Rotary transformer for power transfer to moving shaft - Page 1
Forum section: electronics / projects

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 16, 2021, 10:52:17 am
content: I'm currently looking at ways to get power into a rotating (30rpm) chandelier for a 1-off project. We're concerned about long-term wear issues with slip-rings ( as well as the high cost of good quality ones).I'm thinking about using a rotary transformer, but don't know much about magnetics.Requirements :Max size about 65mm dia x 50mm highPower about 100W, continuous duty.Output voltage ideally 48ish, and load regulation of about 25% would be OKInput voltage : completely flexible.Don't care about efficiency as long as nothing melts!One possibility is Medum power Qi, which can supposedly do up to 65W, but this looks like being at the low end of what we need, and there doesn't seem to be much hardware out there above 30W at the moment.Suitable magnetics seem a bit thin on the ground.Something like this would be perfect, but struggling to find anything off-the-shelf at a sensible priceThe next best thing would seem to be a standard circular pot-core like thisObviously the gaps at the sides would have some effect, but how much ?What sort of core material should I be looking at?I'm thinking frequency in the 50kHz range ?The gap would be as close as we can get within mechanical tolerances - guessing 1-2mmIt would be nice to avoid the need for any feedback for regulation ( i.e. output determined by transformer ratio) - is this realistic for, say, 25% regulation?This paper has some useful info, though I don't understand teh function of L1/CR1 in fig. 1909https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_19.pdf[ Specified attachment is not available ]Any thoughts welcome.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 16, 2021, 11:36:58 am
content: Normal pot cores will probably work pretty well for this.The rotary transformer will likely have a lower (and somewhat variable) magnetizing inductance compared to a traditional transformer.You can reduce the required reactive energy that the bridge needs to provide by using a resonant topology, but dealing with parameter variations in the transformer, both due to manufacturing tolerance and due to rotation, movement and temperature is not a trivial task, so I would suggest starting with a non-resonant scheme to begin with.The simplest approach that is likely to work well is a plain isolated charge pump. Drive the primary from a full bridge, and rectify the secondary into a capacitor. Load regulation is given mainly by the leakage inductance of the transformer, but 25 % should be achievable. The easiest is to run it open loop, note that the output voltage might rise at light loads, mandating some minimum loading. You can close the loop by sensing the secondary voltage and regulating the supply voltage to the primary bridge. Duty cycle will have a small impact on the output voltage.The next step up would be adding an inductor between the secondary side rectifier and the cap, making it into a forward converter. Load regulation will likely be worse open-loop, but now you can regulate the output voltage by changing the duty cycle of the primary bridge, saving the pre-regulator in case you want to run it with closed loop voltage regulation.50 kHz seems to be a reasonable operating point, depending on your desired input voltage. Most normal power grades of ferrite will be good for this. N87 or equivalent is a classic. There are better grades available these days, but maybe not in the pot geometry. Regular component distributors often have an enormous markup on ferrite cores, I usually get them from a dedicated magnetics supplier like semic.info these days.

author: langwadt
author_rank: Super Contributor
timestamp: September 16, 2021, 11:53:58 am
content: https://nanopdf.com/download/rotating-transformer-for-a-wound-rotor-synchronous-motor_pdf?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 16, 2021, 11:56:59 am
content: Quote from: Wolfram on September 16, 2021, 11:36:58 amThe reason I want to avoid feedback is that any feedback has to pass back over a rotating joint! Obviously there are ways to do this but want to keep it as simple as possible. the output will eventually be going into a DC-DC buck converter down to 5v or so ( after passing through a resistive structure, hence 48-ish volts) , so regulation isn't a huge issue.There will be some minimum load of a few watts

author: voltsandjolts
author_rank: Unknown
timestamp: September 16, 2021, 12:45:45 pm
content: Devil's advocate: Sounds like fun, but could it be a bit too much faff for a one off unit?https://www.bgbinnovation.com/online-shop.slipringpackage/spb14-6-way-300mm-26awg/https://www.ebay.co.uk/itm/153597143918Off the wall idea; maybe a car alternator could be the slip rings and motor in one unit.https://hackaday.com/2020/01/16/car-alternators-make-great-electric-motors-heres-how/

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 16, 2021, 12:49:43 pm
content: Perhaps start with a SSTC design, but with a secondary that has a few turns with a parallel cap?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 16, 2021, 12:55:11 pm
content: Quote from: voltsandjolts on September 16, 2021, 12:45:45 pm

author: voltsandjolts
author_rank: Unknown
timestamp: September 16, 2021, 01:07:14 pm
content: True, but alternator slips rings are robust for thousands of hours at 2000rpmhttps://www.ebay.co.uk/itm/274884305266Anyway, I'll shutup now!

author: Cerebus
author_rank: Super Contributor
timestamp: September 16, 2021, 01:57:31 pm
content: Out of the box idea: What's themechanicalshaft power like (assuming that 30 rpm is continuous)? Enough that you could get your electrical power just by putting a generator coil on the rotating part and have some permanent magnets (or a coil) as the stator?(I still suspect that "Find the right slip rings" is the real and ultimately cheapest answer.)

author: Terry Bites
author_rank: Super Contributor
timestamp: September 17, 2021, 02:52:50 pm
content: Car alternator sliprings and brushes are battle hardenned. But maybe a bit too sparky when dirty, but I think you like that kind of thing...... what about a pair horn push slip rings? Cheap nasty and robust. Or the real thang.https://www.ebay.co.uk/itm/265066949029?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&toolid=10050&campid=5338358731&customid=16318898208085802554912021000008005https://www.ubuy.hk/en/search/?ref_p=ser_tp&q=Taidacent+Through+Hole+Conductive+Rotary+Slip+RingsI like the idea of a magnetic coulping but an uphill struggle for a one off I think.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 03:20:32 pm
content: The slots in the pot core don't matter much. Air gap between faces is much more significant. That's limited by mechanical runout and such.A resonant converter is alright, depending on if you can afford some means of feedback, or primary-side sensing, or dissipate the excess in a shunt regulator (so the primary delivers constant power, ez pz). This is the main difference with gap: if you can't afford a tight gap, leakage will be substantial and regulation will be poor, and resonant will be attractive to deal with the additional reactive power; with a tight gap it's just like any split-bobbin transformer, the regulation might not be great but it's manageable.Also possible with enough gap, and a particular magnetic arrangement; it could be arranged as a ferroresonant transformer, I think, which would effectively be regulated by the saturation flux of the materials, and the driven frequency. This would require a ferrite "washer", to return some flux (magnetic shunt) around the secondary side, I think?An example of feedback might be light-housing an IR LED out from the shaft, and put some phototransistors around it. Sum them up and that's your feedback signal. Does need good shielding/filtering from ambient, and does need to be filtered to remove ripple caused by rotation. Slow control has consequences for output regulation, and in turn, how much filtering (energy storage) you need onboard; an ideal converter has approximately zero energy storage and instant control response.I mean, assuming you can't get an axial position of course, in which case the opto is trivial.Tim

author: sandalcandal
author_rank: Unknown
timestamp: September 17, 2021, 03:38:15 pm
content: I'd agree a resonant topology is likely what you want to efficiently deal with the extra and less controlled leakage inductance that your are likely to encounter. However, you could potentially go with a "simpler" push-pull system and just eat the excess switching losses depending on how much margin (in terms of money, size and weight) you have available to add the extra snubbers and thermal management. Poor regulation can be dealt with using additional stages which shouldn't be too hard with something off-the-shelf (universal mains voltage SMPS?) but again would need to mind margins.I wonder about the mechanical aspects here.* What are the loading conditions?* Will there also be potential non-axial loading e.g. wind or drafts in the installation location?* How big of a shaft do you need for this chandelier?* What will the bearing arrangement look like?* Can the bearing arrangement maintain sufficient runout to stop rubbing under the expected conditions? ​Leakage flux could lead to inductive heating of the shaft if it is conductive and/or magnetic.If runout can be well guaranteed then building a tighter and more efficient transformer/wireless power transceiver could be made easier.

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2021, 03:43:51 pm
content: How large is this chandelier and what is driving it? I would think about integrating a transformer into a multi-pole ceiling fan motor somehow by adding windings to the armature. The 'inside-out' configuration of some of these seems like that might not be too terribly difficult. Might be a bit of work for a one-off, but maybe have a look at a ceiling fan motor and see if that is workable?

author: sandalcandal
author_rank: Unknown
timestamp: September 17, 2021, 04:00:55 pm
content: Quote from: mikeselectricstuff on September 16, 2021, 10:52:17 amLooks like a differential mode choke (sort of?) with a flyback diode possibly to try force power back out onto the input bus capacitor? I've never seen that before in any modern designs. Normally people seem to just let it flow though FET body diodes directly to the input bus.

author: sandalcandal
author_rank: Unknown
timestamp: September 17, 2021, 04:33:43 pm
content: Thinking of this more. If you use a P core with a shaft through the middle (or near the outside surface) and that shaft is magnetic then due to the gap in the cores, flux will be encouraged to pass though and be concentrated in the shaft. Even if you use a non-magnetic but still conductive material, fringing flux near the gap will induce eddy currents in the conductive material that will also cause loss/heating. With a larger but very low flux design at low switching frequency this could be ok but I'd suggest something else.The coaxial core design in the paper you shared is probably the best in terms of a compact, magnetically efficient solution but I think you could do a design that gets away with using P cores if you can tolerate having a thick shaft, though you may have considered it already. You could use a large hollow with the core in the centre. If you make the shaft of plastic (or some other non-conductive non-magnetic material) then you have freedom to make it quite thick if it is an outer layer surrounding the core. Alternatively you could use a metallic shaft but have a large air gap (or some other non-conductive non-magnetic material) between the inner diameter of the metallic shaft and the transformer core in the centre. There is also the benefit of better protection of the transformer core. The downside is you potentially have a shaft that is visually too fat? though I guess you could potentially hide the wider part with the transformer in the "body" of the chandelier?Edit: Since all you want to do is stable transfer power across a small air gap, without the need for much regulation, doing a series resonant design should be very simple. If you operate at the resonant frequency then gain should remain very close to 1. If the resonant point shifts then voltage will drop if load is high but it shouldn't randomly climb to an unreasonable level if you fail to control it as with an LLC design. You could even add an external "ballast" resonant inductance which would reduce the relative effect of leakage inductance changes.

author: langwadt
author_rank: Super Contributor
timestamp: September 17, 2021, 05:18:28 pm
content: not exactly super cheap at ~32€, but if it is for a one-ofhttps://www.digikey.dk/product-detail/en/tdk-electronics-inc/B65945A0000X022/495-5356-ND/3914532but then, sliprings with a rated life of tens of millions revs isn't much morehttps://www.ato.com/miniature-through-hole-slip-ring

author: uer166
author_rank: Super Contributor
timestamp: September 17, 2021, 06:02:49 pm
content: One big problem that I see is the large mechanical forces exerted by the fields within the gap(s). It may be impossible to properly align and mechanically secure the 2 core halves axially well enough so that one side doesn't suddenly collapse and touch the other side. It's the same issue you have when aligning and securing a steel/permanent magnet rotor inside a stator in a BLDC. I think a motor/generator pair would be easier to do, but a slipring would probably be more reliable. Of course the best might be a wireless power solution where the mechanical forces are benign and the alignment non-critical. Maybe you can get some devkit likehttps://www.we-online.com/web/en/electronic_components/produkte_pb/demoboards/wireless_power/design_kit_200_w/wireless_power_200wkit_page.phpthat can do 200W supposedly readily and don't have to deal with Qi?

author: sandalcandal
author_rank: Unknown
timestamp: September 17, 2021, 06:12:46 pm
content: Edit: This design is a bad idea as there is no capacitor in series with the primary winding to limit the magnetising current.Never mind, just my sim messing up. Still the lack of capacitor on the primary might cause issues? Need to explore further.LTSpice simulation of a SR-ish type converter with a leaky transformer. The .asc is attached. Idea taken from tank XV here:https://www.ti.com/seclit/ml/slup376/slup376.pdfEffect of change load from 50 Ohms (310 W) to 100 kOhms (<0.2W)Effect for leakage inductance change from 70uH to 130uH in steps of 10uHStability looks pretty good overall. 25% Regulation seems achievable with a fixed ~28kHz input switching frequency.Qi wireless chargers are just resonant air gapped transformers much like what is being done here but with more overhead protocols and control.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 06:33:35 pm
content: Quote from: uer166 on September 17, 2021, 06:02:49 pmI am extremely interested in your 1.5T+ saturation pot cores and wish to know more at your earliest convenience!Tim

author: sandalcandal
author_rank: Unknown
timestamp: September 17, 2021, 06:37:35 pm
content: Quote from: T3sl4co1l on September 17, 2021, 06:33:35 pmFYI typical power magnetics don't go much above 200mT even at "low" frequencies of <100 kHz. 50mT to 100mT seems common for most modern applications.

author: uer166
author_rank: Super Contributor
timestamp: September 17, 2021, 06:43:16 pm
content: Quote from: T3sl4co1l on September 17, 2021, 06:33:35 pmI did say it was same issue, but didn't say it was of the same level

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 08:27:46 pm
content: More just to say -- do the sniff test; assembly won't be an issue, it's only pulling while active (unlike those permanent magnets, pesky indeed!), and, if your bearings will have trouble with a few gramforce from a ferrite core, you have much, MUCH bigger problems.(For those that aren't aware -- magnetic force is proportional to flux density squared. So, in the low 100 mT range, you can probably pry things apart by hand. Fridge magnets are in the 10s mT range, using a pattern optimized for close-up holding force. Ferrites top out at 400mT or so, while NdFeB magnets do about 1.5T, and some iron alloys (soft) go up to 2T or so, depending on how hard you want to try to magnetize them. Any higher, you REALLY have to want it -- typically needing superconductors, as you'll get no help from iron pole pieces. These (up to 2T) fields easily cause finger-crushing accelerations, but it's nothing a properly built mechanism can't handle -- the pressure (B^2 / (2 mu_0)) is equivalent to just a few bar. So if you think about shop (compressed) air equipment, that's about the most strength required for any ordinary magnetic assembly.)(As a corollary, this is essentially why coil guns do, and forever will, suck.)Tim

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2021, 08:32:06 pm
content: I would expect slip rings to be more robust and reliable than the electronics required to drive a rotary transformer. I mean there are the car alternator applications already mentioned, and then stuff like rotating signs that used to be common out front of businesses, those use slip rings and would run for years and years. Carnival rides are another application that gets heavy use.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 17, 2021, 10:40:33 pm
content: Quote from: sandalcandal on September 17, 2021, 03:38:15 pmI don't think the mechanics will be an issue - there will be a large ring bearing, leaving about 80mm dia in the centre completely clear. The thing will be about 1.5m diameter and weigh about 30 kilos (all carbon fibre) . Budget is not an issue

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2021, 11:02:23 pm
content: Quote from: mikeselectricstuff on September 17, 2021, 10:40:33 pmThen a slip ring that size with multiple contacts in parallel and AC power would seem to be a pretty easy, reliable solution.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2021, 11:02:41 pm
content: Ah cool you have the center to yourself. A tight fitting pot core should do nicely then, can even hack an opto without too much trouble (but shouldn't need it).Tim

author: langwadt
author_rank: Super Contributor
timestamp: September 17, 2021, 11:10:38 pm
content: Quote from: mikeselectricstuff on September 17, 2021, 10:40:33 pmthen something like this,https://www.ato.com/small-through-hole-slip-ringseems like the simple straight forward solutionrated for 20-150M rotations depending on type

author: sandalcandal
author_rank: Unknown
timestamp: September 18, 2021, 02:26:45 am
content: Quote from: mikeselectricstuff on September 17, 2021, 10:40:33 pmAs Tim beat me too it, a closely fitting pot core should be feasible to make then. I think the magnetics might be ok but I'm not 100% on the converter still.In retrospect that "SR" converter circuit I posted earlier could have issues due to magnetising current inductance not being captured by a resonant capacitor. A much higher magnetising inductance could be used? or you could use some sort of choke/ballast as per L1 in your OP example circuit? Would need to run further checks including time-domain simulations...Whilst this sort of design really piques my interest, practically speaking, a KISS approach using a slip ring may indeed prove the more effective solution still.Edit:I think Sam Ben-Yaakov has a number of good lectures on the topic of wireless power transfer.The circuit topology from your OP is discussed and analysed at 45:45 He refers to it as apush-pull parallel resonant inverter. Searching that term yield lots papers analysing and discussing that topology.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 18, 2021, 03:25:36 am
content: This search string got me some juicy hits:rotary power transfer shaft magneticSo did:Contactless Commutator.First hit was a 1 Kilowatt design. I'll try to link it.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: September 18, 2021, 03:30:34 am
content: https://ieeexplore.ieee.org/document/6647032That is the link mentioned above. Let me know if I need to try to attach the file.Steve

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 18, 2021, 12:08:06 pm
content: In your first post you write you want to buy parts "at sensible prices", and make other price related remarks.Then later you write:Quote from: mikeselectricstuff on September 17, 2021, 10:40:33 pmFrom what I know of your (marvelous) work, it's one-offs art installations or maybe small series, and you have generous budgets to work with.Somehow I have a suspicion that big pot cores are difficult to make. They have to be baked at high temperatures and the material is brittle and may crack easily during cooling. I suspect that additives that reduce the brittleness also influence magnetic properties.I've always been confused about the relative high prices of ferrites in general. Baked clay is as cheap as dirt, and plain steel is a few euro's per kg, but ferrites can easily cost a few orders of magnitude more and I never really understood why. Even if you look at websites like Aliexpress, big ferrites are never cheap.https://www.aliexpress.com/wholesale?SearchText=ferrite+pot+coreAir gaps in magnetics are common, which I also find somewhat contradictory. First you concentrate the field, and then you wreck it with an air gap. Main Idea as I understand is to make the saturation characteristics a lot more gentle. The distance of the airgap has a huge influence on the magnetic properties, so good bearings are essential and the electronics should be able to cope with changing magnetic coupling.As already stated, sliprings can have a long service life, especially if they do not have to be cut into pieces such as with motor commutators. Maybe it is an option to build a spare slipring somewhere in your gadget. If it then fails 30 years from now and the company who made it once no longer exists, then the person who services it just swaps the worn out slipring with the brand new one sitting next to it.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 18, 2021, 12:55:48 pm
content: Every man and his dog is offering an LLC design tool. TI, NXP, Infineon, ST etc. All soon to be eaten by CPC- not that one, the other one, the CPC in the east.The LLC transformer air gap is put to use in the design. The stray field that does not couple to the secondary core increases the primary indcutance.A bigger air gap can cause unacceptable heating in the primary though. For a 100w the transformer is going to be small.There is no need to integrate the LLC indcutor into the transformer if the rotary XMFR air gap complicates things too much. It can be a discrete component. It can even be moved over to the secondary side. Use the regular design tools (maybehttps://www.ti.com/design-resources/design-tools-simulation/webench-power-designer.html) and rearrange the results.https://www.edn.com/power-tips-84-think-outside-the-llc-series-resonant-converter-box/Its all network voodo that I cant be bothered with these days.BTW You can custom cut ferrite with water jet machining. I've had slots machined into thick glass which is about the same hardness as ferrite. It worked really well and it wasnt very expensive. Great place in Uxbridge. One of many I'm sure.You can also stick bits together with a suitable adhesive.http://ferroxcube.home.pl/appl/info/gluing.pdfhttps://gatewaycando.com/blog/post/machining-bespoke-ferrite-geometries-from.htmlTons of accademic stuff out there with XMFR design equations for ubernerds.https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwi1tMq64YjzAhU6QEEAHV_QC4gQFnoECAQQAQ&url=https%3A%2F%2Fcoefs.uncc.edu%2Fmnoras%2Ffiles%2F2013%2F03%2FTransformer-and-Inductor-Design-Handbook_Chapter_19.pdf&usg=AOvVaw3DiQeVV1ZPZZD7dty_jJnEhttps://www.semanticscholar.org/paper/Inductive-power-transfer-system-with-a-rotary-for-Ditze-Endruschat/081530c01555060d6a2d940c060ccabe761bca8ahttps://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjam_bl4YjzAhWGAMAKHUoJBawQFnoECAYQAQ&url=https%3A%2F%2Fwww.ti.com%2Fdownload%2Ftrng%2Fdocs%2Fseminar%2FTopic4LD.pdf&usg=AOvVaw37WK5ns6BWDzHjlAO1oLgN

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2021, 02:48:53 pm
content: Quote from: Doctorandus_P on September 18, 2021, 12:08:06 pmMeh, sintering is a well developed science. More just a matter of, taking the time to tune the shape exactly, in case compensation for differential shrinkage is necessary; and actually making and running the tooling.There aren't many customers for big ones (over a few cm), and there are better shapes for thermal performance (windings have more cooling in an E or ETD), and high power rarely needs the shielding afforded by something like that. Large toroids are also moderately available, have seen 6" ones in stock from time to time. Have used 4" (102mm) ones myself.Additives of any sort, are forbidden; any (nonmagnetic) impurity would collect at the grain boundaries, effectively increasing airgap, reducing mu_r. Or dissolve into the ferrite, causing whatever effect it would (probably more hysteresis?). I'm not aware of anything that has useful impact on brittleness of the fired body, though if you mean for the green (pressed powder) part, there are organic binders to help with that (which burn out, at modest expense to shrinkage).Neat aside: ferrite is among the ceramics that exhibit ductile deformation, albeit to a very limited extent. Evidence is found the scratches left by grinding: smooth tracks suggest material being peeled away, while jagged tracks suggest brittle fracture. You see a bit of both in micrographs, IIRC.But it's also a rather soft compound (Mohs hardness about uh, 5 is it?), nor very strong (say, comparable to above-average concrete), not at all like a porcelain (mostly silica glass, quite strong, even a bit flexible -- in the elastic range that is). So, probably any flexibility it might have is balanced by the poor strength, so it's still pretty brittle overall.QuoteYeah well, baked clay doesn't have technical let alone analytical chemical purity.I don't know exactly how tight the formulations are, but every new grade that pushes losses say 10% lower than the previous generation, must be worth a fair amount of development to achieve. It's not like they're doing that every single year. On top of that, they have to match the exact composition in production, using good purity feedstocks with careful process control.As a highly refined, technical material, I'd say it's one of the cheaper things, and indeed the generally cheap elements involved, I would say, are probably partly why.Consider, for example, an equal volume of epoxy or other fancy plastic resin -- the costs are probably ballpark similar, of course the cheapest (say ABS, etc.) will be less, but the fancy stuff like epoxy, uh UV-cure 3D printing goo, etc., those are highly refined and modified substances, ultimately produced from dirt-cheap (literally) feedstocks (i.e., petroleum for the most part).QuoteIndeed, it's all about having the right amount.Note that energy storage goes as B^2 / (2 mu). High mu means less energy.But low mu also hurts, ultimately because copper has some resistance. If it had zero resistance, we could just let the current circulate forever, whatever the flux density. But with resistance, energy stored in the inductor has a clock that ticks down very quickly, and we need to compromise between resistance and reactance.Ideally, we'd have a metal with about 1/20th the resistivity of copper, or less. This gives slow enough time constants (~ms), in air-cored inductors, that are useful for typical power conversion purposes (if the cutoff frequency is ~1kHz, then a Q of ~100 might be expected at 100kHz, i.e., efficiency ~99%).Alas, we can't have that outside of cryostats, so we settle for permeable cores. We need to concentrate the field to make copper more effective, but not so concentrated that our maximum energy storage is hindered.There are some places where a low-mu (distributed airgap) ferrite would be helpful, but mostly it's better to have the regular stuff in stock, and cut it as needed -- since we can make our own airgap of almost any arbitrary size by cutting.And again, optimizing production quantity is a big part of the cost of anything. Big parts aren't so much expensive by sheer volume, as they are by the much smaller scale on which they're produced. Or, if they run batches of them less often, there's more warehousing cost to store the extra, or more opportunity cost effectively paid by the customers waiting for them when stock runs out, etc...So also, it's probably better to modify standard cores, which will also then fit in standard bobbins, don't have to do anything custom at all, it's real nice.And we do suffer some from that, like the intense fringing field around the gap, increasing losses or necessitating a more expensive winding (fine stranding?). It's all a compromise.In contrast, when we don't want energy storage, we do indeed want mu as high as possible, and as little air gap as possible. Transformers should simply be an instantaneous ratio between windings, without storing energy themselves: maximum magnetizing inductance and minimum leakage inductance. This is the case where we especially work at concentrating that magnetic field.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: September 19, 2021, 12:40:39 pm
content: Another thought. Coaxial solenoid transformer.Concentric windings can be rotated independently. One solenoid inside another. That opens up a lot of options including air core at higher frequencies.courtesy of M Faraday

author: sandalcandal
author_rank: Unknown
timestamp: April 22, 2025, 12:13:19 pm
content: Is there an update somewhere on how this ended up going if it did go somewhere?

author: mzzj
author_rank: Super Contributor
timestamp: April 22, 2025, 01:18:44 pm
content: Quote from: mikeselectricstuff on September 16, 2021, 12:55:11 pmTypical car alternators seem to last 200 000 to 400 000km or around 3000 to 6000 hours.On average they rotate at roughly 5000rpm, 300000 rotations in just one hour. at 3000hour you should be at 900 million rotations.Should leave some to spare your requirement is only 1M+edit: damn this was some necro-thread

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 22, 2025, 06:24:50 pm
content: Quote from: sandalcandal on April 22, 2025, 12:13:19 pmproject never went ahead - pretty glad actually as it was looking to a be a PITA for variouss reasons

author: IconicPCB
author_rank: Super Contributor
timestamp: April 22, 2025, 07:59:57 pm
content: The alternator idea is worth exploring both as a motor and transformer. All in one.Get rid of rectifier bridge. Drive stator with three phase power. Set up a friction brake on the motor shaft so as to be able to load the motor mechanically.Use the slip rings to harvest slip energy and Bob's your uncle.Google slip power recoveryEDIT: bugger too early in the morning to recognise this is a two page post

author: Langdon
author_rank: Frequent Contributor
timestamp: April 22, 2025, 09:50:29 pm
content: How does the TGV connect to electricity?Use what it uses.What about good ol' carbon brushes?In theory, a 2 copper ball bearings might be the perfect solution.Perhaps some axial ball bearings, whose steel balls have been replaced with brass balls would work.