high current DIY inductors - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 03, 2020, 04:54:56 am
content: I'm working on a large power inverter, for a DIY solar power system. I have found fairly cheap sources for the power semiconductors and capacitors, but the inductors seem to only be available at crazy prices.I found this old thread on DIYing large inductors:https://www.eevblog.com/forum/projects/big-inductors-diy-style/My requirements, however, are quite a bit more reasonable - on the order of 500uH-1mH at up to 60A or so. (2 of them, one per phase, along with a bunch of much smaller inductors elsewhere in the unit.) The switching frequency is 15-20kHz and size and weight are not that important, low losses and low cost are.Thus far, it seems like rewinding a few big transformers would be the most economical route. Or is there a reasonably cheap source of magnetic cores?

author: BravoV
author_rank: Super Contributor
timestamp: January 03, 2020, 05:05:37 am
content: Bought this from local re-cycler, it was salvaged from a big telecom big rectifier (AC to DC converter).Measured using LCR meter, DCR=0.04Ohm, inductance 502uH@100 Hz and 460uH@100 kHz.Shot with TO-220 chip as size comparison.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2020, 07:21:47 am
content: A pile of T300-52D's would do.Tim

author: jbb
author_rank: Super Contributor
timestamp: January 03, 2020, 07:44:27 am
content: A stack of E65 ferrite cores can get huge core areas with common parts.For this application frequency range, nanocrystalline / amorphous iron materials can be quite good. They have the high saturation flux density of iron with much lower core loss. But they will be very noisy acoustically.What topology are you looking at? Moving from 2 level to 3 level can be very beneficial; it reduces switching losses (may allow higher frequency) and can straight up halve the inductor value required.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2020, 02:01:51 am
content: Quote from: jbb on January 03, 2020, 07:44:27 amI did a lot of reading about amorphous iron cores and they do appear to be the perfect kind of core for my application. Is there a guide for how to select the size of core and what turn count/gap size to use? I also read that those cores are fragile, is that really true or it that only in comparison to the conventional iron cores that are very rugged? All the amorphous iron cores I could find available in small quantities are horseshoe shaped (sold in pairs) and since the bigger ones are somewhat cheaper per pound, I had the idea to buy a single large pair, wind each phase on each horseshoe, then put them together with a piece of ceramic in between to set the gap and keep the two sides isolated, as well as keep the windings away from the gap area. Would that cause undesirable interaction between the phases if they're running at the same switching frequency but the output currents are not the same?I don't think it would be very easy to do 3 level switching for a split phase design. The power module I'm planning to use (3rd gen Prius inverter, goes for fairly cheap) has 2 3 phase bridges. One I will use as a 3 phase variable frequency output for a thermal storage compressor (only needs light filtering to avoid EMI and standing wave problems) and the other I will use 2 phases for interfacing to the mains (via a 240V plug to a dedicated circuit) and 1 phase for a 120V UPS output. The neutral is connected to the center tap of the DC bus capacitor bank, so each phase is more or less switching at +-200V or so. (There's a control system that senses the current drawn from the grid and commands the inverter to source a current to mostly offset it without netting an export, so it's not a grid tie inverter in the conventional sense.)

author: jbb
author_rank: Super Contributor
timestamp: January 05, 2020, 04:46:21 am
content: Quote from: NiHaoMike on January 05, 2020, 02:01:51 amI don't have a good reference off the top of my head, but the good old winding area product method is quite applicable. There are two things to watch out for:just because you can go above 1 Tesla doesn't mean you should, check your core lossesthe cores are laminated, and the laminating material takes up some room, so the effectiveironarea is somewhat smaller than simply multiplying length and widthQuoteI would say that they are alottougher than ferrite, but should still be treated with caution. Probably less rugged than big old silicon steel.QuoteNo go. You'll need one core set (2 halves) per inductor. I also recommend using a bobbin to hold the windings down. For C cores, you can improve the DC resistance a bit by using two bobbins, and putting half the turns on each bobbin (still just 1 phase!); this reduces the average turn length while still letting you fill the winding area, and decreases resistance.Remember to consider AC resistance at the switching frequency (i.e. skin andproximity effect). At 18 kHz, skin depth is about 0.5mm. Using 0.5mm diameter cores for the winding would require many parallel strands (certainly more than 20!) and be a nightmare to wind, so you might need Litz wire...Gap can be set using a shim as per usual.QuoteOK, 2 levels it is. Great that you're looking at some thermal storage - are you doing mechanical engineering on that side too?Also, what modulation are you planning to use on the full-bridge output? The classic 'bipolar' mode only works as 2 level, whereas 'unipolar' produces 3 output levels and might help (at the expense of more common mode filtering).Oh, be careful here. Are you using the Prius DC link caps, or applying your own? In any case,make sure there are bleed resistors to discharge the capacitors.Otherwise they could give you a lethal surprise.The Prius DC link caps will likely be quite small, because the expected loads (3 phase motor/generators) are constant power types. When outputting to a single phase grid, you will get 'lumpy' power output at twice the line frequency. So you might need to add quite a bit of external bus capacitance to buffer this energy flow.(Spitballing capacitor calculation, I could be very wrong...60A outputAllow 15V DC link ripple; 15V / 60A = 0.4 Ohms120Hz ripple; Xc = 1 / (2 * pi * f * C)Hence C = 1/ (2*pi*120*0.4) = 3300uFDon't forget about capacitor ageing, so you may need nearly double that!)If adding your own caps, you will need to be really careful about stray inductance.QuoteAgain, think hard about your caps. You could structure your output to provide a 120V - midpoint - 120V output, just like a split-phase system.Do you really need this output? It's quite cumbersome.QuoteRemember that you will need anti-islanding sensing. If you choose to never export to the grid you must be very confident that your control system manages this reliably.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2020, 06:35:35 am
content: Amorphous/nanocrystalline isn't great for inductors because the fringing field at the air gap is vicious. The material is only available in strip and powder forms, and you usually see the former.Can be okay at low frequencies (under 10kHz maybe), and yeah, mind delta B. Bmax is wide open (~1T as mentioned), so the advantage goes to DC bias with a small ripple fraction, as with other low-Q chokes (laminated and powdered iron, for their respective frequency ranges, this one falling inbetween).Tim

author: jbb
author_rank: Super Contributor
timestamp: January 05, 2020, 07:04:16 am
content: Quote from: T3sl4co1l on January 05, 2020, 06:35:35 amThat's a good point. The last inductors I did had quite small gaps, so fringing wasn't a major issue. I have seen mention of designs with multiple smaller air gaps in series, which looks very irritating mechanically.I'm sure the problem gets extra-terrible if we think about a nanocrystalline core with a large gap and a foil winding

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2020, 02:22:47 pm
content: Quote from: jbb on January 05, 2020, 04:46:21 amThe stock capacitor bank is not tapped so there will be an external bank for the reasons mentioned. And it will be on the order of 2x 15mF total capacitance mostly because of the ripple current limits of electrolytic capacitors. For bleeders/indicators, I plan to use the old "LED + resistor" trick, taking advantage of the fact that modern LEDs need little current to give a visible glow so they wouldn't add much to standby draw. And there obviously will still be procedures to check for voltage before servicing, albeit with a few more steps between the final multimeter check and adding some shorting jumpers, since shorting such a large capacitor bank with any significant voltage will make quite an arc flash.The large capacitor bank will also double as "nearline" energy storage, giving smart loads seconds to respond to small power variations. The control side will do UDP multicasting in order to signal the power status to remote smart loads. I plan to run it without batteries initially, then decide how much battery capacity to get.I'm treating the two main phases as separate inverter outputs since the load may be unbalanced. That actually solves a problem seen on at least some "zero export" inverters - because they do not independently control the output currents, an unbalanced load will cause an export condition on the less loaded phase with an import to balance that on the more loaded phase.The UPS output is fairly small, certainly not the 60A of the other two phases. Most likely 20-30A at most. (Sorry for not clarifying that.) Since there will be a contactor to disconnect the two main phases, I did think about adding some more circuits to confirm the contactor did open and allowing them to double as "low priority" UPS outputs, but decided against that due to the control complexity and that 120V, 30A of UPS output is a lot.QuoteThe control system will be designed to not completely offset the load so the grid going down will quickly pull the voltage to zero. There will also be voltage and frequency checking as well as a N-G loop continuity check.\Quote from: T3sl4co1l on January 05, 2020, 06:35:35 amCould that be solved by splitting the winding so that there is no winding over the gap? (The inductors will be packaged into a metal box to keep the magnetic fields from interfering with nearby circuits.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2020, 04:26:17 pm
content: Quote from: NiHaoMike on January 05, 2020, 02:22:47 pmNah, it's fringing through the material itself -- where it's fringing in plane is fine, but through plane is where all your losses happen.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2020, 05:45:34 pm
content: Quote from: T3sl4co1l on January 05, 2020, 04:26:17 pmWould it help to add some powdered iron pieces between the amorphous cores and the gap?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2020, 06:53:50 pm
content: Uhhh.... of what permeability?Replacing air gap with low mu means needing much more gap, which, eh, I'm not sure, it might help. Effectively it's a wider gap so should fringe even more widely, but the mu will keep some flux in it, less fringes from the base core. But maybe the increased distance, the wider fringing, turns out worse anyway.High mu isn't any good, obviously. Air gap is needed to generate magnetizing inductance.Or if you mean replacing the core faces with an isotropic material and still using an air gap... I wonder if you're saving much at all by using nanocrystalline. It's a hell of a lot more expensive than just using powder in the first place.You're welcome to run it in FEMM, if you can replicate the materials correctly.Alternately you could wrap the air gap with heavy copper, which forces the flux to flow within the shield rather than fringe out around it (you leave a slit, so it's not a shorted turn). This is going to cook -- the question is, will it cook as hot as the core alone? I'm not sure how to calculate losses in either case, so FEMM might be the better method again.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2020, 08:50:12 pm
content: Quote from: T3sl4co1l on January 05, 2020, 06:53:50 pmSo what's the advantage of using amorphous iron over powdered iron in the first place? I was getting the impression that it can run at higher flux levels compared to powdered iron before the losses become excessive, thereby reducing the core size for a lower overall cost. Or do you mean that if I use powdered iron as well as amorphous iron, the powdered iron becomes the bottleneck and the advantages are lost?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 05, 2020, 10:26:27 pm
content: Quote from: NiHaoMike on January 03, 2020, 04:54:56 amMagnetics, Inc. Kool-Mu is likely going to have the best combination of cost vs. energy storage vs. losses. Amorphous/Nanocrystalline (Metglas, FineMet, etc.) will give slightly lower losses and require less volume for a given energy rating, but costs a lot more for said energy rating, needs to be discretely gapped for chokes (so, lots of fringing effects and relatively hard saturation), requires a bobbin to hold the windings and banding to hold the cores together. Basically, a lot more cost and labor for a slight reduction in losses/volume.Powdered iron will be shockingly bad here - high losses and not really all that less expensive than Kool-Mu. Gapped ferrite will have minuscule losses but cost more and require a lot more volume for a given energy rating. Magnetics, Inc. XFlux might be a good alternative, though. I'd double check the losses for the flux swing, but it should still be okay at 15-20kHz and <100mT delta-B.That said, 1mH * 60A = 1800mJ* which is going to require a massive core area regardless of the material. Magnetics, Inc. makes large E cores out of Kool-Mu which are relatively cost-effective though I have mostly used stacks of toroids. My most recent design was a PFC choke that needed to be 250uH and handle a peak current of 33A at 50kHz. I used a stack of (3) 77736 40u cores with 25t of #8 Litz; each core cost about $10.50, IIRC. Not bad, all things considered, but you need at least 2x the inductance 2x and the current... To paraphrase Jaws, "you're gonna need a bigger boat."* - or 3600mJ in magnetics-speak.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2020, 06:57:33 am
content: I wouldn't say shockingly bad. The lossy powdered irons (#26, 52) are fine in the low 10s of kHz and low to modest ripple fractions. The lower loss ones (#8, etc.) are okay at higher frequencies / ripple fractions. They're cheap, so if you don't mind using a little more space, it's an easy way to store energy.Kool-mu handles even higher ripple fraction in turn, though not all /that/ much, I've cooked one before...Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 06, 2020, 01:54:25 pm
content: Found these inductors for really cheap:https://www.newark.com/hammond/195e50/bracket-mount-choke-2-5mh-50a/dp/50H6592?CMP=AFC-OPThey're only 50A (I can lower the output current rating to that) and very heavily discounted for some reason. (Anything to beware of? I haven't tried ordering so it's possible the shipping fees would make it not as much of a bargain as it appears.) Would these be suitable for my application or would they be too lossy at the frequencies I'm working at?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 06, 2020, 02:30:33 pm
content: Quote from: NiHaoMike on January 06, 2020, 01:54:25 pmThese are strictly for 50/60Hz choke input filter applications; losses will become prohibitively high around 400-1000Hz, depending on the lamination thickness and grade of electrical steel used.EDIT - but that's a helluva deal so I ordered one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 08, 2020, 01:03:59 am
content: What source would you recommend for getting a few of the Kool Mu cores? Searching Digikey or Mouser doesn't give any results for cores, just (small) complete inductors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2020, 08:48:47 am
content: I think Amidonhttp://www.amidoncorp.com/used to show stock of items other than their main catalog (which BTW is Micrometals powdered iron, most of Fair-Rite's catalog, and Magnetics ferrite toroids). Probably you can still RFQ, but no telling stock...https://elnamagnetics.com/used to have online inventory checking but seems they dropped it.https://lodestonepacific.com/has inventory checking but it's kinda broken?.. old fashioned RFQ and purchasing.Tim

author: floobydust
author_rank: Super Contributor
timestamp: January 08, 2020, 08:55:22 am
content: I've gotten samples fromMag Inc.andMMG Canada Limited.Würthoffers inductors and custom magnetics.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 08, 2020, 11:25:42 am
content: Quote from: NiHaoMike on January 08, 2020, 01:03:59 amPretty much anything involved with magnetic components - wire, cores, bobbins, tapes, etc. - is still operating in the pre-internet era, with all pricing done by RFQ and with oppressive minimum order quantities. The one partial-exception to this isDexter Magnetics- they do have an actual web shop but quantities rarely seem to be accurate and the search function is terminally stupid. Still, you can probably, eventually, maybe find some Kool Mu cores to order from them.Otherwise, a somewhat close competitor to Magnetics, Inc.'s powder materials can be had fromCWS Bytemark. I've used them for one-off (and not "state of the art") type applications with good results.EDIT - it looks like Mouser carries a few sizes of Hitachi FineMet amorphous/nanocrystalline cores now... sit down before you look at the price, though.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 11, 2020, 05:09:23 am
content: Is fringing losses with amorphous cores really a big deal at 15-20kHz? The following shows a gapped amorphous toroid for PFC and inverter applications.https://www.nanoamor.com/pfc_choke_coresAs a guideline, at 15-20kHz, what would be reasonable J/kg values for amorphous, iron powder, and Kool Mu cores, assuming the core geometry makes efficient use of materials as most do? I'm starting to get the impression that for small hobby quantities, amorphous cores might actually end up the cheapest option.Someone suggested salvaging the stators out of the hybrids as well (as in where the transmission casing is damaged but the stator itself is still usable) but I'd prefer to not do that since it sounds messy and time consuming, plus probably would not be cheaper after factoring in buying the entire transmission and then selling it minus the stators. (Of which, from the pictures, the stators appear to be made from laminations, how do they keep the losses down?)

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 11, 2020, 12:35:13 pm
content: Quote from: NiHaoMike on January 11, 2020, 05:09:23 amFringing effect is proportional to gap gap length and current to the first degree. The fringing field from the gap induces eddy currents in the windings closest to the gap (roughly out to a distance equal to the gap length) and those losses go up with frequency, of course, but a counterbalancing effect is that wire diameter (or foil thickness) have to be reduced with increasing frequency which limits the magnitude of the eddy currents in each wire/foil.Quote from: NiHaoMike on January 11, 2020, 05:09:23 amThe amount of energy stored in the core is almost irrelevant; most of the energy is stored in the gap (whether discrete or distributed).Quote from: NiHaoMike on January 11, 2020, 05:09:23 amThat doesn't sound practical at all. IIRC, early Prius models used a bidirectional buck/boost converter in between the battery and inverter which would have a rather beefy choke (or 3?) that would be better suited to your use.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 11, 2020, 01:31:44 pm
content: Quote from: MagicSmoker on January 11, 2020, 12:35:13 pmThere was a reference to fringing losses in the core material itself, is that a problem at the low frequencies I'm working with? I'm thinking the fringing losses in the windings could be solved by sectioning the windings so that they're away from the gap area.QuoteThe iron powder and Kool Mu cores have the air gaps integrated so there should be a value for the maximum energy that can practically be stored in them. For the horseshoe amorphous cores, the gap is chosen by the designer but in the end, there's still going to be a value for how much energy an inductor made from that core can practically store.QuoteThose are only on the order of 250uH, which is fine for DC but for AC, the large filter capacitor (about 300uF) would pull a lot of reactive power.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2020, 01:47:53 pm
content: Quote from: NiHaoMike on January 11, 2020, 05:09:23 amThey don't give losses.Impossible to tell.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: January 11, 2020, 03:41:37 pm
content: NiHaoMike, great problem and discussion.1/ Laminated 60 Hz inductors and cores cannot be used on 20 kHz SMPS as the core losses are too high regardless of type of steel or lam thickness.2/ Amorphous cores have low losses but are super costly, you would need a custom cut core.3/ The best source of IN STOCK low cost powdered iron cores in all shapes, EE, EI, toroids is MicroMetals in Anaheim Calif. These are specified to work in SMPS.founder Jim Cox is a Ham and super engineer.https://www.micrometals.com/The website includes many selection and design app notes and guides and includes free design software.I have used their parts and info back to 1980s. They have samples available as well!4/ Ferrite like the CoolMu are also fine but need a gap, the powdered iron have a distributed gap.Gap is needed for any DC inductor.All oldline ferrite mfg (Philips, FerroxCube, Siemens, Stacore) have disappeared or moved prod to China.Hope this can assist your interesting design!Good Luck,Jon in Paris.Just the ramblings of an old retired EE