Axial flux PM alternator efficiency?

[Rob Beckers]

Does anyone have experience with the DIY axial flux type permanent alternators? The type I mean is with a double PM rotor, and coil stator sandwiched between the rotors, as in http://www.scoraigwind.com/pmgbooklet/itpmg.pdf for example.

I've been reading that these get quite warm when in use, despite their relatively large size (all relative to power of course). That would lead me to believe their efficiency is not great, and that is what I am trying to find out more about. Does anyone have measurements, or a link to measured efficiency numbers? What is the main issue affecting efficiency for this type of alternator?

Thanks!

-Rob-

[Mark Parsons]

Hi Rob,

I am also very interested in this same subject. I tried a single rotor axial flux experiment on my Banki turbine. I couldn't get enough flux to get to cut-in at the low RPM so I integrated a store bought PM alternator (Delco style). I have the parts (neo magnets, magnet wire, steel disk blanks) to build an axial flux machine. I have been seriously considering the option.

The heat generating losses inside any motor or alternator are: 1.) I^2R (amperes squared * ohms) in the windings, 2.) Eddy currents from stray flux crossing any non-moving conductor, 3.) Friction. Overheating failure is caused by the inability to shed this heat adequately.

I believe item 1 - I^2R is the main culprit with axial flux overheating issues. The flux path is relatively small to keep the diameter (inertia) constrained causing more turns of smaller gauge wire to meet the low RPM with relatively high voltage requirements. This is further exacerbated by the high charging currents created by direct coupled battery charging due to the low battery internal resistance.

I'm not sure that the efficiency of a well made DIY axial flux machine is significantly less than other commercial options at the relatively low power ratings. The overheating failure mode is likely due more to the inabilty to shed the heat with construction techniques like smooth cast stators and rotors with poor heat transfer characteristics, inadequate heat sink radiating surfaces and lack of blower fins like standard commercial made motors / alternators.

I've read quite a few Otherpower.com posts that indicate 50 - 70 amps of charging current from 10' diameter turbines. A 10' turbine in a 20MPH wind with good wooden carved blades is likely able to produce over 1,200 Watts of shaft power. At 900 watts (70 amps * 12.9V) delivered to the battery, the stator must then dissipate the other 300 watts (1,000 btu) plus. A high thermal gradient is created if unable to shed this heat effectively.

Some more data from an experienced individual with one of these machines would be very helpful.

Mark

[Rob Beckers]

Been doing some more reading and thinking on the axial flux generator issue, and it seems to me that the inherent problem of these alternators is that they have very little copper for the power that is demanded of them. Compared to a more traditional alternator (if there's such a beast) the stator coils have no overlap, they're all placed side-by-side. Doing so leaves not much room for copper. Consequently it would seem that inherently these alternators will suffer from resistive losses in excess of traditional designs.

I'm no expert, so this is all speculation on my part. If there are any experts out there I'd love to hear more about it!

-Rob-

[Mark Stirling]

Rob, there are commercial axial flux motors that include cooling tubes between the coils on the stator. This could also be done with Hugh Piggott's design if needed.

A few angled air cooling holes drilled through the pm backing plates might prove to provide sufficient cooling in overheating is a problem.

One interesting approach that is being investigated is field weakening to control output as speed increases. This can be accomplished either electrically by a coil that weakens the pm field or mechanically by increasing the air gap.

The appeal of Hugh Piggott's approach is its extreme simplicity. A toroidal approach to winding will get you more copper in the right places but is more difficult for amateur construction.

There are some university studies being done on efficiency but I have yet to see anything yet that yields practical real world results for comparison.

[Rob Beckers]

Hi Mark,

(By the way, welcome to the forum!)

Since posting that original message I've been looking at alternators that are used in commercial small wind turbines. My perspective has changed a bit from this.

By and large, axial flux alternators seem to be restricted to the DIY segment. I suppose this is mostly because they are simple to make with minimal tools. I know about one commercial small turbine that uses one (something English, forgot the brand), but they are few and far between for commercial use. For good efficiency an axial flux alternator would get large compared to a conventional radial alternator. Then there's the need for twice as many magnets vs. a radial alternator. Those neodymiums are not cheap, so that's a major factor for a commercial design.

Besides the above, it seems that making a zero-cogging radial flux alternator, using laminates in the core, is not all that difficult. They are high-efficiency too; I've recently seen one that claims 94% efficiency for a 2.5kW alternator. Winding them is a lot more work vs. an axial alternator. Then again, it makes more economical use of the available magnets and copper.

An interesting hybrid that I've seen a few times is the arrangement that uses magnets in a radial fashion, a toroidal core from laminates, with windings around the core. So the flux (at least the part doing the work) is at a right angle to both axial and radial (What is this called?). Southwest's Skystream works that way. There's also a 20kW turbine using a similar alternator. The advantage is that this can be wound by machine.

Interesting stuff! Anyway, I came across axial flux and they sounded interesting, hence the inquiry.

-Rob-

[Jeff Birkle]

Hi Rob,
Here are some pictures of one of my axial flux stators, and a full alternator assembly.
I'll start a new thread.
Jeff

[Rob Beckers]

Hi Jeff, thanks for the pictures. Nice work!
They're great alternators; easy to make and as you say they work well. I know 'm from Hugh Piggott's design. Certainly a lot easier to put together than messing with laminates and windings on those.

-RoB-

[Steven Fahey]

Hi Rob,
I built an Axial last year, and tested it using a lathe and a scale to simultaneously measure input and output power. I wouldn't call the test fool-proof scientific, but I was both pressed for time and making do with what I had that day.

I made a write-up because other people wanted to see what I did; I can give you the link if you want to see it (still getting used to this forum's interface).

[Jeff Birkle]

I have done some extensive testing with these alternators on my milling machine as well.
The last bit of testing I would like to do is steel plate thickness tests, relating to output.
I would be interested in seeing your results. Where is your post?
Jeff

[Steven Fahey]

The last bit of testing I would like to do is steel plate thickness tests, relating to output.
I would be interested in seeing your results. Where is your post?
Jeff

Here it is:

http://www3.telus.net/faheydumas/Wind_Turbine/AXIAL_FLUX_Testing_V2.pdf

A suggestion, for those who want to study any kind of electrical device, without building endless iterations:

http://femm.foster-miller.net/index.html

FEMM is an excellent tool for figuring out where the lines of flux goes, how it's captured, and which small changes have big or small effects.

Have fun!

[Stewart Corman]

Steven,
Just read thru the entire pdf ...first rate analysis and projections!
your presentation format was excellent ..

Am I correct that you only measured as max efficiency of 50%+, and it goes downhill from there?
This is generator efficiency only, then add rotor max 59.3% Betz x rotor efficiency
If so, then a commercial unit like a Skystream which delivers overall Cp=0.45 is unattainable with a DIY AFPM similar to yours since 50% x 59.3 = <30% to start with


BTW, only one comment ...could you add a second scale on x-axis in mph for WS ...never saw kph used, m/s is more commonplace on other side of the pond

Just thought of something else ... not exactly sure what you are measuring with the spring scale ..it is showing a torque, but of what??

OT - what word processor did you use before saving as pdf?
what drawing program for the wiring diagram?

Stew Corman from sunny Endicott

[Steven Fahey]

Steven,
Am I correct that you only measured as max efficiency of 50%+, and it goes downhill from there?
This is generator efficiency only, then add rotor max 59.3% Betz x rotor efficiency
If so, then a commercial unit like a Skystream which delivers overall Cp=0.45 is unattainable with a DIY AFPM similar to yours since 50% x 59.3 = <30% to start with


...could you add a second scale on x-axis in mph for WS ... not exactly sure what you are measuring with the spring scale ... what word processor did you use before saving as pdf?...what drawing program for the wiring diagram?

Stew Corman from sunny Endicott


Wow lots of questions!

Okay, yes, about 50% electrical efficiency - which is about what you expect when you delve into the theory of it all. I'm not handy enough with the terminology to 'splain that one.
Then when you get the prop bolted on, the wires run, and the batteries shed their heat, etc., etc., my hand calculations of the total system efficiency is about 10-15%. Meaning 6 Watts of wind through the prop for every 1 Watt out the battery leads.

I live in Canada. I use KPH because everything else I read is in KPH, too. Don't you love standardization?:rolleyes:

The spring-scale measurement of torque is so-so accurate. To do it "right" you need a stator support that rotates freely (say, in the tailstock of the lathe), keeps the stator centered when it turns, and use the spring scale to hold on to that. I wasn't going to make something like that for just one test, so I did the next best thing. Pivoted the stator on the tool rest, and the sum of reaction forces between that pivot and the spring scale gives you a number. With a bit of crunching, torque falls out the analysis, but nobody that read the article ever liked my way of doing that. Keeping the stator centered on the rotor was tricky, too.

I did the write up in MS Word, imported photos and diagrams as necessary, and printed the document using PDF995. It's a driver that you can "print" to, and it spits out a PDF file. I also use MathCAD for algebra, analysis, and graphing. I use AutoCAD for drawing. All stuff I have access to through the office; admittedly not common practice.

Do NOT confuse Cp with electrical or system efficiency, like you did above. I don't know a thing about the Skystream, but it's entirely possible for it to have a Cp of 0.45, lofty though that is. The term Cp is a purely aerodynamic one - Wind speed power versus prop torque or power. What that power is used to do - eg. pushing water or electrons, is another matter.

[Stewart Corman]

Steven,
just a few clarifications:


The spring-scale measurement of torque is so-so accurate

I am talking far more basic ...torque of WHAT?
what does this measurement attempt to describe??
not commenting on your technique ..that is fine


Do NOT confuse Cp with electrical or system efficiency, like you did above......The term Cp is a purely aerodynamic one - Wind speed power versus prop torque or power


Nope, beg to differ here ...the NREL reports show for a given WS, how much juice gets delivered to the load resister ...power in vs power out = Cp ...whole system
http://www.nrel.gov/wind/pdfs/33452.pdf

thanks for the references to your composition techniques ...MathCAD only one I am not familiar ...I can spit out a pdf directly from Word

Stew

[Mark Parsons]

Greetings Steven,

I too must commend you on your pdf presentation, very well done! I suspected that the efficiency of the axial flux alternators was about 50%. Certainly way below commercial alternators typically in the 90% range.


Greetings Stew,
Quote:
The spring-scale measurement of torque is so-so accurate

I am talking far more basic ...torque of WHAT?
what does this measurement attempt to describe??

Steven has mounted the stator loosely, the spring-scale is measuring the mechanical torque induced into the stator due to the electrical power being drawn. Power = Amps * Volts = Torque * Speed.

Regards,
Mark

[Jeff Birkle]

Nice post Steven!
I have done a lot of testing myself. I have figured that bench testing is close, but not quite the same as in real conditions.
Your alternator is 12 1" x 2" x .500" neos, 9 coils? what is the wire size (18)and turns per winding?
If you were to use the same size props, and go to a smaller wire with more windings for the 24 volts, you would end up with the same cut in-cut out wind speed and the same max watts, only difference being line loss.

From what I understand, the Betz limit is a convex curve related to wind speed exponentially increasing.
PM alternators have a more straight with a slight concave curve, due to a single unchanging amount of flux (electromagnets can change flux), meaning Cp is displayed for one specific RPM-wind speed, or an average for the entire RPM-wind speed range. Therefore changing low, or high RPM-wind speed performance, or even extending RPM-wind speed range will effect overall output, but may not change Cp.

I am comparing output of my home-brew 10 footer, to the Bergy 1KW, and the Southwest 100 and 200, and the home-brew will outperform the commercial turbines slightly. When I get some time I will post my results too!
Jeff

[Steven Fahey]

Jeff,

If you were to use the same size props, and go to a smaller wire with more windings for the 24 volts, you would end up with the same cut in-cut out wind speed and the same max watts, only difference being line loss.
Jeff

That's a good description of what I should have done when I put that alternator in service. I had the furling wind speed very low and it still burned out. I should have been operating at 24 volts to properly load the alternator, but I'm cheap, and was only using 12V.

Hey I look forward to seeing what you measure from your machine. Have you posted details about it already on the site?

Stew,

Thanks for linking me to that NREL report. I like the analysis methods at the back, and I do see that they are using Cp as an overal system parameter. So I'll grant you that, but I won't get off my high horse, yet. At every stage of power conversion, there is a loss, which can be represented as inefficiency. The Bergey test is a good example. One chart shows the Cp peaking at 0.21, another shows the inverter efficiency as a constant 85% or so. Let's crunch some numbers on that, and we can say that BEFORE the inverter, the generator output divided by raw wind power is about 0.25. You could use the term "Cp" for that, too, if you want, because you have taken the inverter's efficiency into account, too.

Now what if you are also equipped to measure mechanical power in the same test? Then you could compute: generator power divided by mechanical power, and get another factor which describes how efficiently the generator transforms mechanical power into electricity. That is exactly what I represent in my tests, because I was measuring mechanical power and electrical power directly, and simultaneously.

Not done, yet. The prop on the windmill must convert the power in the stream of wind into mechanical power. That's not perfectly efficient, either. It is this stage of conversion where I like to use the term Cp, because I can isolate all the other ones, and focus on this uniquely as an aerodynamic problem.

And then if your head isn't just swimming in letters and symbols already, you can then multiply each of the efficiency factors together, and just end up with the same catch-all factor you started with.

The NREL guys did not take measurements at each stage of the conversion process, so their term Cp includes all of the conversion inefficiency factors I just listed to you. I'm sure the blades of the Excel convert more than 30% of the wind power into mechanical power when it's running in 10 Mph winds, but a subsequent loss of power to the electrical generator and then again through the inverter are what reduce the overall system power Cp to 0.21.

In my background I have read many NACA reports. That's the 1920's to 1960's predecessor to NASA, which did mountains of research on aircraft. I have read propeller efficiency studies, and they are greatly concerned with the conversion mechanical (engine) power into fluid dynamic (wind) power. Backwards from what we're concerned with, but still helpful. I won't go on any more because there are many terms like Cp, Ct, and J - each of which have a different meaning (some have a different acronym) in the windmill community.

[Stewart Corman]

First a reply to Jeff:

From what I understand, the Betz limit is a convex curve related to wind speed exponentially increasing.the Betz limit is a single calculated number = 59.3% and has NOTHING to do with airfoils or rotors, or wind turbines ...it is simply mathematics

so if wind power increases with the 3rd power exponent of the wind speed and we multiply by a constant, then yes, the curve is convex as you stated


PM alternators have a more straight with a slight concave curve, due to a single unchanging amount of flux not "more straight" ..it is a straight line linear function, double the rpm doubles the voltage .. if you are not using a simple resistive load, then current will be altered as a function of the controller, battery charging parameters, and possibly the frequency


meaning Cp is displayed for one specific RPM-wind speed
Therefore changing low, or high RPM-wind speed performance, or even extending RPM-wind speed range will effect overall output, but may not change Cp.if output is changed for any specific WS, then Cp by definition has changed.

How you change rpm performance is a complex issue... pitch angle, load, blade profile, number of blades, solidity
For any fixed rotor geometry design with a fixed load, there is one WS which exhibits max Cp ..it performs less efficiently both above and below that optimum WS. Some designs have "flatter" curves ie the CP is more constant for a greater WS range ...but not necessarily a benefit, since it can lead to a runaway condition!

Note that there is significant data that above a certain WS, the CP starts to fall dramatically ...I can document that for a three blade turbine, this WS is 8m/s and for a two blade it is 12m/s ...has to do with RE# and the transition from bubble separation/reattachment to pure turbulence bubble detachment ...surprisingly is independent of blade length

http://www.mh-aerotools.de/airfoils/bubbles.htm


now to Steven's comments:
Now what if you are also equipped to measure mechanical power in the same test? Then you could compute: generator power divided by mechanical power, and get another factor which describes how efficiently the generator transforms mechanical power into electricity. That is exactly what I represent in my tests, because I was measuring mechanical power and electrical power directly, and simultaneously.
I read Mark's answer to my query on what exactly were you attempting to measure with your force gage on the lathe. I am still not convinced that the numbers generated have any merit as a measurement or what you can do with this data after you get it.

In my original posting of Allison's article, I had noted that he used a home brew "dynamometer" to get a Cp of 0.57:
http://s145.photobucket.com/albums/r203/scorman1/Wind/Allison%20article/

this technique eliminates the electrical generator and all its losses and frequency variability

IMHO, what you are measuring on a lathe, is the reactance force of the coils driving an electrical load from the rotational torque provided by the lathe motor.
You need to monitor the input electrical wattage of the lathe motor w/ and w/o the load and note the difference.

BUT, and here is my bone of contention ...a driven electrical motor is NOT adversely affected by loading ( a linear relationship?) as is a wind turbine, whose aerodynamic properties change dramatically as a function of load applied. So I don't see a correlation between the lathe measurement and any useful parameter of the turbine spinning in free air.

I may be missing something very basic here, but I just don't see it!
Bottom line is the Cp as calculated by NREL ..the whole system at any particular WS.
Newer products such as the SkyStream use a variable load scheme to "hand taylor" the CP performance curve to maximize the output ...I was truely amazed to see that they achieved a Cp=0.45 at only 11mph WS.
The only other fixed blade design which approached this was Claus Nyboe's Windflower at Cp=0.47 but at 17mph

Stew Corman from sunny Endicott


http://www.greenpowertalk.org/attachment.php?attachmentid=459&d=1210080347

[Steven Fahey]

Stew,


IMHO, what you are measuring on a lathe, is the reactance force of the coils driving an electrical load from the rotational torque provided by the lathe motor.
You need to monitor the input electrical wattage of the lathe motor w/ and w/o the load and note the difference.

BUT, and here is my bone of contention ...a driven electrical motor is NOT adversely affected by loading ( a linear relationship?) as is a wind turbine, whose aerodynamic properties change dramatically as a function of load applied. So I don't see a correlation between the lathe measurement and any useful parameter of the turbine spinning in free air.

I may be missing something very basic here, but I just don't see it!


The explanation has to go right down to the fundamentals, then.

[blackboard=on]

The first law of thermodynamics states that (aw jeez, professor!):

Energy cannot be created nor destroyed, only changed.

I have to start with this statement because you ignored some very essential changes in energy's form, but they are crucial to understanding the meaning of "efficiency".

Start with the lathe. It is an electromechanical device that converts electrical energy into mechanical energy. We want to use it to test a Permanent Magnet Alternator, a device that converts mechanical energy into electrical energy.

There is no direct transfer of electrical energy from the lathe to anything else, because it is wired properly, according to Code.

That mechanical energy, produced by the lathe, can be used to turn anything that fits in the chuck. In my case, I put the rotor of a PMA in the chuck. Turn the lathe ON and the chuck and the rotor turn, even if there is no stator installed at the time. In this condition, the rotor's inertia is the only load on the lathe, so the lathe only imparts energy to the rotor for a moment until it gets up to speed.

Now put the stator between the faces of the rotor. The stator can have its output leads connected or not. If you connect the leads to any electrical device, resistors, a battery, inverter, whatever, then it becomes possible for current to flow through that load, once the stator produces some voltage potential. The load will absorb some amount of power. If the leads of the stator are not connected, no power will be consumed, because no current can flow.

To make the stator produce a voltage potential, the rotor can be turned by the lathe. Since we have the stator connected to a load, work will be done by that load. The rotor induced its magnetic flux through the stator coils, the field direction flips back and forth through the stator coils as each pole passes, and the emf generated by this process made it possible for current to flow through the wire in the stator and the load.

When this happens (and I'm finally going to make my point) the rotor does work on the stator. The current in the stator's coils applies back-emf which must be overcome by applying a net power to the rotor, otherwise the rotor would slow down. This power is manifested by a Torque on the stator. The stator wants to start turning, to relieve the back-emf. The mounting of the stator prevents this, and holds the stator steady against that torque.

I devised a mounting system for the stator that provides a measurement of the applied forces.

Torque multiplied by RPM is power. This is the mechanical power that the rotor puts into the stator, in order to produce electrical power.

The amount of mechanical power applied to the PMA must be greater than the electrical power generated. Divide one number into the other and you have the electromechanical efficiency of the PMA.

[blackboard=off]

Whew. My fingers are sore. Your eyes must have gone foggy by now, too.

You can't get a meaningful result by measuring the amps into your lathe. But that's a totally different diatribe.
I can only handle one per day.
Have a good weekend.:)

[Stewart Corman]

Steven,
You have gone into great detail on how a mechanical rotation causes electrons to flow from the coils passing magnets into a load ..that is fine... that and $2 still gets you on a NYC subway.
But it still tells you NOTHING!

A lathe motor does not change it's characterisitics significantly as a function of load ...not true for the wind turbine and it's aerodynamics.

Study the following chart and try to understand the can of worm you have opened up:
501

If you go back to the previous post where I showed the SkyStream performance chart you will note that TSR varies by a factor of 2:1
They have juggled the TSR by varying the load dynamically to prevent stall AND maximize output.
It is quite likely that without any sophisticated controller, that TSR varies by a factor of 7:1 or more ...all that has to happen is that at low WS, TSR =1 (low RE#, low L/D ratio) and it is producing juice, and then gets up to TSR=7 at high WS (high RE#, high L/D) before furling

Since the amount of juice produced by your AFPM generator is related to both rpm AND mechanical torque for a given load ...you now have a defacto "feedback loop". If the turbine is running at a reasonable WS, the torque produced will sustain the AFPM feeding the load. If the wind now slows down a bit and you are on the left side of the torque/TSR curve, then the TSR is reduced, which reduces the output MORE than just the reduction of the power of the wind. Ah, the Cp doesn't stay constant either! It comes to an equilibrium somewhere, whereby the TSR and torque for that WS are producing a different power output at a different Cp. IMHO, the calibration of your lathe torque is irrelevant to this non-linear situation.
What do you do with the numbers ?

The only way I can see how to characterize the torque produced in real free air turbine spinning, is to calibrate the electrical output and rpm vs WS for specific defined loads that go from very low load ( or no load) to stall load ( dead short??)

Stew Corman from sunny Endicott

[Steven Fahey]

Stew,
... that and $2 still gets you on a NYC subway. But it still tells you NOTHING!

A lathe motor does not change it's characterisitics significantly as a function of load ...not true for the wind turbine and it's aerodynamics.

Study the following chart and try to understand the can of worm you have opened up:
501

If you go back to the previous post where I showed the SkyStream performance chart you will note that TSR varies by a factor of 2:1
They have juggled the TSR by varying the load dynamically to prevent stall AND maximize output.
It is quite likely that without any sophisticated controller, that TSR varies by a factor of 7:1 or more ...all that has to happen is that at low WS, TSR =1 (low RE#, low L/D ratio) and it is producing juice, and then gets up to TSR=7 at high WS (high RE#, high L/D) before furling

Since the amount of juice produced by your AFPM generator is related to both rpm AND mechanical torque for a given load ...you now have a defacto "feedback loop". If the turbine is running at a reasonable WS, the torque produced will sustain the AFPM feeding the load. If the wind now slows down a bit and you are on the left side of the torque/TSR curve, then the TSR is reduced, which reduces the output MORE than just the reduction of the power of the wind. Ah, the Cp doesn't stay constant either! It comes to an equilibrium somewhere, whereby the TSR and torque for that WS are producing a different power output at a different Cp. IMHO, the calibration of your lathe torque is irrelevant to this non-linear situation.
What do you do with the numbers ?

The only way I can see how to characterize the torque produced in real free air turbine spinning, is to calibrate the electrical output and rpm vs WS for specific defined loads that go from very low load ( or no load) to stall load ( dead short??)

Stew Corman from sunny Endicott

Oh dear now we're butting heads.

Which is heavier, a pound of feathers or a pound of bricks?

If you are going to make a generator, of any type, turn at a given speed under a given electrical load, you are going to apply "X" torque to its shaft. Period. A relatively constant and predictable property of the generator.

It makes no difference whether your PMA is bolted to a wind turbine prop, a pelton wheel, a gasoline engine, or an exercise bike, as long as the power can be provided to the shaft at the required RPM, then the electrical power will be generated.

This thread originally asked the question "Axial flux PM alternator efficiency?" meaning the alternator alone. There is no prop bolted on to it in the OP's question. We are now off topic (but I don't really mind that very much) by discussing things like CP and TSR. But they are irrelevent when considering the thermodynamic black box of the PMA itself.

So it's not that you're wrong when you want to talk about the prop's TSR being a variable parameter. For goodness's sake I even have a chart for that factor in my own write-up. The thing is that the efficiency of the propeller should not be confused with the efficiency of the alternator. The two factors can be lumped together, but they can also be analyzed separately.

If you really do want to figure out how the prop is matched to the alternator, then you have to create a curve of "required input torque" for the PMA, and an "available output torque" of the prop, both in respect to RPM. The PMA will have one line for every voltage you select. The prop will have one line for every TSR you select. These prop curves really ought to be obtained from wind-tunnel tests unless you're a whiz with aerodynamic calculations. The prop will have a design TSR (ie angle of attack for best L/D) but can operate at higher or lower TSRs with a penalty in efficiency.

That's about the best we backyard tinkerers can hope for. Like I said at the end of my own report, Cp is a very slippery figure, dependent on too many variables to compute without a lot of time, effort, and resources. All tools the NREL guys can use at will, but mere mortals such as I may never see it.

Hey Stew this is a lot of fun, but if we can't get through to each other by now we never will. Thank you for a very engaging discussion, and the documents you dug up are interesting, in their own rights.
Good night.

[Stewart Corman]

Steven,
Now you get my point ..took long enough

If you really do want to figure out how the prop is matched to the alternator, then you have to create a curve of "required input torque" for the PMA, and an "available output torque" of the prop, both in respect to RPM.The above statement is exactly what you need on the lathe.
How much torque is supplied by the motor vs how much is used by the PMA....that is a measure of efficiency of the PMA alone.

You don't know how much energy is used by the motor to develop torque at any particular load, because you haven't calibrated the efficiency of the motor (nor the slowed down rpm unless you were calibrating rpm from frequency of the electrical output?), even if you measured the input wattage to the motor (which you haven't ).



Stew

[Steven Fahey]

I have provided the required input power for a range of speeds on the chart on Page 9. Assuming you are reading Version 2, dated February 24th, 2007, like I am, there are two curves: one for 12V, and one for 24V.
You can read on the chart that at 24V, and at 700 RPM, about 770 Watts of power are required to turn the PMA. Back on page 5, I gave you the output power of 340 Watts under the same conditions. A bit less than 50% efficient. For convenience, a chart for that is provided on page 7.

My torque measurement method is based on the brake dynamometer, an example of which you can see on Figure 2-20 (http://www.tpub.com/content/construction/14264/css/14264_53.htm) here.

I did not need a friction material to provide a resisting load; the load was already available by charging the battery. I measured the load by making the stator free to rotate. The only restraint to the rotation around the pivot was the spring scale. You can see the spring scale in the photo on page 1.

This tiny PMA made absolutely no measurable effect on the speed of the lathe. I installed a reed switch tachometer, with the magnet on the PMA rotor. Even when the alternator was taking in over 1HP, the tachometer read exactly the same RPM as the lathe's setting. It's a 25 or 30 hp lathe, you see.

[Mike Bullard]

Hi Everyone, Great :D forum you have going here.

The axial flux efficiency question has plagued me as well for some years. I offer the following links on the topic.

Japanese research paper abstract: http://sciencelinks.jp/j-east/article/200203/000020020301A0929970.phphttp://sciencelinks.jp/j-east/article/200203/000020020301A0929970.php

Dr. Bumby et al--I have purchased this paper and it was well worth it.http://www.dur.ac.uk/engineering/nareg/staff/profile/?mode=pdetail&id=237&sid=237&pdetail=39825

And here is Piggott's design being testedhttp://www.otherpower.com/dynotest.html
Mike

[Rob Beckers]

Thanks Mike!
Can't find the Japanese paper, but I've downloaded the Bumby paper from my Alma mater (They still let me access the library online, and download the publications that are available electronically). Good design paper for AF alternators! They're also showing very good efficiency numbers.

Have you built any alternators, or are you in the process of doing this? If so, please let us know more. A few pictures would be nice too.

-RoB-