I am making a transformer with Epcos E55 cores of N27 ferrite. The primary will have four windings each rated for 12-15 VRMS at 50-70 kHz. I am hoping to get up to 1500 watts, so the primary windings will need to handle up to
31 amps. The coilformer (bobbin) has 14 pins, of which 8 will be used for the primaries. The secondary will be two windings capable of 250-300 volts each at the nominal 12/24/48 volts input. For 1500 watts, these will need to be rated at about 3 amps. I have figured on four turns for each primary (about 3.5 volts/turn).I have some 7x3x21 Litz wire that is equivalent to about #14 AWG and should be capable of about 15 amps. But it was impossible to solder it to the terminals of the coilformer. I was also unable to use my soldering iron to melt the insulation and make connection to all the strands.
The information I found generally recommends a solder pot, which I don't have:
For determining equivalent AWG size for Litz wire, the following is useful:
I was able to use some 105/40 Litz wire that is equivalent to about #18-#20, and good for about 3-6 amps. By using some liquid flux and high temperature iron, I got what seemed to be a fairly good coating of solder, but I don't know if it actually contacts most of the strands. And, of course, the transformer is limited to about 300 watts. Here is what I have:
I might be able to add another set of windings in parallel, but that would still limit power to about 600 watts. So I have a couple more ideas.
(1) I could use 16 turns for each primary, so with the 105/40 Litz wire I could use them in parallel for 48 volts at 24 amps, or 1152 watts. Close, but no cigar. And the power would be reduced for 24V and 12V. That may be acceptable for my purposes of supplying 250 VDC to a 240 VAC VFD, as well as
500 VDC to a 480 VAC VFD. For the highest power I would plan to use 48 VDC (4x12V batteries), and only 12 VDC for low power.(2) I could use the 7x3x21 Litz wire without soldering to the terminals, and instead use a "self-leaded" configuration where the windings are terminated on the PCB or separate connectors. Actually, I don't think it is really
7x3x21, as #21 is rated about 2.3 amps and thus the wire should handle 50 amps. Each strand is about 0.0035" or 0.08mm which is the same as that for the 105/40. It may actually be 7x3x21x40, or 440x40. That would be about 4x the size of the 105/40 and thus good for about 12-24 amps. It's supposed to be #14 AWG equivalent, conservatively rated about 12 amps.(3) I might use 4 strands of #18 AWG twisted for a "poor man's Litz" equivalent. That should handle 20 amps. If I use two windings of 16 turns each, that would give me 40 amps at 48 volts (1920 W) in parallel. #18 AWG is good for 17 kHz at 100% skin depth.
Skin effect depth at 60 kHz is 12.2 mils. #18 AWG is 40 mils, so the AC resistance would be equivalent to a tubular conductor with an ID about half the OD. So (I think) AC resistance would increase by ID^2/OD^2 or 15^2/40^2 or about 15%.
BTW, I'm also wondering where I can get core compression clips for E55 ferrite cores? Mostly I've seen only clips for much smaller cores, from distributors. I have usually just used Mylar tape pulled tightly around the outer surfaces, and I may try heat-shrinkable tape.
Thanks,
Paul