High temperature thin dielectric or insulating coatings for bare copper coils? - Page 1
Forum section: electronics / projects

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 09, 2020, 08:03:54 am
content: I need to build an insulation layer on some bare copper coil turns. Something like a Kapton tape would work great but the gap between the coils is very small and it would be very difficult if not impossible to get the tape layer between the turns and wrap around it. A polyimide, or polyester-amide coating like that used in high temperature inverter grade magnet wire would be ideal but I am not sure how feasible or safe it is to apply in a homework shop.Some of the properties that I need are:- The copper will reach ~200 deg. C for extended periods of time (essentially continuous)- Ideally can tolerate high voltages (+600 V, depending on thickness of the insulation)- Can wick or be drawn into small gaps (< 0.005” or <0.012 mm) and only leave a thin film coating- Fairly chemically resist especially to oils- Reasonable heat transfer coefficients to try to get heat out of the coils- Safe to apply or use in a home work shop with just a basic organic vapor respirator and latex glovesIt does look like the polymer precusors for polyimide resins can maybe be purchased (this might be only in huge quanities). Has anyone applied a polyimide resin? Is it safe in this context?Any recommendations of coatings, sprays or other chemical processes to use? I found a few papers where polyimide impregnation was used to insulate coils in particle colliders but there aren't many details to the process.

author: daqq
author_rank: Super Contributor
timestamp: February 09, 2020, 09:17:27 am
content: I don't suppose you could rewind it from teflon clad cable? That would solve the isolation as well.https://www.daburn.com/ptfe-wire-2.aspxAs to anything else, well, those are some fairly nasty requirements. Maybe submerge it into oil as a whole and then put it into something?

author: jonpaul
author_rank: Super Contributor
timestamp: February 09, 2020, 09:42:45 am
content: BonjourThe wire size?Application eg heater of fluids?Flammable liquid?If you can replace the bare wire with properly insulated magnet wire use HPTZ or other triple insulated wire, available from Belden or Electrosol.Expect 10...100# minimum order and special ordersBon ChanceJon

author: trobbins
author_rank: Frequent Contributor
timestamp: February 09, 2020, 09:47:10 am
content: Is the copper coil made from a wire? Is the wire circular? What gauge wire? Is the 600V rating related to turn-to-turn withstand, or layer to layer, or ? Is there a hot-spot situation in the coil (eg. due to multiple layers)?

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 09, 2020, 04:30:26 pm
content: Quote from: jonpaul on February 09, 2020, 09:42:45 amI haven't finished designing the coil but the wire size will be ~1.5 cm by ~2.5 cm in a non-uniform shapeQuote from: jonpaul on February 09, 2020, 09:42:45 amThis is going into a motor so the coil will fit around a segmented toothQuote from: jonpaul on February 09, 2020, 09:42:45 amThe most likely coolant is sprayed automatic transmission fluid (ATF) or ATF flowed through a intersitial channel in the coilQuote from: jonpaul on February 09, 2020, 09:42:45 amI am specifically not trying to use magnet wire so I can have non-uniform profiles of the conductor but I have't figured out yet how to do the insulative coating.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 09, 2020, 04:43:05 pm
content: Quote from: trobbins on February 09, 2020, 09:47:10 amThe copper coil is made from a bar that has been shaped and formedQuote from: trobbins on February 09, 2020, 09:47:10 amIt has not been made from traditional magnet wire, either circular, square, or rectangular. There isn't an exact gauge because of the non-uniform profile but the cross-section dimensions are ~1.5 cm by ~2.5 cm.Quote from: trobbins on February 09, 2020, 09:47:10 amThe motor that these coils are going into will be fed by an inverter with a 600 Vdc link. There will be approximately 4 to 5 turns per coil with the voltage distributed approximately evenly over the coils. Ideally the voltage withstand capability will be higher than 600 V to have a double insulation rating even with the slot liner insulation.Quote from: trobbins on February 09, 2020, 09:47:10 amThe middle turns and the portion of the turns located in the middle will be getting the hottest as I am going to spray the ends of the coils with ATF to extract heat and the non-sprayed portion of the coils will conduct heat to the end turns. I have done this type of cooling before and it is quite effective even at vary high current densities (> 30 A/mm^2).

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 09, 2020, 04:45:49 pm
content: Quote from: daqq on February 09, 2020, 09:17:27 amYes, the requirements aren't easy. This is for a unique motor applications and I need a non-uniform profile along the conductor length so regular magnet wire or teflon clad cable is out. The motor is going to be cooled by spraying ATF on the end turns which is very effective. Submerging it would also be effective but I would have too high a drag losses from the rotor.

author: jonpaul
author_rank: Super Contributor
timestamp: February 09, 2020, 05:39:35 pm
content: Check the flammibility and electrical property of the cooling fluid, also réactions with any insulationJon

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 09, 2020, 05:56:30 pm
content: Look up the specs for Glyptol insulating varnish. I know people have used it to coat the insides of race engines and it certainly has a long history in electronics. Comes in red and clear, not sure what else. The thermal conductivity of all insulators that you can safely use is poor. It's just the nature of the materials.

author: Gyro
author_rank: Super Contributor
timestamp: February 09, 2020, 05:58:12 pm
content: Glass fibre sleeving?If it's for motor use (a high vibration, high force application) then you need something to prevent winding 'shuffle'' - tight wedging, high temperature vacuum encapsulation or both. The porous glass fibre would help you there.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 09, 2020, 07:36:56 pm
content: BTW, can you vacuum impregnate? That helps fill everything for mechanical stability and eliminates air pockets that can encourage arcing.

author: duak
author_rank: Super Contributor
timestamp: February 09, 2020, 10:02:13 pm
content: Glyptal 1201 is the original product used in electrical equipment and engines but it's only good for 130 C.Motors can be rated for class H which is 180 C winding temperature. Here's a description:"Class H insulation consists of materials such as silicone elastomer and combinations of materials such as mica, glass fibre, asbestos etc., with suitable bonding, impregnating or coating substances such as appropriate silicone resins. Other materials or combinations of materials may be included in this class if by experience or tests they can be shown to be capable of operation at the Class H temperature."Glyptal makes high temperaure silicone based products but it seems to have aluminum powder mixed in to give it a particular color. It probably does wonders for thermal conductivity but would also make it useless for electrical insulation. I'll bet someone does make a high temp insulating silicone - just a matter of finding it.I wonder if the windings could be packed in silicone thermal grease and then cointained so that the ATF doesn't wash it away?Best o' luck,

author: trobbins
author_rank: Frequent Contributor
timestamp: February 09, 2020, 10:08:21 pm
content: Are multiple coils (each of 4-5 turns) located in a slot?Is the length of turn so long that you really do need thermal transfer in between turns to conduct heat from an enclosed turn? Due to the cross-sectional area of a turn, it appears most heat transfer would be occurring along each copper turn to where you are actively cooling the copper at the ends of the rotor, as compared to through the slot liner, or through presumably an air gap to the stator?With that size of copper, it seems a bit strange from a packing efficiency perspective that you are then requiring such a thin insulation covering.Are there AC losses in the coil due to some form of switchmode generation of currents? If so, then is that a significant loss contributor due to skin and proximity effects?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 10, 2020, 02:08:52 am
content: There are ceramic coatings that can be painted on and fired, also plasma sprayed. I'd start here-https://www.aremco.com/

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 10, 2020, 06:31:48 am
content: Quote from: Gyro on February 09, 2020, 05:58:12 pmUnfortunately, glass fiber sleeving is too thick and won't easily go over some of the non-uniformities of the coil. I am trying to get a really high slot fill.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 10, 2020, 06:33:07 am
content: Quote from: Conrad Hoffman on February 09, 2020, 07:36:56 pmYes, I can probably vacuum impregnate. If anyone has any suggestions for vacuum impregnating polyimides I would like to hear their experiences.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 10, 2020, 06:35:15 am
content: Quote from: duak on February 09, 2020, 10:02:13 pmActually, I am going for a insulation temperature rating higher than class H, 200+ deg. C. Magnet wire with a higher insulation temperature class is pretty easy to source from Superior Essex or REA.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 10, 2020, 06:44:12 am
content: Quote from: trobbins on February 09, 2020, 10:08:21 pmYes in several of the winding configurations that I am looking at (dual layer fractional slot concentrated windings).Quote from: trobbins on February 09, 2020, 10:08:21 pmQuote from: trobbins on February 09, 2020, 10:08:21 pmQuote from: trobbins on February 09, 2020, 10:08:21 pmI need to get the maximum packing factor possible. This is a very high performance machine I am targeting. Every little bit of copper I can stuff into the slot helps.Quote from: trobbins on February 09, 2020, 10:08:21 pmYes, there will be increased AC resistance just because of the high frequency AC excitation (skin and proximity effects). The PWM harmonics will also add additional losses. I am going to try to build in some features to the coil to mitigate these losses.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 10, 2020, 06:45:02 am
content: Quote from: Conrad Hoffman on February 10, 2020, 02:08:52 amThanks for the tip.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 10, 2020, 07:02:39 am
content: Quote from: magicsmoke on February 10, 2020, 06:44:12 amWhat is your anticipated Rac/Rdc for a coil in the total configuration, and is that consistent with previous test rotors using the same drive waveforms? It seems strange that you haven't described this issue, and only detailed a pedantic need to get the last 0.01% DCR reduction by maxing out the packing factor.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 10, 2020, 09:34:20 am
content: Look at a German company, called Elektrisola.They make insulated wires of the best quality and for special applications, they have odd cross-section geometry shapes.https://www.elektrisola.com/home.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2020, 04:33:07 pm
content: Sheesh, so what the hell is this thing? It's no regular hybrid/EV, that's for sure. Mining equipment? CAT, Komatsu, etc.? [Rhetorically. Not looking for a brand, and I suppose you'd rather not say who, which is fine.]Some big bastard in any case.Why ATF, why not coolant (~100C?)? Is the efficiency loss, due to the high operating temperature, really worth the extra what, 0.5% of conductor you're trying to squeeze in here? Is it already low-oxygen copper? How about silver? (Okay probably not, not at these scales...) How about... GOSS or nanocrystalline core material, instead of regular silicon steel? (Alas, probably not feasible in motor stampings. Also the latter might end up as expensive as silver conductors would...)Anyway, regarding insulation, I know there's some kind of silicone based enamel that goes over 200°C. There are power resistors with such stuff. It's hard and brittle, almost ceramic (and for that matter, probably decomposes and sinters into ceramic if you simply heat it high enough), and probably not practical in the thin layers you're looking for.Really, the amount of sliding, the potential for scraping and chafing, rules out a lot of things already. Like, even hard anodize aluminum (not that you can hard anodize copper anyway; heh, Al-clad then anodize?...ehhh...) isn't going to be all that reliable, shoving it into slots in silicon steel.At least, that is, if assembly will be done in the usual way. If you have a different method in mind, like, the slots are slightly oversized, then after assembly, the laminations are crimped over the bars somehow, that might be cool. No idea. If you already have an assembly and inspection process pinned down, do disregard me.There's also high temp epoxies, but I'm not sure how well they do quite that high, and give or take peak temps. You probably want something with a Tg above operating temp, so it retains strength and doesn't expand much. Shop around, see if you can find anything interesting, call up mfg reps. This is, after all, very much an industrial grade application, in need of an industrial grade solution; reps are still the way to go.Also also, what processes are applied to the ends? -- I've seen these things before, they braze the bars to cross pieces to construct the turns. (At least, a local generator brand does it this way.) Which puts closer to 500°C on the bars, towards the ends of the slots. That'll damage even polyimide*, and decompose most silicones.*Just barely. It can actually handle temps like that, momentarily (seconds?), but I'm guessing the bonding process will spend some minutes hot, so that kind of stinks. AFAIK, polyimide deteriorates by becoming more brittle, and probably carbonizing a bit, neither a great look for high voltage insulation. At least it would be after insertion force is applied, though.If nothing else, I would definitely feel more comfortable if you can afford the space to fit fiberglass or mica paper wrapping, and an impregnation step doing something with a high temp resin, with probably a high mineral fill content* as well.*Incidentally, semiconductor packaging for example is something like 80-90% silica fume, which is how it gets a higher CTI, and such low expansion rate. Not quite directly applicable here I think, as the resins top out around Tc(max), 150-175°C, so, not much over 200. They can sustain soldering temps though. Maybe someone makes an even higher temp version?Tim

author: mzzj
author_rank: Super Contributor
timestamp: February 10, 2020, 07:12:45 pm
content: https://acc-silicones.com/products/encapsulants/QSil550Thermally conductive silicone with 275cel max temp

author: Gyro
author_rank: Super Contributor
timestamp: February 10, 2020, 07:25:12 pm
content: If the OP is talking bare copper, then aren't we looking at more than just thermally conductive encapsulants. Surely something is still needed to protect against shorted turns.If glass fibre is too thick, then what?

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 12, 2020, 12:41:10 am
content: Quote from: T3sl4co1l on February 10, 2020, 04:33:07 pmActually it is for an EV traction application.Quote from: T3sl4co1l on February 10, 2020, 04:33:07 pmATF is pretty much standardly used for spray cooling in EV traction applications. In many cases you already have a transmission using ATF so you might as well use it for cooling the motor to. Ethylene glycol/water is regularly used for cooling jackets and for the power electronics cold plate but there is a risk in spray cooling situations because of its conductivity. For this reason EG5050 is not generally used when spraying the end turns. There are other dielectric liquids which can be used but ATF is probably the most common in vehicle applications.Actually the slot fill is really one of the main places that the motor torque density can be increased. Using random windings you maybe get to ~45% gross slot fill and with specialized winding technology to ~50%. Hairpin or bar winding to ~60 to 70%. The higher slot fill roughly translates into either lowerer losses or the ability to create a higher magneto-motive force.Quote from: T3sl4co1l on February 10, 2020, 04:33:07 pmThe assembly is an issue but I am not worrying about that for now as I have ideas how to approach it through segmentation of the laminations that I have used for other projects. I don't want to constrain ideas regarding the insulation.Quote from: T3sl4co1l on February 10, 2020, 04:33:07 pmThe copper is formed as a single piece so no need to braze the end turns on. Though that is a possibility. The end turns could be then dipped in an insulator. This is somewhat similar to how they make the end turn connections on one side of hairpin or bar windings.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 12, 2020, 12:42:22 am
content: Quote from: mzzj on February 10, 2020, 07:12:45 pmThanks for the link!

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2020, 12:39:00 am
content: I would not even try it without using a vacuum during application.My own experience is that a thick application will crack the insulating material or worse unless the temperature coefficient and elasticity are suitable. Potting compounds tend to be very elastic which will not be suitable in this case. Sometimes a more compliant base coat is used before a hard potting compound.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 13, 2020, 06:06:19 am
content: I actually found something that might work for the turn to turn insulation (CTD-201 Polyimide Coating,https://www.ctd-materials.com/wordpress/wp-content/uploads/2014/05/CTD201-Polyimide-Coating-December-2013.pdf). It looks like it is sprayable and suitable for complex shapes. Not sure if it can be made to wick inside which as David mentioned probably requires a vacuum system.

author: magicsmoke
author_rank: Regular Contributor
timestamp: February 13, 2020, 06:07:34 am
content: Quote from: David Hess on February 13, 2020, 12:39:00 amHave you tried anything like this with a jury rigged/non-professional vacuum or VPI system?

author: trobbins
author_rank: Frequent Contributor
timestamp: February 13, 2020, 11:02:56 pm
content: Quote from: magicsmoke on February 10, 2020, 06:44:12 amHave you estimated Rac/Rdc for the operating waveform? I can appreciate you may be able to modify proximity losses, but not skin effect.If effective Rac/Rdc is more than say a few % increase, or you can't confidently model and do some form of prototype test confirmation of AC losses, then I can't see any merit in maxing out the single conductor copper area to aspire to a max slot fill %, to the detriment of a myriad of foreseen and unforeseen risks. Perhaps you've looked at other practical conductor structures and assessed that a single conductor is the best outcome for your operating current waveform.

author: David Hess
author_rank: Super Contributor
timestamp: February 14, 2020, 04:40:03 am
content: Quote from: magicsmoke on February 13, 2020, 06:07:34 amI have always done it with a random vacuum pump and vacuum chamber.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 15, 2020, 12:30:58 am
content: Are you derating the current capability of the copper for the higher operation temperature?Are you accounting for the higher resistance of the copper at elevated temperatures?