Generator with ferrite magnets

[rossw]

What determines the elusive "efficiency" of a generator?

Or, did the post not actually refer entirely to the generator, but the whole generation/transmission/use in its entirety?

3 phase (with its 120 electrical degrees between phases) has been demonstrated to be more effective and efficient at converting electrical power to rotational power than single phase or two-phase can. It produces a smoother torque with less wear and tear, less noise, less vibration and often less parts.

Machines (like lathes and mills) work quieter, with less vibration and less heat, than the same machine fitted with a single-phase motor.
Pumps fitted with a 3phase motor are usually quieter and run cooler than single phase versions - and usually cheaper too.

[ChrisOlson]

That wouldn't tell you anything.  The efficiency curve of a generator changes with loading.  They are most efficient at very light loads and least efficient heavily loaded.  The amount of shaft power developed by simply hanging a weight on the shaft with a rope would be miniscule, and the power dissipated by the stator would also be miniscule - no where near the rated output of the winding.

The only way to test efficiency is with a dynamometer that accurately measures shaft input torque and speed while loading the generator.  I use a calibrated torque motor to do it, others have used lathes with a torque sensor, and various other methods.  My two-phase dual stator unit is 8-9% more efficient fully loaded than the neo three-phase it replaced, in the same diameter.  That means that with the same shaft input power I get 8-9% more power out of the dual stator two-phase.

Asserting that a three-phase is inherently more efficient than a two or single phase, just because of the number of phases, is grossly inaccurate
--
Chris

[lohearth]

I hope Jerry joins the discussion, he"s done some interesting work with ferrite generators. Some times it's been a little difficult trying to find some of his old posts.

[ChrisOlson]

Machines (like lathes and mills) work quieter, with less vibration and less heat, than the same machine fitted with a single-phase motor.
Pumps fitted with a 3phase motor are usually quieter and run cooler than single phase versions - and usually cheaper too.

Hi Ross,

I think that's probably true when comparing three phase to single phase.  However, in the case of two-phase I have found no difference at all as compared to three phase.

As to the entire generator/transmission combination - I have determined that I get approximately 3% power loss in the gearing.  However the gearing allows me to built a lighter, faster, much more efficient generator and the gains in the generator more than make up for the gearing losses, yielding me a net, roughly, 6% gain in overall efficiency of the unit as compared to a reasonably sized direct drive.  You can can build a direct drive with the same efficiency as the geared units I am using, but it becomes a monster that is not even practical to build from a materials, weight and size standpoint.

So when I am doing my comparisons between the neo and ferrite generators, I am using the same transmission with the same gearing on both.  The calculated efficiency gain (by reduction of resistance in the winding) is very close to what I am getting real world on the turbine.  The two-phase dual stator ferrite generator runs significantly cooler than the three-phase neo it replaced.  The bottom line is that more of the shaft input power is being converted to electricity with the dual stator ferrite, while it is being converted to heat with the single stator neo.

And this is the point I'm trying to make here, in that many folks don't understand generator efficiency.  The number of phases has nothing to do with it.  Efficiency is measured by only one thing - power in vs power out.  The more of that power in that you convert to electricity instead of heat, the more efficient it is.  There can be some arguments as to how much copper or other materials you have to use to get, say, a single phase design to be the same efficiency as a three phase, or whatever.  But in the end, the number of phases does not determine how power efficient the unit is.

When it comes to power transmission, now we're talking a different topic.  Two phase power transmission requires four conductors or three conductors, depending on the design of the generator.  Three wire two-phase uses one wire (the neutral) that is twice as big as the other two because that neutral wire carries the vector sum of the current in the other two wires.  Four wire two phase uses four conductors that are all the same size.  Three phase uses three conductors of the same size and transmits the the same power as two phase with less conductor mass.  So from the standpoint of power transmission, three phase has won out over two phase in the commercial world because it is a more efficient configuration from the standpoint of conductor cost.  However, again I want to make it clear that this has no bearing whatsoever on the generator itself.

I have constantly stressed that fact in this thread that my two-phase dual stator unit is the conventional four-wire two phase system.  It requires 33% more conductor mass (four wires instead of three) to transmit the power down the tower that can be transmitted with three phase using the same AWG size wire.  However, when you're talking only 25-27 meters of wire down the tower, using a 8/4 Type SEOW drop cable vs a 8/3 is a minor price to pay for getting 8-9% better efficiency at the generator with the two-phase configuration I designed here.

So in conclusion, do not confuse AC power transmission efficiency with generator efficiency.  The two are not the same.
--
Chris

[joestue]

That wouldn't tell you anything.  The efficiency curve of a generator changes with loading.  They are most efficient at very light loads and least efficient heavily loaded.  The amount of shaft power developed by simply hanging a weight on the shaft with a rope would be miniscule, and the power dissipated by the stator would also be miniscule - no where near the rated output of the winding.

The only way to test efficiency is with a dynamometer that accurately measures shaft input torque and speed while loading the generator.  I use a calibrated torque motor to do it, others have used lathes with a torque sensor, and various other methods.  My two-phase dual stator unit is 8-9% more efficient fully loaded than the neo three-phase it replaced, in the same diameter.  That means that with the same shaft input power I get 8-9% more power out of the dual stator two-phase.

Asserting that a three-phase is inherently more efficient than a two or single phase, just because of the number of phases, is grossly inaccurate
--
Chris

The efficiency drop you speak of is simply the ratio of the resistance of the machine to the resistance of the load.

You can perform the test I described with any means of power input.
short the stator, drive two machines with constant torque and you will find the slower machine has less electrical resistance and is thus more efficient.
The test is only valid for comparing identical three phase vs single phase stators.

[ChrisOlson]

The test is only valid for comparing identical three phase vs single phase stators.

Of course, but the test still doesn't prove that three phase is more efficient than single phase.

What I'm saying is that if you have two generators, one single phase, and one three phase, and they both have the same internal resistance against the same load, the efficiency will be identical.

The loading characteristics of the single phase may not make it as smooth as the three phase at the same current frequency.  But that has nothing to do with efficiency.  It's like comparing a V-8 four-stroke Otto Cycle engine with a single cylinder unit.  The V-8 has four power strokes per revolution of the crankshaft and the single cylinder only has one power stroke for every two revolutions.  But that does not make a V-8 cylinder block configuration inherently more thermodynamically, mechanically, or volumetrically efficient than a single cylinder configuration.
--
Chris

[bob g]

i don't know about your air core pancake alternators with neo magnets and all that, however i do know that for a given frame size and rpm, those alternator built for
60hz operation will always be more efficient wound as three phase.

from what i recall the stator utilization factor is much higher with three phase than it can be with single phase, so the efficiency of the machine is higher.

typical efficiencies for a 50kva alternator might be around 85% if wound single phase, and as high as 90%  for the same machine wound as three phase.

it has nothing to do with loading as tested at the factory.

i agree though that three phase transmission is more efficient than single phase, as are three phase motors and most other loads that can utilize three phase power.

what is unclear to me, is why would aircore neo pancake/axial alternators be any different than the more common alternators that have been around practically forever.

again from memory, if i recall correctly is was tesla who calculated and then proved that three phase generation is more efficient than single phase, he later also calculated and proved that 5 phase was a bit more efficient yet, however the increase in efficiency was too small to work out economically and three phase won out.

anyway i for one would like to hear more on how a machine wound single phase can be just as efficient as it would be wound three phase?

bob g

[ChrisOlson]

what is unclear to me, is why would aircore neo pancake/axial alternators be any different than the more common alternators that have been around practically forever.

Permanent magnet air core axials are not reactance limited like iron or laminated steel core machines are.  There is only one thing that determines the efficiency of an air core axial - internal resistance of the winding.  Whether it be single phase, two phase, three phase or ten phase, the amount of power you will get out of it for a given shaft input power is determined by the unloaded voltage of the generator minus the loaded voltage divided by the resistance of the winding.

The number of phases does not make one bit of difference.  I've ran them all on my torque motor - single, two and three phase - and there is no difference if the resistance is the same.

For low frequency power like most of the turbine generators run at, single phase is pretty rough running while two and three phase is smooth.  But if you get the freq up to 60-100 Hz the single phase works just as good as low freq three phase too.

I don't know exactly how it became assumed that three-phase is inherently more efficient than single or two.  Tesla never proved any such thing.  He invent the three phase system in 1887 and the reason it eventually won out for power transmission is because the phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load.  This makes it possible to eliminate or reduce the size of the neutral conductor as compared to the two-phase three wire system.  The other advantage is that three phase produces a magnetic field that rotates, which simplifies the design and cost of electric motors.  You don't need start windings and so on with three phase.

But when it comes to power generation, there is no inherent efficiency advantage.  The efficiency is all in the generator design and you can have a very efficient single phase generator and a very inefficient three phase, all because of design.  A single phase of "x" efficiency will usually be a bit larger than a three-phase of the same efficiency.  But once you go to polyphase even that difference is erased.
--
Chris

Edit:  I was looking thru my Picasa program for the pictures that I took of a single phase 12 pole 24 volt that I built and tested last summer with neo rotors.  I can't find the photos.  It had 14 turns of wire in each coil and the resistance was ridiculously low.  It had better than 25% improved efficiency over the benchmark three phase delta stator that I was comparing it to.  But at 2,200 watts and 40 hz it vibrated so bad that it actually made a rumbling noise.  I was going to build another one with 7 turns of wire per coil and try it at 80 Hz to see if I could fix the vibration problem.  But I never got around to it after I got sidetracked building ferrite generators.

[oztules]

Thinking thinking........ I'm struggling with this  Chris

"What I'm saying is that if you have two generators, one single phase, and one three phase, and they both have the same internal resistance against the same load, the efficiency will be identical"

I'm guessing that we are talking about the heat lost in the stator here.

The problem I have is three phase is different to single phase.

In single phase your winding must take 100% of the load.... but it does this in just one EMF pulse per half cycle. It has peak currents about 1.4 times the current you see in your current meter. The loses are I^2 R. If the current pulses have very high peaks they will have correspondingly ^2 higher wattage loss.

With three phase, we have three pulses of current in the same time, so I suspect the peak currents are much less, and each phase carries it for less time, as the other phases take over during the same half cycle.

Also the duty cycle ( to use a term not right but useful none the less) is less per phase.

In Delta, each phase group has only to provide 2/3 of the current, as the "active" phase at any one time gets another 50% from the other two.. effectively lowering the operating resistance from your static measurement.

In star, each phase group has only to run a 66% duty cycle, so has 1/3 less heating loss than a 100% phase group.

For these reasons, it would seem thinner wire than you were anticipating for your heat loss problems, may have been in order to get the winding you needed into the space you had. The single phase had to be heavier just to compete.

I'm suspecting that when you were designing your coil thickness and turns, you expected the wire to necessarily be as thick for single as for three phase...... and I'm not sure that was fair/correct when you were mulling the spacial geometry

Did that make any sense at all?



...............oztules

[ChrisOlson]

I'm suspecting that when you were designing your coil thickness and turns, you expected the wire to necessarily be as thick for single as for three phase...... and I'm not sure that was fair/correct when you were mulling the spacial geometry

Did that make any sense at all?

Well, sort of.  What I found out was that by calculating the theoretical efficiency of the single phase, then testing it, the calculations are borne out with an actual load test and is no different than three phase (or two phase).  The polyphase is smoother for battery charging and yields a smoother running generator.  But my theory is that if you can run a single phase at high enough frequency even that won't be an issue.  I had intended to try that but it would require a different setup than what I currently have to be able to use it, so I shelved that project and went to the dual stator design instead to fit the copper I needed in the space I had.

In reality, three phase (or two phase) is no different than single phase, except you have two or more phases displaced electrically from the first one to provide a smoother output.  If you look at this, if single phase is such a dog, why has it won out over even three phase for residential power?  Three phase is used for power transmission on long distances because it reduces copper mass.  But once you get to the substation the three phase power is split out into single phase and that's what comes into your home.  The four-wire single phase system is used exclusively in North America for residential split phase power, and the three wire single phase system is used in Europe.

The reason single phase has won out for residential power is because it only requires two branch wires to run a load, whereas three phase requires three.

Do you notice significant difference in the efficiency in the motor in your washing machine being that it's single phase?  No.  It's probably 85-90% efficient at full load and has a 1.15 SF just like a comparable three phase motor is and has.  The single phase motor is way more complicated because it has start windings and more than likely start and run capacitors that three phase doesn't require.  It may be bigger than its three phase counterpart because of all the extra stuff inside it to create the rotating magnetic field.  But it is no less efficient.
--
Chris

[joestue]

Compare a single phase induction motor (*run winding only) with a three phase and ask yourself which has more copper.
It is really that simple. both rotors see the same peak flux, they both have the same amount of iron in the core. If you were to pull out the other two phases, and add more copper to the single phase remaining, you might be able to fit 50% more copper in, but this would still not match the three phase winding.

three phase doesn't have any inherent advantage over say, 5 or 7 phases, both of which have been used for large motors but for specific purposes, such as reducing torque ripple to Zero.

in any case, as you're stuck with non overlapping coils. thus obviously the "most efficient" air core alternator would in fact be a solid copper plate instead of a stator, in terms of converting rotating energy into electrical current.
This is why I suggested shorting out your stator and performing a torque-rpm test.
One limitation however is although two coil profiles may perform equally well, one may produce a trapezoidal voltage and works better for battery charging, the other produces a sine wave with a lot of third harmonics and doesn't do as well for indeterminable reasons without a few hundred dollars in test equip

[ChrisOlson]

in any case, as you're stuck with non overlapping coils. thus obviously the "most efficient" air core alternator would in fact be a solid copper plate instead of a stator, in terms of converting rotating energy into electrical current.

Not really stuck with non-overlapping coils, as my two phase dual stator generator has overlapping coils.  I tried initially to get them to fit in one stator and couldn't get the stator thin enough for it to work with ferrite magnets, using the size wire I wanted to use.  That's why I went to dual stators.  The performance of the thing is phenomenal.

Frankly, I really like the 1:1 pole/coil ratio per phase as compared to the 4:3 ratio used on the flat three-phase designs.  With the 1:1 ratio the coil interconnects are really short and it's easy to build a generator with the resistance that you need to match blade power using the size wire you need for the ampacity.  And it's very smooth running and quiet as compared to the harmonics in a three-phase winding.

So I suppose this single vs two vs three vs whatever phase could be argued ad infinitum.  But I came up with a design for ferrites that works, it's very efficient, very powerful, relatively lightweight, not much bigger than a comparable neo, and it's cheap to build.  And yes, it is two-phase, not three.

So will stacking another phase on it improve its performance and make it more efficient?  I can guarantee you it won't because I got plenty of room in my latest design to build more efficiency into it yet if I want to bolt bigger blades to it.
--
Chris

[scoraigwind]

.............
In single phase your winding must take 100% of the load.... but it does this in just one EMF pulse per half cycle. It has peak currents about 1.4 times the current you see in your current meter. The loses are I^2 R. If the current pulses have very high peaks they will have correspondingly ^2 higher wattage loss.

With three phase, we have three pulses of current in the same time, so I suspect the peak currents are much less, and each phase carries it for less time, as the other phases take over during the same half cycle.
...........

Did that make any sense at all?

...............oztules

Makes sense to me, thanks for putting that part of it on the record.  3-phase can have the same resistance but much lower losses, due to its smoother output (lower peaks mean lower losses).  That's why polyphase is better for generators.  Also 3-phase is good for grid distribution, but this is largely because line voltage is 1.73 times higher than the individual phase voltages (neutral to phase) in the houses where voltage is lower for safety.

[rossw]

If you look at this, if single phase is such a dog, why has it won out over even three phase for residential power?  Three phase is used for power transmission on long distances because it reduces copper mass.  But once you get to the substation the three phase power is split out into single phase and that's what comes into your home.

Utter, monoptic nonsense.

That might be the case in your particular patch of dirt - but it's far from universal.

Even in my off-the-beaten-track patch of dirt down under, where you americans seem to think we're incapable of doing anything to a decent standard, 3-phase power is universal.
Up and down the streets they run 3 wire - (3 phase) power. Houses are generally connected from neutral to one phase only. The next house to the next phase, the 3rd to the last phase, then repeat - to balance the load.

SOME houses choose to get 3-phase power. Those that use lots of power (big airconditioners, workshops etc) frequently have 3-phase connection.

Why do the majority only have single-phase? COST. Pure and simple.  The cost of 3-phase metering is more. 3-phase switchboards are larger and more expensive. Line fuses and main switches are bigger and more expensive.

Here's the first photo I could find of a residential street showing the power lines (it's after a hell of a storm, so disregard the debris)


The top three (thin) wires are 3-phase, 22KV. That's common in our area, they run thin wires along main routes and plonk transformers all over the place, usually on poles, to feed the LV on the next level down.

The 4 thick wires are 3 phases plus neutral, 415V phase-to-phase, 240V phase-to-neutral.

The one remaining thin wire is just for the street lighting.

You can see the takeoff to the side-street is just the LV, but is still 3phase.

[mab]

At the risk of speaking about something I know relatively little about, I can't help thinking that the efficiency of Chris's ferrite machine might simply be down to the fact that it probably runs cooler - the resistance of copper goes up 2% for every 5°C - so if you're running at 70°C you have 20% more resistance in the stator than you measured at 20°C on the bench.

my understanding of neo machines is that the temperature of the stator is the limiting factor to the power you can get out of them, and they are often run pretty hot. Chris's machine is generating power in two stators giving a much larger heat dissipation area (for a given power output) than the equivalent neo machine.

Surely that trumps any differences between 1 and 3 phases.

Sorry if this has been suggested in an earlier post, but I'm not going to read through 200-odd posts to check  .

mab

[Jerry]

I just wrote a 2 paragraph responce and it just went poof. I havn't been on the board for quite some time. But I do remember this happening from time to time.

As I said just a few minuts ago (now in hiperspace somwhere?) I'm surprized this 1,2 and 3 phase debate is still going on. It seems that there is some heavy thinking going on here. Thats good. Some day it will all get sorted out.

The expirimenting and such is a good thing. I had allways hoped that more people would do more testing, expirimenting, trial and error and such.
I did quite a bit of that but I wanted more people to do the same. I figured if more people were getting the same results I was getting I wouldn't be concidered to be a loose wing nut with some goofy ideas.

My ideas were well documented here and I could revisit them. This debate in the long run will have a good outcome. Remember in the begining disc alts were built with one magnet disc and a stotor of laminations.

Jerry

[electrondady1]

having been found guilty of using both ceramic mags and vertical mills , i happily resign myself to
single phase.

[ghurd]

3-ph is far better than 1.  Or 2.
5-ph is a tiny bit better than 3-ph.
7-ph is a teeny tiny bit better than 5-ph.
11-ph is microscopically better than 7-ph.
After 3-ph, the improvement becomes far less % per step.

I am kind of confused about the efficiency.  The efficiency of what?
Going to 2 stators and 4 rotors makes it inherently less efficient, unless specific parameters are limited while others are ignored.
Moving forward with what I perceive as the imposed limitations, then a 3rd stator would increase the efficiency even more, and that would make it a 3-ph machine.

It almost sounds like it is somehow splitting the difference between 1-ph and IRP (Jerry Rigged) 3-ph.  I understand it, but it doesn't make it better in apples to apples comparisons.
G-

[artv]

Hi All,....."The efficiency of what?"......I totaly agree.....
If the winds blowing ,or the sun is shining,....your making power......
To me thats 100 percent efficent...........sorry just had to say it......artv

[ChrisOlson]

I am kind of confused about the efficiency.  The efficiency of what?

Let's just say I'm not too impressed with most of the homebrew book designs when it comes to properly matching a generator to available shaft power.  Most of them involve gluing the biggest neo magnets possible to the rotors so they can use less turns, decide on the number of turns based on cut-in wind speed, then stuff the biggest two-in-hand (or more) winding into it that will fit, wiring the thing three phase wye.

That's not the way to build wind turbines if you want one that performs.
--
Chris

Edit:  Before I hit the sack for the night, I just went to get the meter readings off my newest 4.0 meter creation to illustrate to folks what efficiency is all about, as this is the best performing wind turbine I have ever built.  It has a bit of a vibration problem when it yaws because of the two blade rotor, and I haven't decided how to "fix" that yet (probably a teetering hub), but otherwise it is a very nice running turbine.

Power Logging time was 25.9 hours:
The turbine developed 39.01 kWh, 1,452.6 amp-hours, average system operating voltage was 26.86 volts DC, average output was 1,507.1 watts, average amp output was 56.12 amps, peak amp output was 104.88 amps, average wind speed was 10.2 m/s.

This machine cuts in at about 3.5 m/s, produces about 435 watts @ 6 m/s and lets the rotor spin at 400 rpm at 12 m/s where it is producing around 2,400 watts with the generator at approximately 85% efficiency.  And it has the capability to run at that output continuously without hurting it.  I ran it on my test bench at 70-75 amps for about two hours and the stators got to about 130 degrees with absolutely no cooling air blowing on them other than what the rotors move.

That's what efficiency is all about.  The number of phases in the generator is immaterial.

[Antero]

Very good readings ChrisOlson  !

Is there anywhere details or pictures about your windgenerator ?
Do you use anykindof charge controller ?

Antero

[ChrisOlson]

Is there anywhere details or pictures about your windgenerator ?
Do you use anykindof charge controller ?

Antero, if you read back thru this thread I'm pretty sure I posted the details of the 4.0 meter machine.  I would not recommend building one of those until I figure out how to fix the cyclic loading problem imposed on the machine and tower by the two blade rotor when it yaws.  A couple weeks ago it broke the tower mast in high winds and I just about lost the turbine off the tower stub before I got it shut down.  If it yaws gently the cyclic loading is not a big issue.  So I got a double hinged tail on it now that dampens yaw both directions, but that didn't work.  I still shakes the tower all the way to the foundation when it yaws.

I've been kicking some ideas around on a teetering hub for it, which will fix that problem but again adds some more complexity to the machine.

There is no charge controller on it, and I have three turbines plus a little over 2 kW of solar power.  I use a rather elaborate 7.5 kW (total) auxiliary loading system to control system voltage, that uses two Morningstar RD-1 Relay Drivers.
--
Chris

[midwoud1]

Hi Chris.
Is'n it a good idea to make a yaw system with chain and sprockets a worm transmission and a cordless drill ?
Switch left and right with a small windvane .Select  vane control or manual.
Also good to avoid gyroscopic torque on the mainshaft and it can be set crosswind in storm.
I had once made this with a wind servo and a small hydraulic discbrake on the mainshaft on an early mill. Did good.
It was one of my first windmills with a car-alternator and that was not working very well.
I also have 2 and 3 blade rotors and have less problems with 3 blades because the balance is better divided in the circle
but you know that for a long time.
Good luck with all your projects.
Regards Frans.

[ChrisOlson]

Is'n it a good idea to make a yaw system with chain and sprockets a worm transmission and a cordless drill ?
Switch left and right with a small windvane .Select  vane control or manual.

Hi Frans,

Yes that would be a good idea, and another one I have considered.  Passive yaw systems put a lot of undue stress and wear on the machine in gusty or turbulent wind conditions, which a mechanical yaw drive would eliminate.  So that is most definitely a possibility.
--
Chris

[ChrisOlson]

3-phase can have the same resistance but much lower losses, due to its smoother output

I read this yesterday and wanted to comment, Hugh.

Don't get hung up on losses, lest you get three-phase tunnel vision.  Losses are good, to a point.  The challenge in building fixed pitch permanent magnet wind turbines that really perform is not to reduce all the losses to zero.  The challenge is to match your load to your blade power.
--
Chris

[ChrisOlson]

Even in my off-the-beaten-track patch of dirt down under, where you americans seem to think we're incapable of doing anything to a decent standard, 3-phase power is universal.

Thanks for that comment, Ross.  Very helpful.

The fact is, the US electrical grid system is outdated, grossly overloaded in most populated areas, and for the most part is over 50 years old.  It is a house of cards as witness by the huge power outage in the southwest US a few weeks ago, that grid operators did not even know what caused it.

About 99% of rural power is single phase only, and where three-phase does exist it is 115/230 high leg delta.  In more populated areas where industry uses three phase power you will find 120/208 or 277/480, but even then all residential power is four wire single phase.

I rural areas the three phase transmission lines go to a substation someplace and are split out to single phase feeders for rural customers.  Those feeders in this part of the country are not even reliable, with poles going thru swamps and leaning at a 45 degree angle with the wires holding them up.  When the wind blows they go down and the utility company props them back up with pieces of wood.  Some places you can't even get power.  I've wanted 277/480 three phase for my grain dryer and grain facility for almost a decade.  The last price I got quoted was $165,000 to run lines, and that was seven years ago.

So this is one American that thinks no such thing, as what you said.
--
Chris

[scoraigwind]

I am kind of confused about the efficiency.  The efficiency of what?

Let's just say I'm not too impressed with most of the homebrew book designs when it comes to properly matching a generator to available shaft power.  Most of them involve gluing the biggest neo magnets possible to the rotors so they can use less turns, decide on the number of turns based on cut-in wind speed, then stuff the biggest two-in-hand (or more) winding into it that will fit, wiring the thing three phase wye.

That's not the way to build wind turbines if you want one that performs.
--
Chris

Chris is right, efficiency of the whole system is not the same as the efficiency of the alternator.  There are many different efficiencies to consider.  Best electrical efficiency, best blade efficiency, most efficient use of money, most efficient use of magnets, and so on and on.

The homebrew design books that I have published, and the Dans' one is of the same era, are based on a time when NdFeB (neo) magnets were the obviously most cost-effective solution, and they do offer high electrical efficiency.  They still offer a good solution, but there is going to be a cheaper one based on ferrites (maybe Chris can publish one that can be built in a back shed?) as the price trend continues.

The problem with high electrical efficiency is that you end up with a machine that has a low range of speed.  Once you have reached cut in speed then it doesn't take a lot more rpm to reach full power because you only need those extra rpm to overcome losses.  So the lower the losses, the less variation in speed above the cutin.  Blades, on the other hand, want to run over a wide range of speeds in proportion to the speed of the wind which varies from 3 -12 m/s in some cases (7- 26 mph).  So they would like the rpm to increase by a factor of 3-4 times so as to keep their best tip speed ratio.

Here is where the discussions of increasing the air gap to prevent stall and of putting resistance in the line to keep the blades running in stronger winds start to emerge.  Blades that run too slowly in stronger winds can perform poorly (although it's nice and quiet). The best solution for efficiency (overall) is to use some kind of MPPT controller such as Flux has developed or would be available if you use a grid-tie inverter.  By varying the voltage the alternator works at you can have the best of both worlds (low electrical losses and happy blades).  But for those of us who are suspicious of black boxes and like to connect the rectifier to the battery, we have a dilemma.  Higher electrical efficiency means lower blade efficiency at some windspeed.  You can only tune the machine for a narrow band of windspeeds if you keep electrical resistance low.

OK so if we know that electrical losses can be 'good' where does that lead us?  The neo alternator is compact and higher losses will make it burn.  So in that case using an inefficient line can help the blades or you can even use heaters in series with the battery charger.  I do this in some cases.  The heat may be useful.  it's certainly better than having it in the stator.  And it allows the voltage and speed to rise so that the blades run faster in stronger winds so avoiding stall.  The best solution for you is going to depend on your circumstances in so many ways to do with line length, battery voltage, priorities and so forth.  and maybe if you expect the recipes and the homebrew book to guarantee a simple single solution you will be let down.

Using ferrites does help the blades because they do tend to be less efficient, so they have to be built bigger enough which allows them to dissipate more heat. So things are turning around, and lower electrical efficiency does not seem so bad.  You can use single phase and you can use dual stators, simply so as to make the machine bigger and less efficient than it needs to be be, and actually do well because the blades will thrive on the variation in rpm that this produces.  OK so you can get a wind turbine that works really well and I respect Chris for doing this and for stimulating our minds at the same time.  His speed increase chain drive gives a big advantage too, and I am not sure how many homebrew guys will take on that chain drive approach any more than they would wish to tackly flux's MPPT controller.

For me I prefer to make the most efficient use of material where possible and that's why I bang on about a single stator and 3-phase windings..  I am thinking hard about how the low price of ferrites plays into that philosophy and hoping not to lose the good braking effects you get with neos, but I look forward to getting away from the corrosion issues.  I have plenty of neos on the shelf which is are not helping me to be innovative yet, but I see the way forward.  Whatever the design I will seek the most cost-effective use of materials, and if there is power to be burned in reducing the electrical efficiency I will try to use that to reduce the cost of the wiring line or to heat a heater in my house rather than to heat the stator. 

Chris is still perfecting his ferrite magnet turbine and we'll continue to watch in awe.  And one day there'll be a simpler version available as a homebrew manual.

However

[defed]

Let's just say I'm not too impressed with most of the homebrew book designs when it comes to properly matching a generator to available shaft power.  Most of them involve gluing the biggest neo magnets possible to the rotors so they can use less turns, decide on the number of turns based on cut-in wind speed, then stuff the biggest two-in-hand (or more) winding into it that will fit, wiring the thing three phase wye.

That's not the way to build wind turbines if you want one that performs.

i know that you hate writing and don't WANT to write a construction manual, but i'd love to know your process on determining windings, wire gauge, diameters, etc.  that's one thing i've never been able to wrap my brain around, how to design a stator.  i mean, there must be a logical process, i just haven't ever seen one well explained.

[ChrisOlson]

Using ferrites does help the blades because they do tend to be less efficient, so they have to be built bigger enough which allows them to dissipate more heat. So things are turning around, and lower electrical efficiency does not seem so bad.  You can use single phase and you can use dual stators, simply so as to make the machine bigger and less efficient than it needs to be be, and actually do well because the blades will thrive on the variation in rpm that this produces.

That's probably better put, Hugh, than anything I could come up with to explain it.  I think too many folks are hung up on the generator efficiency thing, and lose track of the big picture.  I have found that you can build turbines that perform without having to use an expensive MPPT controller, or resistance in the line to get it run, if you throw some of the conventional homebrew ideas on how turbines are built out the window.

The first thing is to design for peak efficiency at the average wind speed you see on your tower, NOT to get peak efficiency at cut-in and then it goes downhill from there.  The ferrites REALLY help with that.  The generator is not near as "stiff" as a neo and it lets the turbine run.  After building three of them now and getting them a bit "tweaked" to better match my blade power, in my humble opinion, it is the only way to go.  It is the first time I have ever been able to achieve an almost flat TSR curve from 6 m/s to 12 m/s with a fixed pitch permanent magnet wind turbine.  That makes the machine incredibly efficient because the rotor extracts more power from the wind.  And when you extract more power from the wind, you can turn it into electricity!  If you don't grab that power from the wind because of a machine that runs up against a stall problem, you will never get it to your batteries.

@ defed:  the answer is lots of experimenting.  Hugh can come up with designs on paper that work pretty close to what he figured they would.  I sort of know enough about it to come up with the idea, then I build it and see if it works.  Once I find out how it works, then I "tweak" it to make it work better.  The bad thing about my way is my stator collection that I have.  I would venture to say that if anybody needs a stator I probably got it in stock in my shop.
--
Chris

Edit:  I decided to add another comment because there's a lot of wailing and gnashing of teeth over this dual stator thing and efficiency, etc..

My goal was to build a generator with ferrite magnets for a medium size turbine that matches the neo in output.  I achieved it, and then some.  Hugh wrote:

For me I prefer to make the most efficient use of material where possible and that's why I bang on about a single stator and 3-phase windings.

And there is nothing wrong with this approach.  It provides, probably, the least cost solution.  In my design and thought process, cost was an issue, but as long as it was cheaper and more durable than neos, and still matched the neo, then I would consider it a success.  My other hobby is NTPA tractor pulling.  So I took this approach:



You get about 1,800 hp from a blown 575 KB Hemi on methanol.  You need 5,000 hp?  Both three of 'em on there.  It works.

[ChrisOlson]

i know that you hate writing and don't WANT to write a construction manual, but i'd love to know your process on determining windings, wire gauge, diameters, etc.  that's one thing i've never been able to wrap my brain around, how to design a stator.  i mean, there must be a logical process, i just haven't ever seen one well explained.

I'm going to try to answer this.  And please note my ideas on how to do this are not in any of the books.

I start with a pole and coil layout.  It might be a 12 pole 9 coil three phase with neos.  Or in this case it was a 10 pole 10 coil single phase (which evolved to a two-phase after I tested the single phase and determined how much power it makes).

If you know what blades you're going to use, and are familiar with their performance characteristics, you can figure out how many watts they can make at the shaft at various wind speeds.  If that know that, you know how many watts load you need to match the shaft power.  That tells you what size wire you have to wind with to handle the shaft power.

For 24 volt with a run resistance of around .12 ohm I like the stator to have about .3 ohm internal resistance.  This gives very good electrical efficiency.

So now you know what size you're going to use and you know what the resistance has to be.  The number of turns required to get that resistance is already figured out for you.  You can tweak the shape of the coils a bit, or whatever.  But the number turns is pre-determined to get the resistance, within a couple either way, depending on how you tweak coil shape and size.

Depending on what magnets you use, determines how fast you have to spin it to get the voltage.  With neos it takes less rpm and the voltage rises very rapidly.  With very weak magnets (ferrites) it takes lots of rpm to get the cut-in voltage, and it takes lots more rpm to get the next open volt beyond cut-in to start pushing any amps.  The amount of power an air gap axial makes is determined by the open voltage minus the loaded voltage (or "clamp"), divided by the resistance.  So all this determines what gear ratio you're going to use.  If you use a higher gear ratio it makes the generator "stiffer" and puts the blades into stall.  If you use a lower gear ratio it makes the generator more "loose" and lets the turbine run.

The trick is designing a generator that takes LOTS of rpm's to build each open volt.  And that's why I've gone to using gearing.  I can built a generator with tremendous electrical efficiency (low resistance with big wire to handle lots of amps).  But it takes LOTS of speed to get the volts out of it.  That speed is good.  It lets the blades run at their optimum TSR.  You just have to select the gear ratio to do that, because this very efficient generator needs WAY more rpm's than the rotor can safely (or efficiently) turn.

So, like I said, this is not in the books.  The books don't deal with geared turbines.  And getting this efficiency from your turbine with a direct drive is not impossible - it's just really hard.  The size of the thing, and the number of poles you have to use (magnet cost and rotor size) gets really big to get the same efficiency you can get out of a geared unit, with the same amp capacity.  And let me caution that before you go rushing out the door convinced that geared turbines are the only way to go, you can't just slap a set of gears on any old blades and generator and make it work.  There's a "sweet spot" in gearing because you're dealing with torque loss in the drivetrain traded for speed vs efficiency gain in the generator.  All this has plus electrical losses has to match shaft power for the blades to be at their "perfect" TSR over a wider range of wind speed.  That's where some experimentation comes in to find that "sweet spot".  You can calculate it all you want.  But in design the testing phase is what tells the story and determines the final outcome.  The only way to test it is to build it.
--
Chris

[scoraigwind]

...how to design a stator.  i mean, there must be a logical process, i just haven't ever seen one well explained.

I have been using the same technique more or less for over ten years and so the results are predictable (if boring at times since I have figured out the 'best' shape of coil and spacing of the magnets to minimise the use of NdFeB).  The process is described in my recipe book.  I put it in rather as an afterthought because the book is limited to 64 pages long (the way it is bound) and I had worked hard to get all that info into the pages and ended up with some pages to spare.  So I set out the process on pages 54 to 57 as concisely as I could.  Too concisely for most people as it turns out but some people have read it and emailed me questions, and I help them out.  Most people are happy to have the answer to their particular stator needs and that's OK by me.  It would probably need another book to explain the design process properly.

The main idea is the get the best match for tip speed ratio in low winds.  But using neo magnets that usually means it doesn't work so well over the full range.  I did a page about this back in 2003.  http://www.scoraigwind.com/axialplans/update.htm  and the subject has been done to death since on this forum.  But ultimately you have two choices.  One is to do something to control the voltage and raise it in higher winds.  the other is to use resistance to adjust the speed characteristic.  the second way is inherently inefficient but simpler.

[Flux]

Not really a lot to add, Hugh and Chris have summed it up.

Probably the biggest confusion through all this discussion seems to be regarding efficiency.  Chris gets excellent results and if his machines do the job and produce the maximum energy into a battery then yes they are efficient but the actual alternator efficiency is not really that high. As long as we keep that in mind then this all makes good sense.

I have said for years that you have to trade alternator efficiency to get the prop to perform. there is more to be gained there than you loose on the electrical side.

With ferrite using fluxes 1/3 that of neo, a direct substitution will need 3 times as many turns and will end up with 9 times more resistance, that is why ferrite machines need to be big and heavy if you keep the same speed restrictions.

For those prepared to go to a speed increasing transmission there is no real problem.  Trying to use ferrite with direct drive will be more of a challenge and beyond 10ft it may need a different approach to keep things sensible.  With ferrite, resistance is such a big factor and there are probably other methods of construction that offer a better solution than the axial design but I won't go off the track here. A major problem with radial machines is that they need machining facilities to get very far, the same is true of speed increasing drives so those with good facilities have more methods available to tackle the job.

Unless you tackle the basic problem of volts needing to rise with wind speed then even if you manage to keep the prop at the peak of its power curve you will not get much over half of what is really available in high winds but as long as things work well in the common wind speeds simplicity will usually win out in the end.

There still must be some half way method between a mppt converter and sticking absolutely to direct connection, for those wanting to exercise their brain cells this must surely be worth the challenge. The Australians seem to be doing well with capacitor boosting and capacitor reactance neutralising with those inherently unsuitable f & P machines. For ferrite perhaps we should start to look at using many more small magnets and get the frequency up high enough to try these methods.  The main problem here is to get the frequency high enough to use sensible size capacitors.

Flux

[ChrisOlson]

Probably the biggest confusion through all this discussion seems to be regarding efficiency.  Chris gets excellent results and if his machines do the job and produce the maximum energy into a battery then yes they are efficient but the actual alternator efficiency is not really that high. As long as we keep that in mind then this all makes good sense.

I'd like to point out that the power efficiency of my dual stator generators is very good - better than 80% at the maximum power the blades can deliver at 12 m/s.  It does drop below 70% at higher outputs but those are what I call "spike" outputs and they're beyond the design furling speed of the machine.  They still happen, as furling is a not positive means of power control.  But they are usually very brief - less than a few seconds in most cases.

Personally I do not trust "black boxes" (MPPT) on a wind turbine that adjust the power curve by changing the operating voltage the generator "sees".  I've flown turbines long enough to realize that they can get pretty wild in high winds.  Even big utility scale machines shut themselves down in excessive wind speeds instead of trying to keep harvesting power from dangerous wind velocities.  It's my opinion that you're playing with fire running a MPPT Buzz Bomb.  A box that is not capable of shutting the turbine down when voltages reach dangerous levels, or protecting itself without depending on some other means (furling, voltage clipping, etc..) is not a well-designed unit, IHMO.  If that box is going to run a turbine on the raw edge, then it better have an internal self-protection mechanism to govern what it does.

So I prefer the mechanical approach because it's less likely to let the smoke out.
--
Chris