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Could have PFC stage boost to 565VDC instead, and then full bridge -> 400VAC sine wave. Don't think it's worth it, as the reduction in core losses will probably be less than the increase in switching losses.
Where did you get the spreadsheet you are using for the calculations?
I'm trying to repeat the minimum turns to stay out of saturation and I'm getting lower numbers. 73 turns producing 1.1T rather than 1.6 at 50Hz. I think the 400Hz route is weird and I'm wondering if keeping the cores split would let you put enough copper on to get decent performance at 50Hz.
Btw what fraction of your winding window are you planning to use?
I also have a probably insane idea to track the 50Hz waveform and switch polarity at the peak to get you pseudo 100Hz. It's ugly, but wouldn't need big capacitors.
That makes two of us :) My formal education in this area goes as far as a lecturer skimming over something along the lines of "Voltage ratio = turns ratio, don't have too few turns or it won't work".
Wastrel wrote ...
Where did you get the spreadsheet you are using for the calculations?
I made it, so it's both poorly laid out and probably incorrect.
Wastrel wrote ...
I'm trying to repeat the minimum turns to stay out of saturation and I'm getting lower numbers. 73 turns producing 1.1T rather than 1.6 at 50Hz.
(possibly dodgy calculations follow)
Erms = 4.44 * f * N * a * Bpeak.
Setting Bpeak = 1.1T, f = 50Hz, Erms = 400V and a = 0.01729m^2 after applying 95% stacking factor, solving gives N = 95 turns.
For a square wave, 4.44 is changed to 4 and N = 105.
Can you post your calculations to compare with?
Wastrel wrote ...
I think the 400Hz route is weird and I'm wondering if keeping the cores split would let you put enough copper on to get decent performance at 50Hz.
I've copied the spreadsheet and adjusted it for a single small (H = 70mm) core to 5500V and 7.5kVA capacity. Paralleling the primaries and putting the secondaries in series would then allow for equivalent capacity. Looks like overall losses will end up being higher.
As the number of layers goes up, the calculation for number of layers becomes inaccurate due to not properly accounting for the reduction in inner diameter. I'll have a go at fixing that later.
Wastrel wrote ...
Btw what fraction of your winding window are you planning to use?
Hoping to use as few layers as reasonably possible to improve heat dissipation and make it easier to wind - a 26mmx14mm cross-section bicycle wheel needs to fit through it for winding.
Wastrel wrote ...
I also have a probably insane idea to track the 50Hz waveform and switch polarity at the peak to get you pseudo 100Hz. It's ugly, but wouldn't need big capacitors.
Chopping the input waveform like that is an interesting idea, I don't know whether it will work. Hopefully someone else can chime in on that.
Wastrel wrote ...
What is the transformer for?
Aim: To wind a transformer with similar capacity and losses as a small pole pig, except smaller/cheaper due to operating at a higher frequency.
I could have sworn the voltage on the spreadsheet was 325Vish, has it been changed? That's the main reason my numbers are wrong.
I have the feeling that the capacitor this is going to need will be huge, and the current requirement will make it expensive.
The energy wasted is quite flat and I think it's to do with how you are calculating copper used. Fixed diameter wire, single winding. I think if you wind more copper on the core the power wasted will drop. Tape for the primary for example. This may make it optimal to use a lower frequency than 400hz and radiate much less total heat.
You know about interleaving primary and secondary windings, right? This will be harder given the secondary voltage.
I'm surprised splitting the cores isn't better, but then the main advantage would be to use more copper. I'd also consider hacksawing away some of the inner ring, which would cost you some area but open up the winding window a lot.
How did you calculate the cross sectional area of core you needed for 15kva? I know you are in the right ball park for 50Hz operation but I'm quite fuzzy on the actual maths for that. You may have much more metal than you need for 400Hz (are proportionally more hysteresis loss).
My transformer theory is weak as I said, I have done a lot of reading for a core of my own and I know enough to know I don't know what I'm talking about. Some things I think are generally true are that toroidal cores are good for low leakage of flux and can have a huge winding window compared to E cores, but that you pay a heavy price if you don't fill most of it with copper.
Something else I think is true is that for a resistive load your copper losses should be highest when the load draws full current, and the core losses should be highest when the load draws no current. This make sense to you?
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Using your formula, and rearranging to find N, I got Erms/(f*a*Bpeak*4.44). using 4 instead of 4.44, f=400, a=0.01729, Bpeak=1.1 and Erms=400, I got 1/(4*0.01729*1.1)= 13.14475turns.
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Ross First let us know: Service factor: 1 for continuous service at full load, 0,5 for work 5 minutes in a period of 10 minutes, 0.2 for working 2 minutes in a period of 10 minutes. (Welding machines transformers are designed for .2~.5 service factor) Second : Do you have intention of add a small fan to cool the core ? Third: Use all units as IS system. m for meter, kg for weight, T for tesla, V for volt A for amper Etc Do not play with gauss, oersted, gilberts, centimeter, inch, pound, once, inch etc etc Third: Define yours design parameters
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Toroidal Transformer Design
