Large diameter wedge magnets on smaller diameter rotor

[dciolek]

Just toying around learning about wind turbines for the first time.  I had some used parts lying around (rotors, a hub and an axle half shaft from a Chevy 2004 Impala) that I thought would make good junkyard stock for a turbine project.  The two rotors sandwiched back to back in the reverse direction and cranked down to the hub with the lug nuts creates a pretty decent air gap that could fit 1/2" magnets and a 1/2" stator with somewhere between 1/16" to 3/32" clearance between stator and magnet all put together (not sure if that is too little or not).  I think I sorta realize that the car rotors are a bit heavy to be ideal vs. just some designed steel plate custom cut for the purpose, but this is what I'm toying with for now (I'm not a welder or skilled in metal works -- so not sure why I'm toying).  However, I am wondering about the magnets to use...

Not sure if bigger is better with this setup or not -- but I noticed that wedge magnets present more surface area in the same angle than bar or discs.  I also noticed that there are these neat 14" OD x 8" ID wedges available (16 pieces complete the loop).  However I have only about an 11 7/8" diameter car rotor to work with (rust probably grabbed the other 1/8").  Is it feasible to use only 12 of these wedge magnets successfully with the car rotor?  That matches up the inner diameter pretty well at 6" ID (6" ID / 8" ID * 16 magnets = 12 magnets) and this should fit on the rotor with the outer edge of the magnets only slightly hanging over the edge of the rotor and separated at the ouside edge slightly due to the difference in diameter and number of magnets used.  Is this a no-no?  Should you never have the magnet all the way to the edge of the rotor -- or covering the entire available sweep dimension (gotta leave almost a six inch hole in the middle for the hub)?  Don't know the trade offs here and thought I'd pose the question.

Seems like more magnet surface area is good -- but my air gap isn't very adjustable (without shimming with washers or otherwise).  Any thoughts?  Should I not be jumping so fast without more knowledge on things like this?

[wpowokal]

While I am not sure what you mean by rotors (language difficulty) you could mean disc brake or flywheel,  others have use disc brake discs, using 12 magnets to 9 coils is classic 3 phase (3 coils/ phase, as to clearance then the least you feel  comfortable with for heavy discs i assume distortion (bending from magnetic pull) is not an issue.

The placement of the magnets as you describe is in order with a couple of buts, the closer the magnets are to each other side by side the more flux that leaks between adjacent magnets instead of passing through the coil to the face opposing magnet but it works. Look here although I suspect you have already http://www.windstuffnow.com/main/dual_rotor_turbine.htm

When one considers the cost of the magnets, having two ideal plates cut is small change. (2nd but)

Oh and one warning!! proceed with this project and you will be hooked on the subject.

allan

[dciolek]

We are probably talking the same thing (disc brakes sounds like the same thing -- I buy them as "rotors" for a car, but its shaped like a disc and it has "brake pads" squeezing on this disc to stop the car).  These rotors are dual plated with fins connecting the two discs and is 1 3/16" thick overall (heavy for what I read people use as rotor for their turbines).  Just so happens that's what I have to play with though, and I thought these parts are available in any junkyard if you had to scrounge them and couldn't weld or cut steel or whatever.  Not sure if I can finish the project under this restriction or not and what compromises would need to be made to avoid custom fabbing anything.

That's exactly the kind of info I didn't have on magnet placement though -- thanks!  If I use the method I was describing above -- the magnets along the inner diameter of 6" would all be touching at the inner corners like in the attached 1/2 scale CAD drawing.  Sounds like it will experience loss of flux due to the proximity -- even though I have more magnetic surface area per angular section of rotor.  So it sounds like it may be "better" (from a $$ per useful flux capacity standpoint) to use rectangular mags that fit the 30 degree section and allow for more separation.  Or maybe the 2" discs like in my first design layout (see attached).

Was thinking you could "overmagnetize" the turbine with too much magnet and too little air gap such that it doesn't turn well (probably all the discussion on air gap and cut in speed and cogging and stalling I guess).  Also sounds like you can spend big bucks on big strong magnets and lose a lot of generating capability by missing on any number of parameters.  Sounds like there is MUCH to learn from trial and error -- although you can't make too many mistakes when magnets cost so much...

Funny thing is though - I won't even be able to operate this beast if I build it because I live in a neighborhood with restrictions on things like that and don't have the space to fly it anyway.  Just really curious about the technology and it may come in handy some day.

[ChrisOlson]

Not sure if bigger is better with this setup or not -- but I noticed that wedge magnets present more surface area in the same angle than bar or discs.  I also noticed that there are these neat 14" OD x 8" ID wedges available (16 pieces complete the loop).  However I have only about an 11 7/8" diameter car rotor to work with

I wouldn't use those big wedges on that small of a rotor.  They're too powerful.  I got a set of those big wedges in the generator for my 21 footer that I haven't finished yet and there's several hundred pounds of force pulling those two rotors together.  Might be in the thousands of pounds of force between those two rotors (20 magnets on each rotor, 22" diameter x 3/8" thick).  I'm fairly certain you could stick one of those rotors to the roof of a car and lift it off the ground.  But I don't know how you'd ever get the rotor off the car again.

I'd use the smaller ones that form a 8" OD circle.  Those will fit nicely on that size rotor and allow you to use less turns of wire than using the 2 x 1 bars.  With wedges you have to make sort of wedge-shaped coils though, which have a longer turn length than winding round coils for round mags.  I've tried both ways in the past and still get better performance from the wedges, though.  And Ed Lenz has does a lot of experimenting and found the same thing - 10-11% more power out of the same generator simply by using wedges vs another shape.

One thing about using wedges is that they don't come with holes to pin them to the rotors.  So you either have to band the rotors, drill holes in the magnets to pin them, or machine a groove in the rotor to within a few thousandths of an inch from the edge for the magnets to fit in so they don't fly off when the glue eventually gets weak.  I prefer drilling and pinning them - they can be drilled with a cobalt drill at very low speed in a drill press but it's time consuming and kind of messy.
--
Chris

[bob golding]

hi, i used those small wedges ed sells on volvo  discs quite successfully for a few years. the volvo discs are 10 3/4 inches i seem to recall. heat  can a problem if your blades are too big. otherwise go ahead. if you are in a city why not try a bicycle generator? plenty  of things to think about with one of those if you just want to dip your toes into the subject.

[dciolek]

OK, a few hints from u guys brings a few improvements:

1) Create a lip around the outside of the rotor.  Helps to hold the magnets in from flying away when at high speed (place outside of of magnets at 11.875" OD on a 12" rotor).
2) Use smaller wedges -- like (16) 22.5 degree 8" OD x 4" ID wedges on a 12" rotor (see attached).  This creates some gap between the magnets to better concentrate the flux intercepted by the stator.
3) Create a method to mechanically separate these rotors from each other down the road.  Realize these magnets are serious stuff -- it will take more than a hard tug to get them apart.
4) Start small and learn more along the way before you jump into the big stuff and build something you can't even effectively use.

All good advice for me - THANKS! Except I'd struggle to do #4!  Knowing me, I always do the super-extra-turbo version of anything (my razor has five blades not two, my kitchen table seats 12 for a family of 4, my car has a supercharger when I can't use that much power anyway, etc).  I know that I SHOULD start small and be reasonable rather than be over the top.  But then I wouldn't be me and I'd lose interest in the effort...

OK, OK though, I'll scale it back to 10' diameter blades instead of 20'.  Actually -- I have no clue how to "size" the blades for this heavy rotor/magnet setup in a relatively low wind area (Ohio).  Not even thinking about blades or towers yet -- just understanding the magnetics and electronics and mechanics for now.  Maybe that's my first mistake.

[ChrisOlson]

hi, i used those small wedges ed sells on volvo  discs quite successfully for a few years.

So have I.  Those small wedges come in either 1/4" or 1/2" thick, and for a 10 I'd use the 1/2" thick ones.  But I've built several with the 1/4" thick wedges too, and they work very well for smaller generators.

The big wedges come up to 1" thick.  You put 40 of those on two generator rotors and you got pretty serious flux.  Those big mags have 6.48 in² of surface area x 20 = a whopping 129.6 in² of magnet area on each rotor.  If you miscalculate even a little bit on the windings you'll end up with a machine that makes 233 volts @ 180 rpm.  Don't ask me how I know.  And I haven't figured out how to fix it yet either because even in delta or Jerry it makes way too many volts.  One solution I came up with is that I need to find a set of Jake 31-20 blades to drive the thing.

So, for dciolek here, I just don't think those are the magnets to fiddle with when you've never built a turbine before.
--
Chris

[dciolek]

I may have to use the 1/2" thick magnets regardless if I stay on the path with the back to back car brake rotors instead of fabbing custom steel plate.  Based on the "fixed" distance by bolting two of these guys back to back with their matching hub and lug nuts (just enough over 1 1/2" apart to leave between 1/16" and 3/32" gaps between a 1/2" stator and the magnets on each side) -- if I went to 1/4" magnets -- I'd have 5/16" - 13/32" gap on each side of stator.  Maybe that's OK?

But I think I've got a bigger design flaw -- weight.  Just weighed the rotor assembly when put all together and it is about 45 lbs (that's what's turning WITHOUT magnets and blades).  Seems like I'd need a gale force wind to turn that bad boy...

[ChrisOlson]

if I went to 1/4" magnets -- I'd have 5/16" - 13/32" gap on each side of stator.  Maybe that's OK?

That's too much air gap for 1/4" magnets

But I think I've got a bigger design flaw -- weight.  Just weighed the rotor assembly when put all together and it is about 45 lbs (that's what's turning WITHOUT magnets and blades).  Seems like I'd need a gale force wind to turn that bad boy...

Not necessarily.  IIRC, the generator rotors for my 13 footer weigh about 24-25 lbs each with magnets on and those are made from 3/8" steel plate.
--
Chris

[dciolek]

I just realized that with the half axle shaft I'm using to connect the blades to the rotors (through the hub bearing and tightened with the axle nut) -- I can remove the blades from the PMA easily and attach a different half drive shaft rigged up to couple to a gear that can be attached to a bicycle.  Viola -- hot swap bicycle generator (just like bob golding suggested earlier).  Although, not knowing much about these things -- I wonder how difficult it would be to actually turn the gear using foot power when it is attached to a 45 lb assembly and held in place by the magnetic attraction of 32 high power magnets.  That would probably put "spinning" (that aerobic bicycle exercise with the heavy fly wheel) to shame...

I suppose since the axle nut holds a tire onto a car at high RPMs forever -- it could do the same thing for a set of blades on the wind turbine?  Or is the vibration and rotation different enough to work that nut loose over time and send the blades flying?  Anyone have experience with that attachment method?