« Reply #30 on: July 23, 2018, 01:38:59 PM »
@ futurist, at least someone is using these!

@mads, my thinking exactly! i'm hoping the 5mm plate will stop them from moving around, if not i'll machine up some acetal standoffs.

Offline Nijin

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Re: Drsstc 3
« Reply #31 on: August 03, 2018, 10:59:08 AM »
@oneKone I used 5mm aluminium busbar for my inverter, they don't overheat with 1500A and they are much more easier to cut than copper ;)
Everything is a fuse if you pass enough current

Offline oneKone

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Re: Drsstc 3
« Reply #32 on: August 07, 2018, 12:58:50 AM »
Cheers for the suggestion @Nijin. I thought for weeks about using aluminium for the bus bars. The only downside is that I would have had to buy 8 meters of it... In the end I bought some copper flat bar, just waiting on it getting delivered now!

Offline oneKone

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Re: Drsstc 3
« Reply #33 on: August 08, 2018, 06:54:28 AM »
Ok..... So I think I may have figured out a MMC! The capacitors will still be the same style as I linked before but 330nf @ 3kv. With this I can get away with 3 in series with 2 parallel strings, but to be on the safe side I might go 4 Series with 2 parallel strings. This will also bring bthe cost down to a much more manageable 200ish aud for the MMC.

As for the heatsink I can get one from rs components for about $105 aud... or get two smaller heatsinks, one for each brick for a total of $50 aud. For the heatsink it comes down to appearance.

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Re: Drsstc 3
« Reply #33 on: August 08, 2018, 06:54:28 AM »

 


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