Author Topic: Single rotor dual stator design  (Read 7386 times)

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capthook

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Single rotor dual stator design
« on: November 06, 2008, 08:32:38 AM »
Hi - long time lurker - first time thread.


I'm looking for input on my single rotor/dual stator design.

Something similar to this thread: http://www.fieldlines.com/story/2006/9/9/175715/7026


Most discussion on this board relates to axial flux/air core design.  I've read a few threads discussing the single rotor design - it seems most recommend against it.


However, my design is for low RPM/high torque and the most important parameter is to

generate as much power as possible with each RPM: cogging, eddy currents, heating etc. are less important an issue.


As such - it seems apparent to me that coil cores are needed, grain-oriented silicon(electrical) steel laminations being the superior material.


The single rotor/dual stator idea is to capture the field from both sides of the magnet into the dual stator coils.  It seems this logic is flawed when considering an aircore design - but I'm of the notion that it is a practical design for a device using cored coils.  


The design is 3 phase and uses the mag/coil formula of:

# mags/2 x 3 = # of coils. example: (12 mags/2)x3= 18 coils

This is diffent than the 12/9 axial flux design - but is accepted given the proper geometry:




The protype:








Small protype design


An 8" inch rotor / 12 mags .75" x .75" N42

With 18 coils each side, 3 phase star (6 each phase) : each side then connected parallel.

Coils: #30 AWG / 1"width x 1"long / 30 ohms

Core: 5/16" grade 2 hex bolt (for now - same size electrical steel laminations later)

Airgap (each side): 1/4"

RPMs: 60


The small wire is so-as to maximize turns to boost voltage.


- - -


What can be done to improve this design?

Or am I way off base?


1. The coils are receiving large flux.  A paper clip will stick to the back of the core.

Should the backs of the cores be linked with iron? (like a cross-bar between each)

Or should it be a full circle of laminations?  Or none of the above or something else?



  1. Should I wind the coils with 2 or 3 in-hand to drop the ohms and use a voltage-boost circuit if needed?  
  2. An approaching N magnet is 5% over the coil while the receding S magnet is 5% over the coil.  Does the geometry need to be tweaked to improve the "leg" time?
  3. The coils/magnets are round.  Should the coils be wound more oval to increase the legs and decrease the "heads/tails" or does the iron core overcome this?
  4. The core width is 5/16" with the overall coil at 1" (11/16" windings) while the magnet is .75".  Is the "small" hole ok because of the iron core?
  5. Each coil has it's own core - or should it be designed more like a traditional alternator with slots/teeth etc?
  6. Wood/plywood for now.  Thinking polyproplny (butcher block plastic?) for later.


The rotor should be non-magnetic right - to prevent "sucking" up the flux I want going towards the coils?


Thanks for reading and I look foward to any/all replies, suggestions, ideas and links.

« Last Edit: November 06, 2008, 08:32:38 AM by (unknown) »

hamitduk

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Re: Single rotor dual stator design
« Reply #1 on: November 06, 2008, 04:11:52 AM »
What is the output and the wrap count?


Hami

« Last Edit: November 06, 2008, 04:11:52 AM by hamitduk »

Flux

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Re: Single rotor dual stator design
« Reply #2 on: November 06, 2008, 05:16:07 AM »
"What can be done to improve this design?

Or am I way off base?"


Yes. This has been covered adequately in the past I really can't find any desire to go through all this lot again for the perfect alternator probably for the perfect VAWT.


See what you can find out about Holmes's lighthouse generator, he got further with your ideas than most but they don't build them now and he didn't make the same mistakes.


Flux

« Last Edit: November 06, 2008, 05:16:07 AM by Flux »

capthook

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Re: Single rotor dual stator design
« Reply #3 on: November 06, 2008, 10:05:01 AM »
"What is the output and the wrap count?"


Each coil is 300' with output 2v @ .5a


- -


"Yes. This has been covered adequately in the past I really can't find any desire to go through all this lot again for the perfect alternator probably for the perfect VAWT.


See what you can find out about Holmes's lighthouse generator, he got further with your ideas than most but they don't build them now and he didn't make the same mistakes.


Flux"


Thanks for the reply Flux.  I have read most of this board (100 hrs+) and consider your posts to be of great value.


I'm not familiar with a Homes lighthouse generator and am unable to find out more on the internet than a few vague picture of the whole device.


While I understand your frustration with explaining why an odd idea may be a poor one and how to improve it - I have spent alot of time on this.


What "same mistakes" am I making?  Why is this design "way off base"?


With the aircore design used mostly here - would it increase the ouput to add cores to the coils of that design?  Would this be a better route than the one I'm attempting?


"This has been covered adequately in the past" - can you provide links/details?  I haven't seen much discussion of this - other than the thread link I gave.


You consider this design idea a poor one?  Why?  What should I do?  


Thank you

« Last Edit: November 06, 2008, 10:05:01 AM by capthook »

ghurd

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Re: Single rotor dual stator design
« Reply #4 on: November 06, 2008, 11:39:56 AM »
I'll take first crack at it.


It seems you are aware of what makes a PMA work, but choose to ignore the pitfalls.

The link you provided tells most of the problems.


Wrap count.  300'?  Is that 300 turns or 300'?

The 30 ohm part is a killer.


The magnetic circuit is not closed.

That is mostly why only 2V.


I can not even guess how you arrived at 2V and 0.5A with 30 ohms per coil.


Cores cause their own problems.

If you like the idea of cores, maybe try an induction motor conversion.


"small wire" makes for high voltage.  And low output.

The high voltage is not there because the magnetic circuit isn't closed, and eddys in the bolts, and the airgap issues.


Voltage boosters slow the bleeding.  They do not heal the wound.

Designing something with the intention of needing a voltage boosting circuit is like cutting off fingers now so they won't get hurt later in table saw accidents.


The device would work a lot better if the distance from the coils to magnets was shortened (a LOT).  Normally I would call it the air gap, but the magnetic circuit is not closed, therefore the airgap is somewhere in the triangle between undefined, leakage, and infinite.


And maybe if the bolts were put in steel plates... It would close the magnetic circuit.  Eddy currents and their related heating and cogging may move up on the list of "less important" issues.


It is not an odd idea.  It is a flawed idea many new people have.

I know a guy who made this a long time ago, and he wouldn't bother making another.

http://www.otherpower.com/images/scimages/742/SMALL_PROTOTYPE_ALTERNATOR_CLOSE_UP.jpg


G-

« Last Edit: November 06, 2008, 11:39:56 AM by ghurd »
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capthook

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Re: Single rotor dual stator design
« Reply #5 on: November 06, 2008, 12:15:30 PM »
ghurd-

Thanks for the reply!



"Wrap count.  300'?  Is that 300 turns or 300'?"

Thats 300' - about 2200 turns each coil



"I can not even guess how you arrived at 2V and 0.5A with 30 ohms per coil."

oops typo- that's 2V @ .05A with 40ohm coil (wrong coil ohms)
(yes - small - but the protype is only 8" rotor - real build would be MUCH larger)

I have also implemented a 1:4 vbelt pulley setup in one build - increase RPMs and output (at the cost of 5-10% as a pulley setup is close to 90-95% efficeint?)



The voltage-multiplier was just a thought if needed to further boost voltage after winding several strands in-hand to reduce resistance.  Is this defeating the purpose and I should just go with a bigger wire in the 1st place>



The pic you linked to is just like what I'm thinking - but with magnets and coils on both sides. (is there a thread discussing this build?)

The pics I posted are sloppy in that the airgap is not set - it is 1/4" - more like the pic you posted.



"And maybe if the bolts were put in steel plates... It would close the magnetic circuit.  Eddy currents and their related heating and cogging may move up on the list of "less important" issues."

Would putting "cross-bars" at the back of the cores to close the circuit offer the same idea but lessen the eddy currents etc.?
Or a "ring" of lamnates?



Will a core design offer relative output improvements?

ie: 10 horsepower input / core design = 50% increase in output but at the cost of 25% input due to iron losses.  Or when comparing the two designs - it is close enough to equal output that a design without cores is preferrable?




Thanks again for the reply!
« Last Edit: November 06, 2008, 12:15:30 PM by capthook »

capthook

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Re: Single rotor dual stator design
« Reply #6 on: November 06, 2008, 12:21:18 PM »
The above number is a 1 coil test: 2V @ .05A 40ohm

So small 8" protype is 18 coils each side x 2 sides


I found the thread linking to that pic:

http://www.fieldlines.com/story/2004/1/4/18573/78409

« Last Edit: November 06, 2008, 12:21:18 PM by capthook »

ghurd

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Re: Single rotor dual stator design
« Reply #7 on: November 06, 2008, 02:02:08 PM »
"about 2200 turns each coil"  I freaked when my last new idea came up with 600 turns.  And the one before it was 1600 turns IIRC.

"typo- that's 2V @ .05A with 40ohm coil"  A free stepper motor beats that.

"The voltage-multiplier was just a thought" & "Is this defeating the purpose"  Cut off all your fingers you want, but if you started with 10, cutting off fingers will not get to 50.

"increase RPMs" in a situation like this is pointless due to the inductance of a 2200 turn coil.  The thing is going to peak out.


The entire design is hopelessly flawed.


"but with magnets and coils on both sides. (is there a thread discussing this build?)"

Yes.  Many.  Probably google search the board for "dual stator".

And maybe Monte350(?) had pertient files.


Laminates would help.  Some.  The airgap is still "huge".

Cross-bars would only help on paper.  What is 1/10,000th of 1/10th of 1%? kind of idea.


Core ideas are way past what you have presented so far.


The 2200 turn, 2V, 50ma, 40 ohm coil, sounds like a tiny stepper motor.


"Will a core design offer relative output improvements?"  Sure.

Just the same as my (newly discovered / no wonder I have issues) step-brother-in-law-in-law's semi will go farther with a gallon of diesel than my neighbors riding lawn mower...  After that 30 ton trailer is added to the mix.


You disregard the 30 ton trailer when it is not convienient for you, then include it on the 'bad' side to make your PMA seem better.


If the things that make it a poor PMA do not matter, then it is a Great PMA.

The actual output says otherwise, because the output includes everything.


The 8" thing makes it big enough.

I don't call it small until it's less than 4". I used to think 5" was small.

And yes, I have troubles too.

But it is easy to smoke those numbers with a 1.5" dia x 0.75" stepper motor.

G-

« Last Edit: November 06, 2008, 02:02:08 PM by ghurd »
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capthook

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Re: Single rotor dual stator design
« Reply #8 on: November 07, 2008, 10:52:57 AM »
The dual-rotor design with the completed magnetic circuit seems the better design approach.  However - the relatively low mass of copper in the current designs I feel is not offering full potential output.  I'll mull that part over and post some ideas.


I already spent weeks on this presented (crappy i guess) design.  I wound 18 coils with 2200 each - took about 1 1/2 hours each to wind neat and tight.


Thanks for the comments and helping me shape my thinking. (and preventing me from wasting even more time and frustration conintinuing the same path)

« Last Edit: November 07, 2008, 10:52:57 AM by capthook »

capthook

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Re: Single rotor dual stator design
« Reply #9 on: November 07, 2008, 06:02:49 PM »
What about doing the rotors like this?  The idea would be to keep the rotor weight down.  Maybe 2 pieces of 1/8" steel sheet cut out in circles a bit wider than this pic shows and screwed together to make 1/4"?  Or 1/2"? Or will the gap between sheets reduce the effect?  Or will it be WAY less effective because the steel mass is not sufficient?


And as to copper mass.  What about increasing coil height? Like 2 in-hand, side by side - stacked ontop of each other?  Yes - all the windings might not receive a full 1.2T flux, but power output should increase?


tx



« Last Edit: November 07, 2008, 06:02:49 PM by capthook »

capthook

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Re: Single rotor dual stator design
« Reply #10 on: November 07, 2008, 06:32:09 PM »
And as to single rotor designs:


This is the best idea I've seen yet - presented by Electric Ed:




There was little/no discussion of this design.


You have the benifit of a lamintated steel core, and a completed, strong magnetic circuit.  And all the windings are perpendicular to the flux path meaning no "dead" coppper.


It would seem the increased power output would greatly outway the iron losses.


Thoughts?

« Last Edit: November 07, 2008, 06:32:09 PM by capthook »

Jeff

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Re: Single rotor dual stator design
« Reply #11 on: November 07, 2008, 06:42:19 PM »


  1. st rule of thumb I learded here: Inside coil size should be pretty close to the outside dimensions of the mags. Kind of ruins the 18 coils idea unless you make VERY thick coils. Which, isn't recommended. See rule #2.
  2. nd rule of thumb I learned here: Air gap...some measure this as the gap between the magnet face on rotor # 1 and the magnet face on rotor #2, and run from 5/8" to 7/8". But I've found most use pieces of a broken-up CD (approx. 0.80") to set the distance between the magnet and coils.
  3. rd rule of thumb: Larger diameter wire in the coils produces more amps and less voltage. Smaller gage wire makes for more voltage & less amps. If you have enough coils, and plan on 3-phase current, you'll have more options getting the needed voltage or amps.


Look at some of Hugh Piggots "smaller" windmills, say 5-8 foot diameter blades. Most of those use 2" Dia. mags, 1/2" thick N42's, and most also use 15 ga. magnet wire.
« Last Edit: November 07, 2008, 06:42:19 PM by Jeff »

ghurd

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Re: Single rotor dual stator design
« Reply #12 on: November 08, 2008, 07:39:35 AM »
As shown, it would need a pipe wrench to turn it.


The other post, with the steel donuts would be a lot better.

But thicker coils and losing flux means less power, and there is a knee in the graph which is the point where the machine goes from "working pretty good" straight to "not working much at all".


Not sure why you are so worried about weight.  It really doesn't matter much.  There are very few and very small losses related to weight.  I test flew a 30+ pound motor with ~32" dia blades and it spins right up, even with iron losses.

« Last Edit: November 08, 2008, 07:39:35 AM by ghurd »
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Jeff

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Re: Single rotor dual stator design
« Reply #13 on: November 09, 2008, 05:12:31 PM »
OOPS! BIG typo on rule #2: Thickness of a CD IS about 0.08" NOT .80" !


Also, the average coil wire guage I've seen here (and I've seen a LOT of them!) are anywhere from 13-18 gage, with the smaller gage (even 15-18 ga.), it's been the norm to wrap 2-4 strands in hand. For 12-24 volt systems, try a single "practice coil" with 40 turns. Measure & record the Ohms before you mount it, or do  anything else with it! Then mount it and run it at as close to the RPM it will normally run at. Then measure Volts & Amps again. Open volts, and volts connected to a battery are two different things! Espescially due to the wide variance of the state of charge of your battery! Try to use one with a reasonably good charge.

« Last Edit: November 09, 2008, 05:12:31 PM by Jeff »

capthook

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Re: Single rotor dual stator design
« Reply #14 on: November 10, 2008, 02:17:37 AM »
Thanks again for the replies (and to the board)

I've learned a ton here!


As such - the decision to pursue the axial flux, dual-rotor design seems almost a no-brainer.  


I've got a bunch of 3/4"x1/2" N42 from previous projects - so I'll be using those.


I'm getting some 8" steel rotors fabricated this week. Is 1/4" thick enough?

And the local guy does torch cutting.  Is that different than plasma? Should I look around?


The windstuffnow site is selling 3/16" steel discs - so I was thinking 1/4" would be ok - but looks like most are using 1/2"?

« Last Edit: November 10, 2008, 02:17:37 AM by capthook »

capthook

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Re: Single rotor dual stator design
« Reply #15 on: November 10, 2008, 02:41:04 PM »
Found a local guy with a CNC Mill.

Quoted $85 for fabricating 2 8" x 1/4" steel rotors - including shaft/bolt holes.


Sound resonable?


Before I make the plunge:


Is 1/4" a waste - I REALLY should go with 1/2"?

« Last Edit: November 10, 2008, 02:41:04 PM by capthook »

spinningmagnets

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Re: Single rotor dual stator design
« Reply #16 on: November 10, 2008, 03:26:06 PM »
As far as the thickness of steel for a rotor, I have read that you should take the exact magnet you are going to use and put it on one side of a steel plate candidate.


Then hold a steel paperclip up to the opposite side, if the paperclip sticks theres some residual magnetism leaking through, and a thicker plate would be better. If the paperclip won't stick, its at least thick enough, if not thicker.


Apparently there's nothing wrong performance-wise with using a plate thats a little too thick, except for extra weight, cost, and difficulty cutting.


If anyone knows of any other criteria for selecting the best steel plate for rotors, please reply!


"I was Poland, and he was a Stuka dive bomber" -Madeline Kahn in "High Anxiety"

« Last Edit: November 10, 2008, 03:26:06 PM by spinningmagnets »

Jeff

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Re: Single rotor dual stator design
« Reply #17 on: November 12, 2008, 10:33:37 PM »
   Again, a "rule of thumb" ONLY! Rotors should be approx. 1/2 the thickness of the magnet. The "rule of thumb" difference though, is the magnetic properties of the steel the magnet is being mounted on! Let's say someone "sticks" you with some stainless...boasting "it'll never rust on you!". Grades of stainless range from completely non-magnetic to "pretty good" magnetic-wise. That's why so many people are using brake discs for rotors!

   I always thought you could actually have someone who works in an injection mold machine shop (like I have), make a mold for some fairly large rotors (let's say 18-24"). Have them made from carbon fiber, with lots of ribs for strength, a good hub, and pockets for magnet assemblies. VERY light, almost indestructable, and could be made extremely accurately. Make pockets in it to fit other metal pockets from a highly magnetic flux material to mount the magnets in, fitting tightly on all four sides and the back of the magnet, leaving only 0.001 - 0.002" of an inch less depth so the magnet stuck out slightly further, but the neccessary size & thickness in the sides, top, and back for maximum flux flow. Mount the magnets in those, and then mount that assembly in the carbon fiber rotors!

   Practically NO issue with location, balance, and the weight reduction would be a great help! Also, the pockets the magnets would be mounted in would allow maximum flux density all around the magnet. If a replacement would ever be needed, you can have exact replicas either already made, or make more much easier as you already know the dimensions.

   Once you have the mold "dialed in" for sizing, shrinkage, etc., make a few thousand of the things! At 4000 pieces (there would be two used per genny), the $200,000 cost of the mold would come out to $100 per genny! Dont forget, that's with holes, pockets, and even tapped holes already molded into the rotors! A good mold should be good for at least 100,000 to 500,000 parts, which brings the cost WAY down! Example: 100,000 rotors = 50,000 gennys. That's $4.00 per genny plus material & handling. Oooooh! It might even get up to $10-$15 extra per genny at that volumn!
« Last Edit: November 12, 2008, 10:33:37 PM by Jeff »