Author Topic: Stacking Stators in PMA (Read 6464 times)

HomegrownPower · « **on:** November 12, 2007, 05:54:58 AM »

Be patient with me guys I will not quit . I may have some radical ideas that prove how badly I need your help but reguardless of why I may fail it wont be from bugging you into answering questions that will never be used because I quit.

Basically all I'm wanting to do is build a super pma lol . Keep in mind I'm not trying to run this pma with wind power . My idea of a pma will be fastened to a conctete foundation/slab and turned mechanically somehow .

My ideas basically follow the book with the exception of stacking stators . Perhaps three magnet plates as opposed to the traditional two plate design . Other changes to the books design are the incorporation of a stainless center shaft with good quality bearings and stainless spacers to seperate them precisly .

Basically I'm wanting to follow the book design ( as I certainly haven't a clue without it ) Building the stators and magplates like the book design ( now that I have that heat sink idea out of my way ) with the major change being the number of stators / magplates .

I'm thinking of a 2 stator 3 magplate design . I call the center mag plate nothing but an exciter plate and had no intentions of adding a steel plate to back the center magplate .

Is this wrong ? If one was to stack stators and sandwish 3 magplates together should the center magnet disk be backed with steel? It just seems like the steel in the center plate would play havic on the pma to me .

Anyone thats considered a design or has info concerning a multi stator design would be very much apprechated .

Thanks
Lonnie

Flux · « **Reply #1 on:** November 12, 2007, 01:11:57 AM »

Think of your dream as two PMAs stacked on a common shaft. You double the output and keep the same frequency. With engine driving and the desire for standard frequency you may be forced to stack stators although it doesn't make as good a use of the magnets.

In this arrangement the centre steel disc would have magnets on both sides and if the disc was thick enough you could make the magnet polarity on either side the same or opposite. As you make the disc thinner then you reach a point when some flux penetrates the disc and it becomes desirable to have the magnets stacked with N outwards on one side and S outwards on the other so they are in series. Once you settle for that configuration you no longer need a steel disc at all and you would have the same effect with 2 magnets stacked together and supported in resin, wood or any non magnetic way.

It still works if you only use one magnet instead of 2 in series but you get less flux in the gap. You could even replace the centre magnet with a dummy mild steel block to carry the flux between the stators but again it would come at a price in flux density in the windings.

In the extreme case you could leave the centre magnetic circuit out and stack the 2 stators together and make it a dual rotor with a very thick stator, but the cooling would be poor.

From this you should be able to see that as long as you keep the magnetic circuits in series you don't need steel in the centre disc. If you keep 2 magnets in series on the centre disc then steel has the big advantage that you can stick the magnets on the surface. If you use something non magnetic then you will need to cut holes in the disc so that the magnets touch.

With one magnet in the centre, steel is not practical if you want both stators to produce equal power, so you are forced to adopt the mechanically difficult option of cutting holes in the centre disc to take the magnet.

For wind power I strongly discourage stacked stators as they are not using the magnets to best advantage and low speed alternators need large diameter. For engine driving I still see little virtue in a PMA, with no facility for voltage regulation unless it is for battery charging, but if you must do it then you can stack as many stators as you want but I still think it is easier to use steel discs with magnets both sides. If you can manage with less magnets then you can use non magnetic discs with magnets in holes. With 2 stators it will be fine, but if you stack many then the steel discs with magnets both sides will be better, The leakage will get a bit strange with long magnetic circuits with several gaps. At this point your stainless shaft may be needed but for 2 stacked stators it is a waste of money .

Flux

Roly SA · « **Reply #2 on:** November 12, 2007, 01:40:47 AM »

Pity i havent seen this discussion before i posted my own similar one.

I also have the idea of have 2 stators sandwiched between 3 rotors. I want to make the centre rotor from composite material with holes cut for the magnets.

The idea is to have a small diameter alternator that can spin around 3000rpm or more. I have small coils planned (still waiting on responses from other members about the size) so the more there are the better.

I dont want to hi-jack this discussion, but any interested members please look at mine as well, as i have other questions. Flux, i would appreciate your input.

http://www.fieldlines.com/story/2007/11/12/82613/291

Thanks

Roly SA

hiker · « **Reply #3 on:** November 12, 2007, 04:46:40 AM »

i belive shadow made a dual stator mill a while back--cant find the story ..

heres one ..............

http://www.fieldlines.com/story/2006/9/9/175715/7026

wpowokal · « **Reply #4 on:** November 12, 2007, 05:14:29 AM »

My Re mate did this (duel rotor), the centre magnetic rotor was of fiberglass, needless to say it failed quite spectacualy, Ok it is fair to say it was flying at my place when it failed.

You must have a solid steel centre magnetic disc as flux said. It has been discussed that a larger diamiter unit would be more cost effective magnet wise.

But we all are free to experiment as I do, sometime I win sometimes I loose but I have fun trying.

allan down under

electrondady1 · « **Reply #5 on:** November 12, 2007, 06:26:29 AM »

i have built and bench tested several multi stator alternators

they are all made with ceramic mags i had collected

a lot of little magnets of various thicknesses.

it was the best way i could think of to use them all up at once.

the center disks are made with resin or acrylic plastic.

the stators are single phase.

i recommend you rectify each separately .

balancing the rotors is a challenge.

getting then to all run true is a nightmare.

it was just an experiment.

i was looking to get 14 volts @60 rpm from each stator using ceramic mags.

it worked .

the whole thing (i call it "the creature") is dismantled and sitting on a shelf

i may reassemble it at some point to experiment with overdriving it.

powerbuoy · « **Reply #6 on:** November 12, 2007, 07:37:12 AM »

I had this pic posted a while ago. Hope it'll provide you with some ideas.

Powerbuoy

ZooT · « **Reply #7 on:** November 12, 2007, 11:09:34 AM »

I wonder how hot that thing gets without even having open sides on the center rotos let alone the stators.......

I've been following the design concepts and issues of the axial flux design for a few years......and things have changed an awful lot, as each problem has been dealt with......and heat buildup in the stator was one of those problems....

HomegrownPower · « **Reply #8 on:** November 12, 2007, 11:47:04 PM »

Zoot

I was also concerned about heat at faster rpm's . Has anyone tackled this heat problem with any success ? Thank goodness Dan B and a few others saved me from my bright idea of a heat sink attatched to my stators lol . Heck I was trying to move the heat on out of the stator lol .

After being advised not to do that I did as one person said to and just rubbed a neo 42 against the aluminum plate . Heck I didn't have to hold it against it to feel the drag . I could hold it a 1/2 inch over the aluminum and still feel the magnetic field .

Thanks for the post

Lonnie

HomegrownPower · « **Reply #9 on:** November 13, 2007, 12:08:16 AM »

Roly

No need to worry about hi jacking a post jump right in . I've given about the same design idea a lot of thought but will admit I'm too new and green to this to be much help .

As opposed to cutting holes in a composit material to place your magnets why not lay them out on a round disk with a thin sheet of paper seperating the disk and the magnets . Then mix and pour your composit around your magnets so it bonds ever so much better than cutting a hole ? Just a thought but if I had to cut the holes trying to fit the magnets tight enough to hold them in place I'd consider it . That would be my choise but I'm familiar with fiberglass and what I can do with it so maybe that makes some difference .

3000 rpm's you have more nerve that I do lol . The stories I've read about them making noise and shaking , vibrating I might chicken out at a lot slower rpm lol

Best of luck and let us know how it goes for you and I'll do the same . Be sure to fasten those mags in good 3000 rpm's is really doing some spinning . I'm going to a lot of trouble ( bearings and a center shaft ) trying to get one true enough to stand faster rpm's , but not 3000 of them lol

Thanks for your post Lonnie

HomegrownPower · « **Reply #10 on:** November 13, 2007, 12:16:32 AM »

Allen

May I ask the reason this failed ? Did the composit magplate fly apart ? Thats one thing that concerned me on even the steel backed plate . I was thinking of a round ring of somekind going around the outter dia of the magplate to help insure this didn't happen .

Please advise why this failed . Was it the disk comming apart or was it a problem with rubbing assotiated with alignment problems , heat or something else ? How long did it run and how did it do while it was running ?

Thanks for the post Lonnie

HomegrownPower · « **Reply #11 on:** November 13, 2007, 12:31:37 AM »

electrondady1

I like the way you think . I'm too trying to use up a bunch of weaker magnets . I can stack ( 4 in a stack ) two stacks beside each other and end up with a magnet block about the same size as my neo's . neo's are 2 inch x 1inch x 0.5 and stacking 8 of the small magnets I have will get me 2 inch x 5/8 x 5/8 .

I can hold a neo in each hand and get my wife to place the stack between them . They have an incredable flux force when they are mated with the stronger neo's . I've never done any of this before but I think they will do a great job of spanning the gap of an extra stator .

I'm curious about this single phase stator construction I'm going to look in the pages here to see if I can find the proper wiring . I already made one magplate with 12 poles so I'm not sure I still have the option of going to single phase I may be locked to using the 3 phase design .

Personaly from what little I know about this stuff I think I could use the single phase current more efficiently and easier also . I'm more interested in trying to use the straight ac produced as opposed to rectifying it and charging batteries .

Thanks for your post any links to a single phase wiring / coils /pole design would be appreachated .

Lonnie

Flux · « **Reply #12 on:** November 13, 2007, 02:01:24 AM »

I think there is some misunderstanding about heat and speed here.

The reason for heat is that the current density is being pushed too high for the size of wire. People are trying to get too much power for the size with the difficult loading into batteries at constant voltage. You push the alternator efficiency too low and more power is lost in the stator than goes into the battery.

With high speeds you can use less turns of thicker wire so the losses are smaller. With a prime mover other than wind ( engine, hydro etc) you only need to run it fast enough to provide the excess voltage over the battery to drive the current you want, the efficiency remains high and stator heating is a far smaller issue.

Any low speed alternator needs to use lots of magnet and copper to remain efficient. The matching to a wind turbine actually forces us to keep the electrical efficiency low to keep the blade speed up, so it is cost effective to place this low efficiency in the alternator. If you make a big and expensive alternator you will need to add loss in the line for the same power out of the blades. This is expensive so you try to compromise with an efficiency that matches the load and keeps stator heat within reason. Far better to use a more powerful alternator and keep the losses outside but it comes at a high initial cost.

This still applies to iron cored machines, but they dissipate the heat better and can be more severely overloaded without exceeding the wire temperature limits. if you match the air gap machines to the load then this heat problem doesn't get out of hand. Incorrect loading of a motor conversion may not burn it out, but the low efficiency still reflects in the power out.

Perhaps a comment I made earlier about keeping iron in the centre disc of a 2 stator machine may not be out of place. Magnetically I suggested that iron is not necessary but mechanically you need to remember that these discs ( all of them ) are fairly well stressed axially by the magnetic attraction forces and the limit to diameter of axial machines is probably decided by the strength of the discs and their stability against the magnetic attraction.

In theory the centre disc is balanced in pull, but the condition is totally unstable and you can't manage to keep this force balanced. There is a big problem during assembly but it never goes away completely. Any composite material will need to be completely rigid axially as well as holding the cf forces on the magnets. If you can satisfy this in proximity with a fairly hot stator then ok.

You can mount the magnets in holes in stainless steel discs, but I fail to see the advantage when they can be stuck on to mild steel that provides mechanical strength and carries the flux. Using 2 magnets on the centre disc gives more flux than the single magnet in a composite or stainless disc.

For wind I strongly advise against stacking stators, it is not cost effective, has worse cooling and gives a lower output for volume of copper and magnet.

If you must produce engine driven PMAs ( or for faster hydro) then stacking may be sensible but don't ignore the mechanical challenges.

Flux

electrondady1 · « **Reply #13 on:** November 13, 2007, 07:03:18 AM »

homegrownpower,

if you are determined to use your small ceramic magnets there is another option that you may consider trying.

place all your matching neo magnets on one larger steel disk (twice the dia.) giving you 24 poles

place all your ceramic magnets on another matching steel disk.

you can then build a conventional single stator alternator with neos on one side and ceramic mags on the other.

DanB · « **Reply #14 on:** November 13, 2007, 08:39:15 AM »

Just a comment. This is certainly not a new idea - it comes up all the time and generally people are discouraged from doing it. Flux has said that this is not a good use of materials and (maybe I missed it) but I don't think anybody has explained why in this thread yet.

I would never stack alternators like this unless I had to keep the diameter small.

You stack two alternators and you'll get twice the power at any given speed.

If you put all the same magnets and same copper into a single machine (two magnet rotors 1 stator) you will get 4x the power at any given speed.

You probably know this but if you can't get your head around why... consider a simple alternator with 12 magnets/rotor and 9 coils and it will make 'x' amount of power at a certain speed.

Then consider the same 9 coils placed around 'half' a stator twice as large as the last one (but it's still only half a stator) and two magnet rotors twice as big in diameter each with 24 magnets on it. Each time around that 'half stator' is seeing twice as many pole changes (kind of like running the 1st alternator at twice the rpm) and those same 9 coils will now make twice the power they did on the smaller alternator. And... you still have room left for 9 more coils! Twice the magnets, twice the copper - 4x the power. This is why it rarely makes sense to stack things like you suggest.

You seem keen on saving money by using old magnets you have laying around (I think you said they are weaker...) - you'll likely still be buying other stuff though (Copper/resin etc). I expect at this point you would be much better off to get good magnets, build a single rotor machine and at the end of the day you'll likely save money and get more power. Copper/steel/time etc... is not cheap - it's ashame to invest too much and then compromise it all with bad magnets.

HomegrownPower · « **Reply #15 on:** November 13, 2007, 12:52:12 PM »

Dan B

Thank you for taking the time to explain that so a green horn can understand it . I am keen on saving money thats why I down sized my magnet disk from 12 inch to 11.25 . I already had two free steel disk from other junking operation . Another big reason I have the assembly to mount my pma into also making my machine work cost almost zero to date . I did have to compromise in dia though to use my ( on hand material ) .

I see and understand what your saying and really appreachate the information . Again my magplate dia was compromised do to the size of the housing it will all fit into and plates available to me so I really had no choise holding down cost .

I took flux's advise on the number of turns and wound my coils small 25 turns . I think he recomended 25 to 30 turns for wire 13 awg . With his later explanation of this heat problem at faster rpm's I'm very proud I was smart enough to follow his advise on that as heat ( over heating is a main concern to me ).

If I experiment and determine I can have a better pma by removing one stator and the center ( weaker ) magnet plate then I'll gladly remove them . My main reason for two stators is not to make the PMA twice as strong . I could be completley wrong in my thinking but I'm trying to use twice the copper to produce the same output with less heat perhaps less rpm's .

I am perty darn thrifty in my project building . I have to be I only have a fixed amount I can alot twords something like this . I bought 32 neo 42 magnets 2 inch x 1 inch x 0.5 for 133.00 U.S , ebay item 110081970650 . I bought a thousand feet of awg 13 for 80.00 bucks lol 17.5 pounds it's worth that as scrap to me ebay item 250177217954. When a stator burns out I can toss it in the stove . Burn off the fiberglass and sell the copper and recoupe my cost . To me thats thrifty enough lol

I wound the coils for 2 stators and have half of it still left . I'd about bet money I could relist the remaining 8 to 9 pounds left over and recoupe my cost making the stators practically free . However I'd rather keep it around and work on one thinner stator with much more copper in it . Thats another project but I'm thinking on winding a stator more like a alternator on a car . If you ever took one apart you know what I mean theirs actually 3 sets of coils fixed to the outter laminated ring not one . It's been along time but it seems like slightly different size wire was used also . Thats not what I was wanting to tinker with someday . The aspects of their design I 'd like to play with is overlapping the sets of coils like they do .

Point is even if I experiment and determine I get no gain using two stators ( less heat is a gain to me ) I can still pull it apart remove one stator and the weaker magnet plate and still have one hell of a single stator PMA at a cost for all that many couldn't build the double disk single stator PMA for . Plus I get the experience from it all and the extra stator for back up in the even I burn one out .

Thanks again for your post and explaining that so I could understand it . Wish I hadn't been limmited to the size restrictions I was but the savings of using materials I already had were to substantual to ignore them .

Thanks again for taking the time to explain that

Lonnie

HomegrownPower · « **Reply #16 on:** November 13, 2007, 02:15:10 PM »

Flux

You have convinced me I think I'll go with a steel backed center disk . Mechanical strenght and holding things true are a major concern to me and I can see the advantage of having the steel plate . I was first thinking a steel plate would break the flux field between the two stronger neo's ( trying to use the center magplate ) to more less jump the gap of the second stator but depending on both neo's to supply the flux field . However I think the extra strenght will be a good trade off if that did happen and acording to you it should still be stronger than the 2 neo's and the weaker mag's in a series . That means going to a shop to get my center hole drilled . I can grind the outside good but the center hole on the third plate I'll never get perfectly in the middle without a shops help . Costly but well worth it to me to keep things good and true .

Thanks for the more detailed explanation on that and the over heating problem . I realise you probably said it one way or another several times but you hit the bell this time . I get a much better grasps of the heat issue now . THANKS

I was wanting to use more windings in my coils lol . Then I thought about your advise and followed it . I made them 25 ( I think you advised 25 to 30 )turns where I probably had room for 35 to 40 . I wanted to fill them up with copper so bad ( I had the wire ) lol but I resisted that urge . I knew you perty well understood what I'm shooting for in a pma and new you gave that advise for a reason even if I couldn;t understand it but still the urge was hard to ignore lol

I may very well find out the pma puts out better and stays cooler with one stator and two magplates . If I do oh well I'll rearange them quite easily . I'm not sure if you were joking about adding a fan blade to my center shaft but thats my plan . My housing that holds the pma assembly has plenty of room lenght wise to add a fan blade from a small car to help cool things .

Thats the reason for pouring my stator so air can pass through the coils as opposed to resen filled coils . Plus saving a little resen will help also . I will keep everyone posted how this goes .

One question I need to start figuring out . As I build this I'd like to test it in different stages and at different rpm's . I'd like to test it at several different rpm's I can establish with a drill press if it will work like I think it will .

I'll never figure out these math formulas and so on that many here can just pop out . The best I can do is to set my drill press to a certain belt setting to turn a certain rpm and then test and write down the results .

I'm looking for a small hand held thing to test volts , watts or what ever you test for then with luck I might get a formula and figure out the rest . Keeping cost in mind is their any products out there I need to buy ? Any ones that are junk I need to stay away from ? I have no idea about size either . If you could recomend a certain product I'd appreachate it .

THANKS again a million for hanging in their and explaining all this to me . I know I've asked a million dumb questions but wouldn't even have a slight grasp of this without everyones help .

Thanks Lonnie

HomegrownPower · « **Reply #17 on:** November 14, 2007, 06:38:20 PM »

electrondady1

I can't change the dia of the pma due to having most of the parts and need to use them to save the bucks of fabracating them . I'm going to do as Flux advised and use a steel plate with magnets on both sides .

I've already had to squeeze the 12 inch machine into a 11 and 1/4 inch disk . Thats primarly because of having 2 mag plates that size , the housing to mount the shaft assembly into and even a lid for keeping rain out so it just made practical sence to adapt the PMA to fit it . If I ever wanted to have a belt tentioner for different size pullies for different rpm's I have the cabinet the housing sets on already made for doing just that .

Since I'm throwing in all loose magnets on the place trying to use them I think I'll throw in the 8 neo's I had left over . That way I could put 4 neo's on each side of the center plate with two sets of the weaker magnets in between each neo.

With all 32 neo's and 16 groups of the other magnets almost the same size as the neo's it will have to have an incredable flux field . At least it seems like it would have to I wouldn't want my finger inbetween the plates lol .

Attatched is a pic of my toy so far . Looks like completion will be pushed back a week or so . I'd be interested in seeing this stack of coils you made if you could do that .

Thanks for the replys and have a good one Lonnie

electrondady1 · « **Reply #18 on:** November 15, 2007, 07:09:17 AM »

some times we take on projects and they begin to have a life of their own.

we make a series of compromises that changes the original concept.

you are saying you have a total of 32 2x1x.5 neos ?

you can put them together any strange way way you like,

as long as you realize the optimum configuration would be to place

16 of each on two 14"steel disks.

the forces you are dealing with are not trivial !

please take precautions so that you are not injured.

i think its a mistake to mix neo and ceramic mags on the same disk as you describe.

looking at your housing it appears as if the bearings are not as yet mounted permanently.

you could adjust there placement to accommodate larger rotors.

you already have perhaps $300.oo invested in powerful magnets

why not spend the $50.oo and get the right size rotors.?

Roly SA · « **Reply #19 on:** December 31, 2007, 05:38:06 AM »

After reading this i decided to design a larger diameter stator, with 9 coils to fit in about a 12" disk. Since i plan on doing a high speed alternator, it will be better to use thicker wire and fewer turns in the coils. I also want to drill a hole just big enough to fit in the gap of each coil, to get air moving through the stator and dissapate any heat somewhat.

Another idea i have is to cut a ring as backing for the magnets, and use struts/spokes to connect it to the centre axle/hub. this will hopefully bring the rotating weight down somewhat. not sure if this will create any balancing issues or not.

This will hopefully show what im talking about, please forgive the crudeness of the picture, as i only had paint available to draw it.

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Author Topic: Stacking Stators in PMA (Read 6464 times)