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E55 Ferrite transformer and Litz wire

Started by P E Schoen September 24, 2016
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:

http://www.hmwire.com/pdf%20files/Soldering_of_Litz_wire_231507.pdf

http://www.newenglandwire.com/~/media/Files/Litz%20Wire%20Termination%20guide%204th%20Edition.ashx?la=en

For determining equivalent AWG size for Litz wire, the following is useful:
http://mwswire.com/litzwire_tool.php

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:

http://enginuitysystems.com/pix/electronics/E55_Transformer_3081.jpg
http://enginuitysystems.com/pix/electronics/E55_Transformer_3082.jpg
http://enginuitysystems.com/pix/electronics/E55_Transformer_3083.jpg

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.

http://www.powerstream.com/Wire_Size.htm

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%.
http://www.hsmwire.com/skin_effect.php

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 

On 9/24/2016 6:03 PM, P E Schoen wrote:
> 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: > > http://www.hmwire.com/pdf%20files/Soldering_of_Litz_wire_231507.pdf > > http://www.newenglandwire.com/~/media/Files/Litz%20Wire%20Termination%20guide%204th%20Edition.ashx?la=en > > > For determining equivalent AWG size for Litz wire, the following is useful: > http://mwswire.com/litzwire_tool.php > > 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: > > http://enginuitysystems.com/pix/electronics/E55_Transformer_3081.jpg > http://enginuitysystems.com/pix/electronics/E55_Transformer_3082.jpg > http://enginuitysystems.com/pix/electronics/E55_Transformer_3083.jpg > > 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. > > http://www.powerstream.com/Wire_Size.htm > > 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%. > http://www.hsmwire.com/skin_effect.php > > 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
It looks like you are trying to use what you have on hand which is OK, but it's not OK when you are trying to get a lot of watts out of a limited area transformer. Looks like you should get some 260/38 for the primary Look at https://www.newenglandwire.com/newt-catalog.pdf Start at pg 130 (actual number printed on page) and see 50kHz to 100kHz #38 wire on pg 134. I have melted solder in a deep spoon on the stove to dip the end of litz wire in. YMMV. I also use http://www.powerstream.com/Wire_Size.htm For wire #, size, area, amp capacity, etc, very handy page. Mikek
P E Schoen wrote:
> > The information I found generally recommends a solder pot, which I don't > have:
For limited use, buy a cheap Chinese solder pot for under $20: 11)V: http://www.ebay.com/itm/391024009808 220V: http://www.ebay.com/itm/381739017427 -- Never piss off an Engineer! They don't get mad. They don't get even. They go for over unity! ;-)
"Michael A. Terrell"  wrote in message 
news:JJ6dnfWKaeAcpXrKnZ2dnUU7-U3NnZ2d@earthlink.com...

> P E Schoen wrote:
>> The information I found generally recommends a solder pot, which I don't >> have:
> For limited use, buy a cheap Chinese solder pot for under $20:
> 110V: http://www.ebay.com/itm/391024009808
> 220V: http://www.ebay.com/itm/381739017427
That's a good idea. I figured they weren't too expensive. The solder probably costs almost that much! Meanwhile, I was able to wind another transformer with the heavier #14 AWG equivalent Litz wire. For this I used two windings of 5 turns each, just to get an idea of how well it might work, before adding more windings. I was able to solder the ends, probably not quite completely, by setting my iron to 480C: http://enginuitysystems.com/pix/electronics/E55_Transformer_3088.jpg I made a rough little tester using an IRS2453D and some small MOSFETs, and I adjusted the frequency to 80 kHz. http://enginuitysystems.com/pix/electronics/E55_Transformer_Tester_3090.jpg I was able to run the transformer from a 30V supply with only about 50 mA open circuit current draw, so I know it will be OK for nominal 24-28 VDC for each winding, or 48-56 VDC in series. With another pair of primaries I might be able to achieve the 25-30 amps needed for close to 1500 watts. This is the output waveform on the other primary: http://enginuitysystems.com/pix/electronics/E55_Transformer_30V_3091.jpg http://enginuitysystems.com/pix/electronics/E55_Transformer_30V_3092.jpg Thanks, Paul
"amdx"  wrote in message news:ns79j7$5mn$1@dont-email.me...

> It looks like you are trying to use what you have on hand which is OK, > but it's not OK when you are trying to get a lot of watts out of a limited > area transformer.
> Looks like you should get some 260/38 for the primary > Look at https://www.newenglandwire.com/newt-catalog.pdf > Start at pg 130 (actual number printed on page) and see 50kHz to 100kHz > #38 wire on pg 134.
> I have melted solder in a deep spoon on the stove to dip the end of litz > wire in. YMMV. > I also use http://www.powerstream.com/Wire_Size.htm > For wire #, size, area, amp capacity, etc, very handy page.
I found a 150 watt solder pot for about $16: http://www.banggood.com/KLT-350-50mm-110V-150W-Solder-Pot-Titanium-Alloy-Melting-Soldering-Pot-p-1059894.html They have smaller models for as little as $9, but all others are 220V: http://www.banggood.com/search/solder-pot.html I was able to wind another transformer with the #14 AWG equivalent Litz wire, two windings of 5 turns each, and I was able to tin the leads to some extent by setting my iron to 480C. http://enginuitysystems.com/pix/electronics/E55_Transformer_3088.jpg It tested pretty good. I used an 80 kHz square wave from a 30V power supply and it drew only 50 mA open circuit with 60V P-P on the other winding as a secondary. It draws only about 10 mA at 20 volts and 20 mA at 24 volts, so I'm just about on the edge of saturation. http://enginuitysystems.com/pix/electronics/E55_Transformer_30V_3091.jpg Thanks, Paul
P E Schoen wrote...
> Michael A. Terrell wrote ... >> P E Schoen wrote: > >>> The information I found generally recommends a solder >>> pot, which I don't have: > >> For limited use, buy a cheap Chinese solder pot for under $20:
A solder pot is absolutely necessary, along with a big container of flux to dip the litz into first. I don't recommend a cheap pot. You may forget it and leave it on and you DO NOT want any trouble!! Newark sells the Tenma 21-3511 for $25. Not too expensive. -- Thanks, - Win
On 25 Sep 2016 09:31:20 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>P E Schoen wrote... >> Michael A. Terrell wrote ... >>> P E Schoen wrote: >> >>>> The information I found generally recommends a solder >>>> pot, which I don't have: >> >>> For limited use, buy a cheap Chinese solder pot for under $20: > > A solder pot is absolutely necessary, along with a big > container of flux to dip the litz into first. I don't > recommend a cheap pot. You may forget it and leave it > on and you DO NOT want any trouble!! Newark sells the > Tenma 21-3511 for $25. Not too expensive.
I've soldered smaller (26 equiv) Litz with a chunky Metcal tip. One trick is to buy thermal-strip wire, not the classic Formvar type enamel. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 25.9.16 02:03, P E Schoen wrote:

> 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.
In the tube era, about half a century ago, we handled the Litz ends: 1. Put a wad of cotton into a metal lid, 2. Pour some denaturated spirits on it, 3. Put the spirits on the wad to fire, 4. Heat the Litz end in the fire till the insulation burns, 5. Push the hot Litz end with a screwdriver to the wad, 6. Pull out sideways and tin the copper ASAP. -- -TV
John Larkin wrote...
> > I've soldered smaller (26 equiv) Litz with a chunky > Metcal tip. One trick is to buy thermal-strip wire, > not the classic Formvar type enamel.
Ahuh, litz gets interesting at sizes #44 and smaller. BTW, that's lower-case litz. -- Thanks, - Win
On 25 Sep 2016 10:21:40 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>John Larkin wrote... >> >> I've soldered smaller (26 equiv) Litz with a chunky >> Metcal tip. One trick is to buy thermal-strip wire, >> not the classic Formvar type enamel. > > Ahuh, litz gets interesting at sizes #44 > and smaller. BTW, that's lower-case litz.
Are you going to be a netcop spelling nazi when you grow up? -- John Larkin Highland Technology, Inc lunatic fringe electronics