Custom transformer calculation - Page 1
Forum section: electronics / projects

author: Eagle_85
author_rank: Contributor
timestamp: August 18, 2020, 08:33:25 pm
content: Hi. I have an application for a sort of rotating transformer. Where the core rotates and one coil rotates/winds with it while the other coil is stationary and sliding on the rotating core.I know the basic primary/secondary windings = primary/secondary voltage. But for my application i just need a sort of isolation transformer with the same voltage on both the primary and secondary side. All other transformer calculations like calculating the minimum number of turns needed for a given frequency and power level is like dark magic to me.As i understand it you can use less turns if you go up in frequency right? And for my application i obviously want as few turns as possible because i can only have one layer of turns.So what frequency and core material can i use to transfer for example 12 volt and 20 watts? with maximum 30 turns on the coilsI have tried to illustrate what i am after here:The ting on top is a pulley. And the round core is not attachet to the square return path part of the core and will rotate freely. The top coil winds/unwindes with the round cores rotation while the bottom coil stays stationary and just slides on the rotating core. Maybe it is better to just rotate the bottom coil with the bobbin than to rotate the core, but that is not important for the question here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2020, 08:46:35 pm
content: Why does it need to rotate?I can't imagine where such a thing would be used, but I guess it could make a 100%-flex (elastic) "sliding contact". Not sure that enameled wire would be so good on abrasion (the wire will inevitably rub against itself as it un/winds, or against the former if it's made with a threaded form, which would be the better idea as that allows some separation of the turns in-place). Copper certainly isn't setting any records on stiffness or strength, but it's not bad at least (assuming full-hard wire is used). A steel cored wire maybe, or perhaps some alloys, would be the next best option in terms of strength without sacrificing too much conductivity (but, while phos bronze, brass and such are stronger than copper, they're much less conductive too).The usual rotating transformer uses two pot cores, or similar shapes. It's radially symmetrical therefore doesn't matter how they rotate with respect to each other. Windings are fixed with respect to each side, no flex, sliding or abrasion to worry about. Spacing needs to be kept fairly tight, nothing a few bearings can't provide.Tim

author: Eagle_85
author_rank: Contributor
timestamp: August 18, 2020, 08:54:16 pm
content: Quote from: T3sl4co1l on August 18, 2020, 08:46:35 pmBecause the purpose is to transfer electrical power from a stationary object to a slider without direct electrical contact because i want the wires to be isolated.So this transformer is on the slider part, and the "winding coil" will drive the slider back and forth. And yes there will be a threaded part on the core where the winding coil sits and there will be better isolation sheath on the coil wire than enameled wire for better abrasion resistance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2020, 09:00:11 pm
content: How much isolation is required?Why not a traditional sliding contact, or flex cable?Tim

author: Eagle_85
author_rank: Contributor
timestamp: August 18, 2020, 09:12:09 pm
content: Quote from: T3sl4co1l on August 18, 2020, 09:00:11 pmBecause this application will be in salt water so it needs isolation both to prevent corrosion to the wire and to not "leak" power trough the conductive salt water. And the wires needs to be tight at all times, i can not have a loose flex cable dangling from the slider as the slider path is really long.But my question is about the electric specification here, not the mechanics. Like i saidhow can i transfer for example 12 volt, 20 watts with a max of 30 turns? What frequency and core material do i need for that?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2020, 10:17:14 pm
content: Oh, interesting. So that would preclude open slides, but doesn't rule out insulated flex, or sealed slides.(For example, a two-lip rubber seal pinches closed around each electrical rail, insulating them from the salt water. Where the sliding contact enters, the rubber is pushed away a bit and some leakage current flows, but only around the contact, not the full length. Low frequency AC is used to avoid electrolysis, and inert materials are used besides (stainless, monel?). This might be a product that exists already, but I'll admit it would be pretty expensive to produce, from scratch, in continuous lengths!)How conductive of a salt are we talking here? You may be limited on transformer size because the salt will make a big shorted turn.How long is the axis?I could certainly give you a fast and wrong answer. 12V RMS sine, 1kHz, 30 turns, Ae = 300 mm^2, 3C90 ferrite, checks out. But then, you'll need some pretty damn thick wire if the path is "really long". And higher frequencies and voltages may not help you due to induction losses along the wire pair and salt water.How about shaking the thing back and forth to keep a turbine spinning?...Tim

author: Eagle_85
author_rank: Contributor
timestamp: August 18, 2020, 10:46:20 pm
content: Quote from: T3sl4co1l on August 18, 2020, 10:17:14 pmYes the length and the fact that the "axis" is a free hanging wire rules out flex connections.Quote from: T3sl4co1l on August 18, 2020, 10:17:14 pmSeawater.Quote from: T3sl4co1l on August 18, 2020, 10:17:14 pmAround 25 meters. So yesreallylong...Quote from: T3sl4co1l on August 18, 2020, 10:17:14 pmI was thinking about using brass wire for strength, but still keep a ok conductivity. And the same isolation sheathing as on those cut resistant cables for aberration resistance. If that is possible to obtain.I had a classmate in university that did a "powerline network" over a normal rubber insulated mains cable as an umbilical trough 200 meters of seawater for a ROV project to transfer video from a IP camera and control signals. And that worked out fine. And i am guessing that network runs a pretty high frequency modulation on top of the 220 VAC to get a god bandwidth. I remember the professor had the same comment about induction losses in the seawater making the network not working, but in practice it worked just fine. But the professor somehow had no objection to putting a cable with mains voltage in the sea water....Just a small part of the wire will be in the sea water usually. So i dont think induction losses will be a big problem.Quote from: T3sl4co1l on August 18, 2020, 10:17:14 pmThe slider needs to be stable when standing still and have smooth motion when moving so shaking it back and forth is not an option. And attaching a turbine capable of providing 20W of power is to complicated and cost prohibitive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2020, 10:51:16 pm
content: But wait, then if it's free hanging, what holds tension on it?Tim

author: Eagle_85
author_rank: Contributor
timestamp: August 18, 2020, 10:56:25 pm
content: Quote from: T3sl4co1l on August 18, 2020, 10:51:16 pmIt is attached on both ends. And the "axis" the slider is riding on is a stronger wire with high tension.The application is pretty similar to providing power to a cable car. Only that part of the wires of it is submerged in seawater, and you dont want a loose power cable dangling under the main wire. And you dont want big cable reels on the ends or on the slider (cable car) part. And finally it will be standing still for long periods, So a generator on it will not work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2020, 11:21:43 pm
content: Heh, another mechanical method: have say four wires supporting the thing, top and bottom wire tensioned for support and alignment, middle two wires for power and position. The two wires wrap around spools that are geared together, so that common mode motion (both wires pulling in the same direction) causes the sled to move, while differential motion (both wires pulling in opposite directions) delivers power to the sled; a generator then furnishes the 12VDC or whatever. Wires can be simple stainless steel (coated if desired) and no wire storage is needed as the wires are circulated continuously through the endpoints. A pair (or quad?) of servo motors provides motion.The umbilical is fine, because the wires are closely paired and relatively little external field is able to induce to the water. Signal bandwidth may not be great over such a distance, but clearly it's enough to send, BASE10-T or something like that I guess, or any of the modem standards (DSL, DOCSIS?), etc. Moreso if the cable is shielded.Nothing wrong with putting relatively high voltages in sea water, under insulation; the big boys do that all the time with 100s of kV (and 1000s of amperes). They also prefer multiconductor cable, because losses to sea water are a big pain, especially over 10s of km. Putting all the wires together in one cable helps cancel out the external field. (For 100s of km, or between national grids, e.g. UK to France, a DC link is preferred.)Certainly, I've only seen umbilical cables in ROV footage, but that's very shallow knowledge of the field overall and I have no idea what other things they might use.Tim

author: Eagle_85
author_rank: Contributor
timestamp: August 19, 2020, 08:35:32 am
content: Another question:How do i buy the the ferrite core the easiest way? Have someone make a custom core? And if so what company and how much will it cost? Or is it easier to buy some standard square and round pieces of ferrite and glue them together? And in that case what is a god source for standard ferrite pieces?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2020, 05:35:57 pm
content: Ferrite can be ground, it's a soft ceramic. Mfgs and suppliers offer grinding services, though usually for flat faces to meet a dimension or air gap, not sure about circular cuts. If nothing else, a machinist would be able to figure something out.https://www.digikey.com/product-detail/en/ferroxcube/UR64-40-20-3C90/1779-1461-ND/7651791This is the sort of thing I was thinking about. Just use two spools so the core doesn't have to spin.Tim

author: poorchava
author_rank: Super Contributor
timestamp: August 19, 2020, 08:55:00 pm
content: If I understand correctly, there will be a gap between the rotating part and the C-shaped part. Introduction of a gap decreases Al quite badly, so you'll have to wind more turns to keep the idle current reasonable. Also unshielded gaps will tend to radiated noise like mad due to fringing field.You could make it a flyback, but that requires that the gaps be absolutely stable so that the primary inductance doesn't go too low, as the gap increases.

author: Eagle_85
author_rank: Contributor
timestamp: August 19, 2020, 09:06:13 pm
content: Quote from: poorchava on August 19, 2020, 08:55:00 pmYes it is probably better to rotate the bottom bobbin and not the core. So airgaps in the core can be eliminated. I just sketched it up in Fusion 360 pretty fast without thinking to much about it.Is gluing ferrite pieces together an acceptable method of making a core? And if so what glue is best to use?

author: langwadt
author_rank: Super Contributor
timestamp: August 19, 2020, 09:11:04 pm
content: how about using a multicore cable to get more "turns" and just run it through two toroids?

author: Eagle_85
author_rank: Contributor
timestamp: August 19, 2020, 09:16:33 pm
content: Quote from: langwadt on August 19, 2020, 09:11:04 pmI don't understand the concept. Can you make a sketch illustrating it?

author: langwadt
author_rank: Super Contributor
timestamp: August 19, 2020, 10:24:51 pm
content: something like this

author: Eagle_85
author_rank: Contributor
timestamp: August 20, 2020, 07:28:56 am
content: Quote from: langwadt on August 19, 2020, 10:24:51 pmLike a current transformer? Will that be able to transfer enough power effectivly?

author: langwadt
author_rank: Super Contributor
timestamp: August 20, 2020, 08:53:55 am
content: Quote from: Eagle_85 on August 20, 2020, 07:28:56 amI have no idea, but I'd think mechanical sound takes priority over efficient

author: Eagle_85
author_rank: Contributor
timestamp: August 20, 2020, 10:52:11 am
content: Quote from: langwadt on August 20, 2020, 08:53:55 amI was just thinking that a current transformer is just designed to take out small signal voltages to measure the current in a cable and that it was not able to transfer any significant power. I dont know, but that is just my limited understanding of current transformers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2020, 04:59:08 pm
content: A transformer is a transformer, doesn't really matter how you're using it.Power companies use CTs for parasite-powered fault detectors that hang on power lines. Those only need a few watts of course (though they'll absorb hundreds under those momentary fault conditions). CTs are also used for mechanical power in power and industrial application (i.e. powering a relay). 20W isn't unreasonable, you just need enough cable to carry the current.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2020, 09:38:55 am
content: Rotary transformers are a real thing and do not require any winding and unwinding.https://en.wikipedia.org/wiki/Rotary_transformer