Energy harvesting using a brushless generator

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
super-multipolar2.png

Ok, so I want to build one of these things so I can test it. What I'd like to do is simply select a commercially available coil, and attach six of them to a structure similar to the one shown in the image. I have no problem fabricating and shaping the necessary structural hardware, but I can't find what I want regarding the coils. The device would be quite small, smaller than 2 inches in its outer diameter, so the coils need to be small too.

Question, where can I find said coils already wound? I already searched Digikey, and the only thing I could find is the plastic part used to wind the magnet wire around.

B65542BT1_sml.jpg

Would it be possible to somehow use a small telephone transformer for this application?
 

MisterBill2

Joined Jan 23, 2018
10,592
Definitely worth looking into it... mind sharing any clues as to how I should be looking for it? i.e. what would the right search keywords be for me to use?
A series regulator to switch off the voltage when it exceeded some preset value would work well and not dissipate a lot of power. So there would be less stress on components and they would run cooler and last longer. And probably the series elements could be cheaper as well.
And the generators that use a rotating permanent magnet are usually called alternators because they produce AC and need an external rectifier. The more efficient systems use all of the output voltage to power an inverter circuit that serves as a regulator. A lot more complex, but about as efficient as can be.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
A series regulator to switch off the voltage when it exceeded some preset value would work well and not dissipate a lot of power. So there would be less stress on components and they would run cooler and last longer. And probably the series elements could be cheaper as well.
And the generators that use a rotating permanent magnet are usually called alternators because they produce AC and need an external rectifier. The more efficient systems use all of the output voltage to power an inverter circuit that serves as a regulator. A lot more complex, but about as efficient as can be.
Thanks, I'm currently looking at some energy harvesting ic's out there, that feature a polarity rectifier, such as this one. Right now I'm not sure exactly how this thing is going to end up working, but I'm beginning to get the idea. An important requirement is low cost....
 

MisterBill2

Joined Jan 23, 2018
10,592
Thanks, I'm currently looking at some energy harvesting ic's out there, that feature a polarity rectifier, such as this one. Right now I'm not sure exactly how this thing is going to end up working, but I'm beginning to get the idea. An important requirement is low cost....
A small stepper motor would also work as a variable reluctance generator and they are often available quite cheap as surplus, as well as in scrapped printers. And most of them come with a gear already attached.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
A small stepper motor would also work as a variable reluctance generator and they are often available quite cheap as surplus, as well as in scrapped printers. And most of them come with a gear already attached.
Thanks for the suggestion. But it's a requirement that the device itself be gearless.

I'll definitely give a miniature step motor a try, though.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
I've gone through several YouTube videos regarding this type of generator (which is used mainly in windmills) and I'm beginning to realize that this venture is going to be a bit more challenging than I thought.

I've updated the thread's title to better reflect what it's all about, and to see if I can draw more interest in the discussion.

I hate going empirical on this, but I think I'll start testing several magnet-coil configurations and see what works best. What I'd like to know is how to calculate power output taking into account a magnet's strength, a coil's number of turns and rpm's applied to the device.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
With difficulty I suspect. Rate of change of magnetic flux through a coil determines the generated current, but will be highly dependent on magnet strength, pole geometry and rotor position. Rather too many unknowns.
Yeah, I had thought as much... it's probably easier to use FEA to simulate it than trying to calculate this thing. And easier than FEA would be to build several prototypes and test them, which is what I intend to do.
 

shortbus

Joined Sep 30, 2009
9,073
Question, where can I find said coils already wound?
Been thinking about this. They may be a little bigger than you are wanting but would be cheap to get to experiment with. go to an auto wrecking yard and get some relays from a car. Then take them apart to get the coils from them. You then should have a cheap coil on a bobbin ready made that can then be used to size the stator core from. If they work out, then using new PCB mount relays in the same way will get you a smaller version, at a higher price. But the car relays would be good and cheap for experimenting.

Again using car parts I would look at the reluctance sensors from the 1980's GM crank position sensors. The only problem with them would be they won't meet your size requirements. They have the magnet surrounded by a coil, like in an electric guitar pickup. Which may be something else to experiment with, the coil surrounding the magnet instead of the way you showed in an earlier post. The rotor would be made the same. When I first saw the picture I thought you were making a SRM, switched reluctance motor, as that is how they look inside.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
Alright, here's my plan.

I stripped naked an inductor that I had laying around, and after careful consideration, I think that it could be used as a coil element in a semicircular arrangement.

Capture.JPG

Maybe I could put 6 or 8 of them together pointing inwards, with opposite pairs connected in series, and place a rotating magnet in the center. Or, plan B, would be to drill a hole through each inductor's axis, and insert a small cylindrical magnet in it. Then use a toothed gear-like rotating element instead.

Which arrangement do you guys think would be more effective?
 

shortbus

Joined Sep 30, 2009
9,073
Without either a central magnet or other metal in the core of that coil(inductor) I don't think it will give very much output if any. Personally I'd go with a magnet for the core, then your rotor only needs to be iron/steel. Doing it that way makes each coil like a guitar pickup, with the rotor teeth being like a string.

IIRC, your looking for a low loss DC from this. Is using three phases(6 coils) the right approach? Doesn't that increase the voltage lost to rectification?
 
You don't need to go through the entire thread to understand what I'm aiming for. Essentially, everything you need to know can be found in post #22, and post #31.

Ok, so I want to build one of these things so I can test it. What I'd like to do is simply select a commercially available coil, and attach six of them to a structure similar to the one shown in the image...​
Alright, here's my plan.

I stripped naked an inductor that I had laying around, and after careful consideration, I think that it could be used as a coil element in a semicircular arrangement...



Please be advised that (as I understand your project) the magnetic properties of ferrite core materials are woefully inconsistent with your aims! -- I suggest (non-grain oriented) silicon steel (e.g. 'Arnon', etc...) as a far superior choice...

Sincere apologies should I have 'missed' something or merely 'echoed' the post of another!:oops: -- My time isn't my own as of lateo_O:rolleyes:

Very best regards
HP:)
 
--Addendum--

@cmartinez

Here's a 'crazy' thought... It occurs to me that appropriate coil forms might be prepared via careful sectioning of an armature salvaged from a small DC motor (of the sort used in toys and inexpensive tyre- inflators, etc) -- Granted a "jeweler's/surgeon's touch" you might even keep the windings intact...:)

Best regards
HP:)
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
Without either a central magnet or other metal in the core of that coil(inductor) I don't think it will give very much output if any. Personally I'd go with a magnet for the core, then your rotor only needs to be iron/steel. Doing it that way makes each coil like a guitar pickup, with the rotor teeth being like a string.

IIRC, your looking for a low loss DC from this. Is using three phases(6 coils) the right approach? Doesn't that increase the voltage lost to rectification?
Shortbus, I think your idea is definitely worth considering. All of the literature I've found out there deals with the kind of design that places two magnets between two halves of a toothed stator. But also these designs consider much larger dimensions than I'm contemplating. Using coils wrapped around individual magnets like a guitar pickup makes a lot of sense to me... (and I also happen to play the guitar... well duh! ... why didn't I think of that before?)

Maybe that sort of design is too expensive to consider at large dimensions, but a lot more practical at smaller ones.

In graphic terms, here's my plan:

upload_2018-3-30_18-1-3.png

Questions:
  • What would the outer ring's best material be? Magnetic, or non-magnetic? ... for instance, steel or aluminum? I'm guessing the best choice would be to use magnetic, to better focus the magnetic fields inwards... but it's just a guess.
  • Instinct tells me that the magnets' poles should be oriented alternatively. That is, pointing inwards one magnet's north, and the next one's south, and so on. But I'm not sure as to why that should be.
  • If I were to use 1/4" diam x 1/4" long magnets, would it be ok if the coils consisted of 150 to 200 turns of ga #28 magnet wire? ... this is just an estimate (read: wild guess) on my part, and I'm not sure if there's a "rule of thumb" out there that I should follow.

Btw, I plan to connect the device's output to a special energy harvesting chip, probably the LTC3109, which is an Auto-Polarity, Ultralow Voltage Step-Up Converter and Power Manager. I'll be using its output to charge a supercapacitor and run an ultra low power MCU.

 

shortbus

Joined Sep 30, 2009
9,073
Hi C. I'm no expert on any of this, so I'm just relating things from experience.

What would the outer ring's best material be? Magnetic, or non-magnetic? ... for instance, steel or aluminum? I'm guessing the best choice would be to use magnetic, to better focus the magnetic fields inwards... but it's just a guess.
The reluctance pickups used as cam and crank sensors, that I've replaced and then took apart to see what was in them, don't have any magnetic "return path". By this I mean they don't use a ring or surround to give a path for the magnetic field. So don't think that is needed. So aluminum would probably work, since they just use plastic in the cam/crank sensors. Again think about a guitar pickup, they, other than Humbuckers don't use a ferrous plate to connect the magnetic field.

Instinct tells me that the magnets' poles should be oriented alternatively. That is, pointing inwards one magnet's north, and the next one's south, and so on. But I'm not sure as to why that should be.
I'd say it depends on what you're wanting voltage wise, AC or DC. Alternating the magnetic poles would give AC, Like a car alternator. Having them all the same pole should give DC, like a older cars generator.

If I were to use 1/4" diam x 1/4" long magnets, would it be ok if the coils consisted of 150 to 200 turns of ga #28 magnet wire? ... this is just an estimate (read: wild guess) on my part, and I'm not sure if there's a "rule of thumb" out there that I should follow.
From years ago here on the forum one member, Praonevou was his name but the spelling may be off, was using a free "magnetics circuit simulator" but I don't recall the name of it. This may be it but I don't really remember.
https://quickfield.com/glossary/magnetic_circuit_reluctance.htm Maybe a deeper Google search can help you. He did some simulations for me when I was first looking into making a SRM.

Can I ask what the end result is your looking for? Is this to measure RPM or power one of your machines? On another topic, how did you make out with changing the drill motor gears? You haven't updated that lately, unless I missed it.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
Many thanks, Shortbus. I agree that a guitar pickup would be an excellent reference for what I'm trying to accomplish. I think I'll even buy a cheap-o one, and take it apart... see what's inside. My goal is to power up an ultra-low power MCU while also charging a supercapacitor for its eventual use on something else.

As for the drill thread, that project's still very much alive. Right now I'm having a couple of plastic/aluminum samples of the actual gears generated, since they seem to be of special design and do not comply with standard tooth dimensions. When I finally get it right, then I'll work on the modified versions.

@#12, I found this thread opened by you a while ago, in which you showed interest in energy harvesting techniques. If you feel like joining in, all you have to do is read posts #22 and #36 to get up to date with what's being discussed.

And another thing, from what I've seen documented online, this device will produce an AC waveform regardless of the magnet's orientation. This because it appears that a current is induced in one direction when the gear's tooth approaches the magnet and then it's induced in the opposite direction when the gear's tooth moves away from it.
 
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shortbus

Joined Sep 30, 2009
9,073
As for the drill thread, that project's still very much alive. Right now I'm having a couple of plastic/aluminum samples of the actual gears generated, since they seem to be of special design and do not comply with standard tooth dimensions. When I finally get it right, then I'll work on the modified versions.
Oh, your going about the gear thing different than I would have. Since you were having to re use the gears from the first and third reduction, they would already be correct. Then by having the center to center distance of the second and third reduction, you would just pick what ever gears that make up that distance. And by picking those gears they would be the correct pitch for that pair. Almost any seller of gears gives the pitch diameter of the different pitches and tooth sizes. Over the years I've made many things work by doing this without having to buy or make special gears.

Most gears in drills like that are stub tooth for some reason, and stub tooth gears are not listed in a lot of catalogs.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,554
I've just learned that guitar pickups are normally made of #42 awg magnet wire, which is exceedingly thin, about 0.003". And that makes sense. But neither Mouser or Newark seem to sell it that thin, but it's a staple on Amazon. And affordable too, at about $20.00 dlls for a 13,000 feet spool.
 
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