Power Transformers: The Lost Art Of Custom Winding - BSFEEChannel #5

[bsfeechannel]

Well, guys, now it is time for testing out all the boring theory and design considerations of our previous videos. I hope this helps those involved somehow in power transformer designs and/or specifications. As always, comments are welcome.

[cvanc]

Interesting stuff and a nicely made video, thank you for posting.  But why the synthesized voice?

[T3sl4co1l]

Interesting stuff and a nicely made video, thank you for posting.  But why the synthesized voice?

Probably a fan of ChemPlayer

Tim

[Cerebus]

Yup, that synthesised voice is annoying enough that I could only stand 2¹/₂ minutes of it.

It's worth noting that kraft paper is neither good insulation, nor is it non-flammable. I would strongly recommend not following this as an example but instead using polyester insulation or 'fish paper' that is specifically designed for this purpose.

Also, there should be some clearance space left between the bobbin and winding on each layer of the winding to meet the necessary insulation values for clearance and creepage and it should be filled with insulation. As it stands the clearance and creepage distances between one winding and the next are going to be tiny (something on the close order of the radius of the smaller wire) and the 'isolation' provided by this transformer is going to be minimal.

As I say, I only watched 2¹/₂ minutes of it, so don't assume that the rest is kosher

[bsfeechannel]

But why the synthesized voice?

Thank you for your interest.

I say a lot of ahs and ums all the time, which disrupts the fluency of the video. To reduce the time of audio editing I hand the talking to the computer.

 

[bsfeechannel]

It's worth noting that kraft paper is neither good insulation, nor is it non-flammable. I would strongly recommend not following this as an example but instead using polyester insulation or 'fish paper' that is specifically designed for this purpose.

Yes. I agree absolutely. That is why I stress that this is not a step-by-step diy video. Using paper as an insulator will make yours what is called a NEMA/UL Class A insulation device. Its maximum hot spot operating temperature must not exceed 105ºC. I confess I am not comfortable with using paper. It tends to attract moisture and lose its insulating properties with time. To minimize that effect, after this video I potted the whole transformer with insulating varnish, which is a standard procedure in this case.

But, as the title of the video implies, this is a "lost art". I could only find instructions in old books of yore and they recommended paper. I decided to test how good it would perform on this one-off even knowing that modern materials like polyester, which I use, are employed.

Also, there should be some clearance space left between the bobbin and winding on each layer of the winding to meet the necessary insulation values for clearance and creepage and it should be filled with insulation. As it stands the clearance and creepage distances between one winding and the next are going to be tiny (something on the close order of the radius of the smaller wire) and the 'isolation' provided by this transformer is going to be minimal.

Do you think a split bobbin would solve that problem?

As I say, I only watched 2¹/₂ minutes of it, so don't assume that the rest is kosher

Too bad you could not stand my robotic voice. I am sorry for that. At the end of the video I check whether our xformer will take the "heat" or burst into flames.

[johansen]

no lost art, just lack of knowledge.

you want a transformer?

give me the minimum efficiency you need at the nominal load.

the rest of the details i could care less about (if they are fixed, not variable)

i might say: if you make it 2% less efficient i could give it to you for less than a tenth of the price.. and you might be astonished.. when in reality, it would have cost a twentieth of the price to re-design the rest of the product not to require one!

[GeorgeOfTheJungle]

Thank you bsfeechannel. I liked the video, gave it a thumbs up and subscribed to the channel. The robotic voice is not exactly like AvE's, but that's alright I don't care.

[Cerebus]

Do you think a split bobbin would solve that problem?

No, the issue is the creepage/clearance path from the wire at either extreme of the winding, past the insulating layer, to the wire at the edge of the next layer of the winding under the insulating layer. The standard solution on small run transformers is to just run a narrow strip of insulation (e.g. adhesive polyester insulating tape) between the edge of the winding and the edge of the bobbin, using enough layers to match the thickness of the winding layer. Obviously there are more 'engineered' solutions for when you're making 10,000 transformers at a time.

Remember, you're not just insulating the primary from the secondary but also the layers of each individual winding, a split bobbin will only deal with the issue of primary to secondary insulation. There can be a significant potential difference between one layer and the next of a single winding; that is, you can think of each winding as an autotranformer with 'taps' at the points where the individual layer windings end. Sure, if your winding has three layers and 30V total, that's only 10V a layer and highly unlikely to cause insulation breakdown or tracking problems, but if there's a mechanical failure of the insulation, say vibration wearing off the enamel and the edge of the insulator, that's 10V potential across a small contact resistance and that spot is going to get hot fast. Add a buffer zone of more insulation between the winding and bobbin and it's much less likely to happen.

[Cliff Matthews]

I wish the author had given comments before I shelved my isolation transformer project. https://www.eevblog.com/forum/projects/diy-1kva-isolation-transformer/

[cdev]

A related transformer question.

Does anybody know the easiest method to cure old transformers which have dried out and hum, short of total immersion in a bath of shellac with a vacuum pump?

Specifically I've read that one may be able to drop just a few drops of "methylated spirits" on one (which is called denatured alcohol in the US), then let them dry out for a good amount of time and that that may help cure the hum.

As I've only read this once, from one source I don't know if I should trust it. (I have an old -1980s- Sorensen/Xantrex XT triple power supply which works well, but it has three large power transformers and an annoying hum I would like to cure.)

[bsfeechannel]

No, the issue is the creepage/clearance path from the wire at either extreme of the winding, past the insulating layer, to the wire at the edge of the next layer of the winding under the insulating layer. The standard solution on small run transformers is to just run a narrow strip of insulation (e.g. adhesive polyester insulating tape) between the edge of the winding and the edge of the bobbin, using enough layers to match the thickness of the winding layer. Obviously there are more 'engineered' solutions for when you're making 10,000 transformers at a time.

Remember, you're not just insulating the primary from the secondary but also the layers of each individual winding, a split bobbin will only deal with the issue of primary to secondary insulation. There can be a significant potential difference between one layer and the next of a single winding; that is, you can think of each winding as an autotranformer with 'taps' at the points where the individual layer windings end. Sure, if your winding has three layers and 30V total, that's only 10V a layer and highly unlikely to cause insulation breakdown or tracking problems, but if there's a mechanical failure of the insulation, say vibration wearing off the enamel and the edge of the insulator, that's 10V potential across a small contact resistance and that spot is going to get hot fast. Add a buffer zone of more insulation between the winding and bobbin and it's much less likely to happen.

I understand that the last turn of each layer will be very close to  the first turn of the previous layer and that the potential difference will be equal to the sum of the potential of two consecutive layers. It would be nice if we could provide some means of avoiding these two turns getting into physical contact with each other for the reasons you pointed out.

But I  need to see an example of what you mean to really grasp the concept of the solution you propose. Please pardon my ignorance.

MOTs come to my mind now. They are high voltage transformers (in excess of 2kV) but you almost never find insulation between their winding layers. They don't even have a bobbin. Just pieces of impregnated press span paper separating the windings from the core.

[T3sl4co1l]

I've taken apart transformers like that, before -- they use a special "spacing" tape, which is also permeable (plastic or glass fiber felt, as it were), to make each winding layer into a sort of cavity where the wire rests.  This keeps it away from the inside face of the bobbin, meanwhile the insulating tape is able to span full width across the bobbin (because it extends over the filler tapes), giving full creepage insulation.

Varnish is also considered an impregnant compound, which eliminates creepage.  However, because it's solvent filled, it easily leaves voids, so one must inspect the parts using suitable methods (hi-pot, AC test, partial discharge), which may be more expensive than simply making the design oversize.

Resin-coated wire, baked into a rigid form, is how MOTs are constructed.  The voltage between any pair of wires is relatively small, due to the consistent layering and high aspect ratio.  The creepage around the outside is sufficient, because the wire is triple insulated (enamel + resin + varnish; though I would not expect any of those three to actually withstand direct application of 2kV, so this is kind of specious) and the height of the winding is adequate.

Hmm, offhand I don't know of any rules for trading insulation against creepage.  I think insulation always has to be adequate (thick enough to meet the rules, and void free), even if some amount of air gap is guaranteed to be present, otherwise it counts as nothing.

Tim

[bsfeechannel]

no lost art, just lack of knowledge.

Precisely. I should have placed lost art inside quotation marks.

you want a transformer?

give me the minimum efficiency you need at the nominal load.

the rest of the details i could care less about (if they are fixed, not variable)

i might say: if you make it 2% less efficient i could give it to you for less than a tenth of the price.. and you might be astonished.. when in reality, it would have cost a twentieth of the price to re-design the rest of the product not to require one!

Simplicity that comes from mastering complexity is what I call wisdom.

I started this series of videos on power transformers because I used to specify my custom transformers to a local shop and some of them did not perform as I expected. When I discussed the parameters with their tech person the solution usually involved a much bulkier and more expensive transformer. I was not sure if they were skimping on materials, or trying to rip me off, or both.

I then realized I did not understand exactly what influence each parameter of a transformer had on its performance. In fact I did not understand exactly why transformers really work.

The series is me trying to teach myself how to properly design a power transformer, at least at a basic level. And in this video I check what I learned.

In the words of William Stanley Jr., one of the pioneers of the power transformer: "[A transformer] is such a complete and simple solution for a difficult problem. [...] It handles with such ease, certainty, and economy vast loads of energy that are instantly given to or taken from it. It is so reliable, strong, and certain."

Indeed.

[bsfeechannel]

I've taken apart transformers like that, before -- they use a special "spacing" tape, which is also permeable (plastic or glass fiber felt, as it were), to make each winding layer into a sort of cavity where the wire rests.  This keeps it away from the inside face of the bobbin, meanwhile the insulating tape is able to span full width across the bobbin (because it extends over the filler tapes), giving full creepage insulation.

Now you Mr. Cerebus got me very curious about this expedient, since I have never seen it in transformers I have come across. I will certainly pay more attention to this detail from now on. Thank you.   

[Cerebus]

I've taken apart transformers like that, before -- they use a special "spacing" tape, which is also permeable (plastic or glass fiber felt, as it were), to make each winding layer into a sort of cavity where the wire rests.  This keeps it away from the inside face of the bobbin, meanwhile the insulating tape is able to span full width across the bobbin (because it extends over the filler tapes), giving full creepage insulation.

Now you Mr. Cerebus got me very curious about this expedient, since I have never seen it in transformers I have come across. I will certainly pay more attention to this detail from now on. Thank you.

I think that particular gem was down to Tim, actually; although I outlined the problem that particular solution fits.

In fact I did not understand exactly why transformers really work.

"Paging Prof. Maxwell; James Clark Maxwell to the lobby phone please."

It's not quite clear if you feel that you still have something to learn about the basic "why" of how they work (as opposed to the practicalities). If you do, I can heartily recommend the series of lectures on You Tube that Prof. Walter Lewin gave on electricity and magnetism for this MIT 8.02 physics class. Highly entertaining and informative with great demonstrations (worth the price of admittance in themselves) and the clearest guided journey through Maxwell's equations that I have ever seen.

If you can tell someone what Lenz' law is, and how it's crucial to making a transformer work and stops them from catching fire seconds after you plug one in, then you probably know enough not to need to watch Lewin's lectures, but I'd still recommend them for the entertainment value.

[Cliff Matthews]

Series playlist here: https://www.youtube.com/playlist?list=PLyQSN7X0ro2314mKyUiOILaOC2hk6Pc3j

[bsfeechannel]

Now you Mr. Cerebus got me very curious about this expedient, since I have never seen it in transformers I have come across. I will certainly pay more attention to this detail from now on. Thank you.

I think that particular gem was down to Tim, actually; although I outlined the problem that particular solution fits.

I meant "You AND Mr. Cerebus". Sorry for the typo.

"Paging Prof. Maxwell; James Clark Maxwell to the lobby phone please."

It's not quite clear if you feel that you still have something to learn about the basic "why" of how they work (as opposed to the practicalities).

I had to go as far as to revise what I thought I knew about calculus [hence my past video on the subject], Maxwell and related stuff, which I saw years ago at college.

One thing that bugged me is how exactly does the primary "know" that a current is flowing through the secondary? The standard explanation is that to maintain the principle of conservation of energy the primary reacts with a current to satisfy that condition. That is true, but what is the mechanism behind that reaction that gives support to the principle? The answer, although obvious, was not conspicuous to me and it is described in my previous video "Power Transformers: A Simple Circuit Model".

Then it came to the practicalities. I actually received training on how to build a transformer, but it was so many moons ago that I could not remember many details. Plus standards and practices evolve with time. That is why I am excited to know that modern techniques are devised to tackle them.

My goal is not to become an expert in building power transformers, but at least to know what I am talking about, and occasionally build myself one if suitable.

If you do, I can heartily recommend the series of lectures on You Tube that Prof. Walter Lewin gave on electricity and magnetism for this MIT 8.02 physics class. Highly entertaining and informative with great demonstrations (worth the price of admittance in themselves) and the clearest guided journey through Maxwell's equations that I have ever seen.

If you can tell someone what Lenz' law is, and how it's crucial to making a transformer work and stops them from catching fire seconds after you plug one in, then you probably know enough not to need to watch Lewin's lectures, but I'd still recommend them for the entertainment value.

Oh yes, I watched his videos on youtube as part of my path in search for "enlightenment". Fantastic!

[Cerebus]

...they use a special "spacing" tape, which is also permeable (plastic or glass fiber felt, as it were)...

I was going to comment on this a day or two ago but decided to hold off until I could find some photo examples or similar. The finest examples of these tapes are made of Nomex. So, just now I hit ebay and searched for "Nomex tape". I didn't find any of the 'collared' variety you were discussing but I did find the plain variety, including a box of 10 reels, 3cm x 33m of Nomex transformer insulating tape going for a buy-it-now of £5 which I immediately snapped up. So I'm feeling very smug at getting an instant reward for taking the time to try and be helpful.

Here's an image of the kind of insulating scheme we were talking about. If one looks carefully one can see the gaps between the end of the windings and the bobbin.

[ace1903]

Seems that I parts are not installed at all. Newer saw transformer that has only E type laminate.
Think that without I part parameters will be terrible. Checking isolation with instrument on 200 ohms range 
is also pointless to me. Maybe I didn't catch all the details but this doesn't look as "Lost Art" to me.
I expect this to result in power heater instead of transformer.
Would be nice to see final measurement on parameters and noise that this transformer will produce.

[Cliff Matthews]

Quote from: ace1903 on Today at 02:02:40 PM

Seems that I parts are not installed at all...

See 6:30 and give a little credit. Without I lamination's, L frame would clamp and bend inward top and bottom.*edit (video res is only 480p, but testing images look like the I-lams got hammered-in off camera)

[T3sl4co1l]

FWIW, I have seen I-less transformers before.  It's fine as long as you don't mind somewhat higher losses*, and half the A_e (which would be a total killer for power transformers, but these were made for moderate frequencies, signals or pulses, so less of a problem).

*The losses would be higher because, at the edge of one lamination, where it would normally be butted against an 'I', the field has to fringe sideways into the lams sandwiching it.  The divergence concentrates losses, and the through-plane component in the field direction induces eddy currents in the sheet area.  Anisotropic materials (e.g. GOSS) may also incur higher losses, although, as GOSS cannot be fully oriented in stampings, there's a lot of sideways field anyway and the effect will be small.  (GOSS is best used in a stripwound configuration, which for an E bobbin windup, is made with alternate shells of strip that clasp around the bobbin.  Alternately, cut cores are used.)

Tim

[ace1903]

Indeed measurements are OK, seems that video does not include part where I segments are inserted.
Hammering I parts is not good practice in something that pretends to be Art example.
It is up to up-loader to give us answer if I parts are inserted or not.
Nice video but sintetized voice makes difficult to follow. I missed lot of details due to that.
 

[bsfeechannel]

The I's are not shown because I forgot to turn on the camera (Doh!). But I can guarantee that no laminations were harmed in the making of this episode. They were inserted as gently as the E's shown.

The hammer is made of rubber and it is just used to tap gently around the core to set the laminations in place, as the last laminations have to be tightly inserted. You don't want your transformer buzzing like a bumblebee.

As for the voice, I will see if Dave is available for a job as a narrator. His high pitched voice will surely cut through the mix.

[bsfeechannel]

Here's an image of the kind of insulating scheme we were talking about. If one looks carefully one can see the gaps between the end of the windings and the bobbin.

Let me see if I've got this straight.

You leave a margin on both sides of the winding and apply various layers of some kind of filler tape (nomex) to fill the space. The tape must be narrow enough to fit into the margin and must be wound so that its height is equal to the diameter of the wire for the first and last wire layers and twice as much for the intermediary layers. The insulating layer must then be applied from the bobbin "wall" up to the end of the wire layer and then the next wire layer must be of course wound on to the top of that insulating layer until it reaches the margin again. And so on and so forth.

I attached a drawing if my writing be confusing.

[Cliff Matthews]

Grab some old disposable SMPS pulse transfomer and pull the tape off. You'll probably see what your looking for along the sides of the bobbin where the leads come out (like double-sided sponge tape). IMO it would seal out moisture too.

[Cerebus]

Yup, exactly like that. Nomex is scarily expensive and so used for the very highest temperature classes only, you can get a 180 C rating with cheap, easy to find polyester insulation. You're looking for a creepage path of around 4mm on most common classes of transformers, so 2mm each side of the main insulating layers between the bobbin and winding for 2k5 V isolation (from memory). See standards documents for figures applicable to one's target usage (i.e. EN 90950 for computer and telecoms equipment).

[T3sl4co1l]

"Margin tape" gets a lot of hits, "filament reinforced electrical tape" seems to be a good description as well.

3M type 46 seems to be the stuff, https://solutions.3m.com/wps/portal/3M/en_US/ElectricalOEM/Home/Products/ProductCatalog/~/3M-Filament-Reinforced-Electrical-Tape-46?N=5561499+3293718001&rt=rud
and 44.  Looks like Permacel and P. Leo are also popular mfgs.

Tim

[bsfeechannel]

Grab some old disposable SMPS pulse transfomer and pull the tape off. You'll probably see what your looking for along the sides of the bobbin where the leads come out (like double-sided sponge tape). IMO it would seal out moisture too.

Yes. I have some computer SMPSs lying around here doing nothing. I think I will sacrifice one of them to the gods of knowledge. Thank you for the tip.

As a side note, SPMS transformers are subject to high voltages due to their switched mode of operation (The sudden interruption of current across its primary for each cycle produces high voltage spikes). It makes a lot of sense, now that you mentioned SMPS, to provide that kind of protection, even with the use of RC snubbers and flyback diodes.

Yup, exactly like that. Nomex is scarily expensive and so used for the very highest temperature classes only, you can get a 180 C rating with cheap, easy to find polyester insulation. You're looking for a creepage path of around 4mm on most common classes of transformers, so 2mm each side of the main insulating layers between the bobbin and winding for 2k5 V isolation (from memory). See standards documents for figures applicable to one's target usage (i.e. EN 90950 for computer and telecoms equipment).

That's what I thought. Thank you for calling my attention to that aspect. That is one technique I will try to employ on my next project.

"Margin tape" gets a lot of hits, "filament reinforced electrical tape" seems to be a good description as well.

3M type 46 seems to be the stuff, https://solutions.3m.com/wps/portal/3M/en_US/ElectricalOEM/Home/Products/ProductCatalog/~/3M-Filament-Reinforced-Electrical-Tape-46?N=5561499+3293718001&rt=rud
and 44.  Looks like Permacel and P. Leo are also popular mfgs.

Tim

Cool! Following your tip, I googled that too and found this:



Seems to go along with the lines of what y'all been trying to say.

[johansen]

yeah that's basically how every power transformer has been wound for a long time.

but not only is it for clearance and creapage but the space between the wire and the core is also there to reduce the capacitance in some cases

it may also be there to keep the wire away from problem areas, such as air gaps

for example, read here https://engineering.dartmouth.edu/inductor/shapeopt.shtml

[tautech]

I'll drop this link in this thread for the OP's reference............and others.
Practical transformer winding
http://ludens.cl/Electron/trafos/trafos.html