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Old 16th June 2008, 02:51 PM   #1
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Default core power ratings for 9kW DCDC

Hi,

This isnt really anything to do with audio, but this forum is one of the best SMPS design forums on the web!!

I need a controlled DC 300v power supply for a plasma cutter. This will need upto 30A, which is 9kW.

The largest SMPS ive made is a full bridge 100kHz 2kW unit so this is a jump.

The input will be single or 3 phase 230vAC, rectified to DC. Output will be 300vDC. Output control im planning is via a hall effect sensor on the output DC terminal.

I could use the direct mains rectified but this is very dangerous without an isolation transformer. 9kVA 50Hz transformers are not cheap, i think SMPS is the way to go, i also can control things without using resistive heating elements etc.

This will feedback to the SMPS IC, and ill have a Pot, which together with the hall effect sensor, will control the maximum output current. full duty will correspond to the 300vDC open circuit i need to srtike the arc. The current limit will back off the duty and cut the voltage. Ideally, for plasma cutting, the current will be such that about 200vDC is present.

Are there any commonly avaliable cores for handling this power?

For the 2kW unit, i made it from two 1 kW stages and paralleled the output.

I would be v-happy with a single 9kw stage, happy with two 4.5kW stages, and border line with three 3kW stages.

Does anybody have any core recomendations?

im happy with higher than 100kHz, ive made upto 500W smps at 250kHz!! but i prefer to keep 100kHz as a maximum.

Thanks,
Stuart
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Old 16th June 2008, 03:34 PM   #2
Alexsch is offline Alexsch  Israel
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Well lets see...

6 pulse 3ph rectified 230Vac gives 563Vdc. Derating of 30% places you in 900V IGBT range. So you cannot switch at a high frequency anyway.
Core type U93/76/30-3C94 Gives Al=6.4uH, Ae=8.4cm.

Working at 50KHz full bridge and 85% DC:

N*B=114,000.

For Bpp=3KG you have 39T, on primary and 24T on secondary.
Ploss=21W U+U core set.
Lp=5mH.
Im=1.91A
Lo= 90uH for dIL=10Ap-p.
Do NOT use CT secondary, you will loose a lot from Trr. Use either a full bridge rectifier or better still, split the secondary and use 2 of them.

Use phase-shift PWM with IGBT. Current mode will be better here, it allows DC coupling of the primary.
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Old 16th June 2008, 06:44 PM   #3
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Hi,

Thanks for that, was planning on using only a standard uncontrolled "3 wire" bridge rectifier, the AC supply will be a star (Wye) supply with no neutral, so the voltage i get will be lower than indicated 230x1.414=325v as im basically rectifying 3 AC supplied. (unless ive got my 3-phase thinking the wrong way round)

Im happy enough to take 9kW from single phase AC, its just that 3 phase is avaliable to me and it makes smothing to DC much easier.

I had looked into IGBT but ive never gone there before, if the voltage im dealing with is 325v then MOSFET still works, but is pushing it.

IGBT's do come in handy full or half-bridge packs which have insulated heat-sinks and easy termination.

What core power are you calculating for? I see you say "Bpp=3KG" should KG be kW, meaning Bpp=3kW?

Are you suggesting using 4 "U" core half to make an "E" core? Id looked to this but bobbins get hard to find.

Also, something to bear in mind, a 50%-75% duty cycle is not uncommon for the cheeper plasma cutters so i can push the core etc further thermally - im not running 9kW all the time.

Finally, do you know a supplier (UK, or ship to UK) of this core?

Thanks,
Stuart
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Old 16th June 2008, 07:49 PM   #4
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Just wondering if it would be easier using a push-pull toplogy (similar to the ESP 350w system?)

My thought is, i can with much greater ease utilise pulse-by-pulse current limiting.

Ensuring the coils are both the same does become more critical thought?

Stuart
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Old 17th June 2008, 10:21 AM   #5
Alexsch is offline Alexsch  Israel
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Quote:
Thanks for that, was planning on using only a standard uncontrolled "3 wire" bridge rectifier, the AC supply will be a star (Wye) supply with no neutral, so the voltage i get will be lower than indicated 230x1.414=325v as im basically rectifying 3 AC supplied. (unless ive got my 3-phase thinking the wrong way round)
What exacly you have in mind? A schematic can help a lot...

Quote:
Im happy enough to take 9kW from single phase AC, its just that 3 phase is avaliable to me and it makes smothing to DC much easier.
A single phase usage at 9kW level? Too low. 3 phases are better.

Quote:
I had looked into IGBT but ive never gone there before, if the voltage im dealing with is 325v then MOSFET still works, but is pushing it.
At the power levels we are talking about, first consider IGBT, then MOSFET. You dont nessesary need modules, TO247 or something like that may be enouth.

What core power are you calculating for? I see you say "Bpp=3KG" should KG be kW, meaning Bpp=3kW?

3KG=3kilo-Gauss (magnetic flux level).

Quote:
Are you suggesting using 4 "U" core half to make an "E" core? Id looked to this but bobbins get hard to find.
Actually i had in mind U+U as they are, one against another. No need to construct E core from them.
Bobbins? There are NO bobbins. You go to a winding shop and they will do it for you. Also as extreme solution you may design and MILL a bobbin from a plastic material (Delrine?) for yourself.

Quote:
Also, something to bear in mind, a 50%-75% duty cycle is not uncommon for the cheeper plasma cutters so i can push the core etc further thermally - im not running 9kW all the time.
You prefer to use 50-75% DC? OK. Go ahead. But the output choke will be larger.

Quote:
Finally, do you know a supplier (UK, or ship to UK) of this core?
Well, i am Israeli, we have plenty of our own supply sources. Up to a few days ago i never even knew the option of Amidon+VISA or EBAY. Such a wonder was dislosed for me by Tolik in his thread on "My first 12V SMPS".

Quote:
Just wondering if it would be easier using a push-pull toplogy (similar to the ESP 350w system?)
No push-pulls at this level! What is ESP350?

Quote:
Ensuring the coils are both the same does become more critical thought?
Which coils?
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Old 17th June 2008, 01:52 PM   #6
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Hi,

I've attatched a schematic, i was thinking that i want to beable to run on both single and 3-phase depending on the situation.

To make this a bit easier the unit would have 3 inlets, each with its own AC filter and rectifier. This way, i can plug it into 3 mains outlets, if each outlet is on a diffirent phase, that would be the best thing, if not, provided each is on a diffirent breaker then that would be fine.




I have never used U+U core, i guess you wind the coil on each half of it then pair together.

I have several ETD59. Some of N97(Al=5500) material, some of 3C90 (Al=6000) material and good for up to 200kHz.

These have an Ae min of 368mm.sq and a Ve min of 51200mm.cu

I have seen people on the forum push 2kW through at 75kHz. What would you think at 100, 150 or 200kHz full bridge.




When i referr to "duty cycle" i perhaps was confusing things. I am not talking about the PWM duty, but the duty of operation.

Most welding and cutting tools have a continuous duty, for example, a welder can operate at 100% duty (all the time) at say 80A current, however, at 140A current can only operate at 30% duty. So, 3 mins of welding then a 7min cool down.

Most plasma cutters can be operated for 5 mins before a 2.5 to 5 minute cool down period is required. This is because the components can then be sized much smaller and hence cheeper.




The ESP system, i am referring to the common design http://sound.westhost.com/project89.htm
Attached Images
File Type: jpg ac input.jpg (45.6 KB, 800 views)
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Old 17th June 2008, 02:55 PM   #7
Alexsch is offline Alexsch  Israel
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I dont think that this schematic will work... Also, there is no individual phase return in 3PH systems. Only delta or a Y wiring with the forth conductor (N) may be absent. And you need a soft start for the bridge, so use thyristor semi-controlled 6 pulse bridge.
Check semiconductor companies for 3ph rectifier modules.
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Old 17th June 2008, 06:37 PM   #8
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Hi,

Running through my logic for the AC/DC input rectification.
if running from 3 phase, it will be from star. In most buildings it provides 3 individual single phases (N+L1, N+L2, N+L3).

What ive done is parallel 3 standard bridge rectifiers DC output. I can also paralleling the AC input (as in a single phase situation), OR, having 3 seperate AC supplies, i.e. 3 phase.

I could put this into a single block 3 phase rectifier which makes single phase use slighly awkward.

Also, the sockets in the area i am using it in are all on a diffirent phase, so getting the 3 phase is easy.

I may just stick with single anyway as i do have a 50A single phase outlet avaliable to me which gets used with a welder.

Soft starting the AC supply is a good point given the smothing capacitors and the high voltage diffirence, large currents may ensue!


Whats your thoughts on the power handling capacity of the ETD59 core?

Other posts indicate 2.6kW possible at 75kHz full bridge, what would be possible at higher frequencies, bearing in mind i can push the core further than usual given that it wont be used for any more than a 1-2 minutes before a no-load break of 4-6 minutes?

Thanks,
Stuart
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Old 17th June 2008, 07:19 PM   #9
Alexsch is offline Alexsch  Israel
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On rectification.
I do not fully understand what you have posted, but know this:
A SPICE simulation of any rectifier circuit will save you from a lot of trouble.

On ETD59.
Well lets see:

Ae=3.7cm instead of 8.4cm.

Increase primary turns to 88 and secondary to 55.
Ww=41mm.
41/(88/2)=0.93mm diameter wire (in sandwitch interleave 2 layers) at ~19A primary current. Looks bad.
And also you may use powerful fan only for the trafo. Going to high frequency gives very little. You have a lot of AC current to conduct and the skin effect will demolish any attempt to go beyound, say 150KHz.
By the way, using planar core technology may be of an advantage here.

Is this SMPS is intended for mass production, or a learning project? Anyway its most definitely a bold step into a power-electronics minefield.
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Old 18th June 2008, 12:27 AM   #10
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Sorry to interject:

Quote:
I could put this into a single block 3 phase rectifier which makes single phase use slighly awkward.
9KW from a single phase?
That much power from a single phase might be a bit awkward anyways.
I would think at least 2-phases would be needed,for practicality's sake,if nothing else.
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