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25 to 30 foot diameter turbine
methanolcat:
Where to begin,
I have a lot to think about and do for this new turbine. Not sure on diameter just yet, somewhere between 25 and 30 foot. I'm looking to get into the 15 to 20 kw range.
1. build new tower capable of supporting such a beast. 4 inch square tube 1/4 thick 40 foot long for the legs, and either a 6 or 8 inch tube, schedule 80, mast around 20 feet long. This needs to last the rest of my life with no tower maintenance. It will tilt up just like the one I have and I doubt I will be climbing it much if at all.
2. build a low maintenance gearbox, very much like Chris Olson and fabricator did. 6:1 sprocket and chain. I have started on the gearbox and I think just the gearbox is gonna weigh in around 250 pounds. It has a 3 inch diameter blade input shaft.
3. build a jacobs type feathering hub (centrifugal controlled to keep blade speeds reasonable) maintenance required (grease probably 3 times a year, we will see) All steel for the hub has been acquired and rough sketch drawn up.
4. generator, build new or use current generator. Need to do some discussion on this before deciding.
5 make new blades, been thinking of making some of the GOE222 design but the ones I have now perform very well so I may just make the same blades and upscale them, not sure yet.
I will post pic's and progression as I get going, need to take some pics now of current state of gearbox.
Wide open for suggestions and comments and will most likely need some help from some of the brainiac's and number crunchers here on the board from time to time.
First thing I want to do some serious discussion on is the generator I have and what it can handle if geared up 6:1. Here is what I have as far as output goes in unloaded volts, wire gauge is 16, and I think I will wire it up 3 delta and try to get it running 1800 rpm's or higher using the gearbox.
I need to wire it that way and check resistance and post the numbers.
I guess what I don't know is where is the limit, there has to be some limit somewhere where X amount of magnet can only produce X amount of power no matter what the rpm, so is there a limit and where is it and what makes it the limit, does anyone know?
Also, does anyone know the safe rpm limit for a 25 to 30 foot diameter blade set as that will also play a role in generator rpm's. I have been tempting fate too long with the current 15 footer letting it run sometimes in excess of 600 rpm's.
Janne:
Hi Matt,
Congrats on your new project! No doubt, a machine this size will be a major project, but with your skills it should be reasonably achievable.
About the generator, the thing that you will need to pay attention is, that on higher speeds the current output of the generator will start be reactance limited, that will ultimately limit the maximum power you will be able to extract from the generator. If you can power up the generator in a bench, and measure output currents to a known resistor bank value on different speeds, you should be able to estimate the reactance factor on the generator. My guess is that it will not be a problem, as you can rely on pitch control and (I'd suggest at least) brakes on the main shaft to control the speed of the machine.
Have you though about steering to the wind, up- or downwind? In any case, with machine of this size I'd like to see some form of damping on the yaw, to reduce the gyroscopic forces.
On my late fathers' old design, the automatic shutdown speed for the 12m rotor was 2 r/s, and with the 8.8m diameter rotor it was increased to 2.5 r/s. I'd assume you could go a bit higher, if you make a strong hub design, but I'd limit the design speed to around 200rpm maximum, as that already means a near 80m/s tip speed, and it would still allow you to run the machine with peak efficiency up to windspeeds of around 10m/s.
Other than that, build it strong, add a reliable brake to the slow speed shaft, do some calculations to make sure the tower is up to the forces up to the storm wind speeds, and I'm sure it will be fine.
I hope to see some pictures of this project soon :)
methanolcat:
Janne,
On this reactance limited issue, I tried to find out more by searching but everything I came up with looked greek to me and was hard to understand although I do have a minor understanding of it. Before finding out about it, I thought my only limit would have been the amount of current the windings could carry without burning up.
I can wire the generator 2 delta and put the generator in a lathe at work and spin it it up at a few different speeds and monitor current into a fixed load as you suggested and post some numbers, volts, amps, fixed load resistance and rpm's. If you can help me out from there, it would help me figure out whether or not to use this generator or build a bigger one. I'm gonna need to know this before too long because I am working on the gearbox, of which the generator will mount to, so mounting decisions need to be made.
I want to keep the peak voltage reasonable, no higher than 300v or so I would think, but high enough to keep the amps low as well. Looking at my voltage test chart, maybe I should wire for 2 star. That would put me making around 250v at 1200 rpm's and through the gearbox keeping the prop shaft at 200 rpm's. But the voltage will be lower with a load on it so its all a guess and a lot of experimenting right now. Maybe 6:1 isn't high enough, we are talking about a lot of power with such a big prop. I guess this is why not too many venture to far out from the smaller known designs.
So another question, does the reactance change from one winding configuration to another for example, 1 star, 2 star, 2 delta, 3 delta and if so that may change decisions as well.
According to Alton-moore wind calculator, in about 32 mph wind is where I would be at 200 rpm's (30 foot blades), maybe lower (blade design pending) and there is about 47,200 watts (63 horse power) available at the blade, give or take, thats quite a bit and I'm wondering if 6:1 gearbox is enough.
I'm not sure what questions need answered first in order to proceed forward, maybe I should halt on the gearbox until I do some further investigating.
Anyone have any useful information which could help to shed some light on some of these decisions?
Janne:
About the reactance.. In reality, the windings' ability to carry current will be the deciding factor on how much power you can extract from the generator, in the long term. Reatance would in all probability be a bigger problem, if you intended to use shorting out the generator as means to stop the machine, as it might limit the short circuit current too low to slow it down. Mayby Flux, or oztules or someone else will chime in with their experience regarding iron cored machines? Anyways, with the simple speed test on a lathe the reactance factor would be easy to calculate.
I'd assume that the winding configuration would not change the (percentage) effects of reluctance on the generator. With more coils in series, you will have more reactance, but also the voltage will be higher and the current lower. Many "mights" and "maybys" in the thinking above, it's mainly my gut feeling speaking. So unless someone with more experience can comment on this, I'd recommend running the tests.
Can you go much higher than 6:1 with a single stage chain transmission? It would make sense to find a bigger motor frame to convert to a PMG, so you could stick with relative low gearing. I don't know if it's such a big problem with chain drives, but with gearboxes things start to get noisy above 1500RPM.
oztules:
The synchronous impedance will be all the things, that bundled up together will work to stymie our attempts to get the output we want, from the thing we have got to use for the purpose.
If we use an axial flux, with big magnets, then essentially it is just a resistance game to find your upper limit.... ie how much power can the stator shed in the form of heat to stop it deteriorating.
The next problem we encounter is the inductive reactance, which is a linear function. It is simply the impedance that the inductive characteristics of the stator winding presents to current flow, and is dependent on the stator inductance and frequency. If we know the inductance, and the frequency we are interested in, we know the impedance it will present to the current flow.
The next one is not commonly discussed in windmill forums like these. Possibly because we deal mostly with axial flux designs, which by their construction push this problem to the bottom of the heap..... it's this pesky reactance we speak of when we talk of iron core machines.
With all endeavors in generating electricity from rotating machines, we need flux linkage........ this means we need to get our operating flux to penetrate the coils in order to generate an EMF..... simple enough. we wave a magnet past a wire and get EMF. Thats cool, but we also know that if current flows through a wire (the general idea here), it will exhibit a magnetic field of it's own making. It will be in opposition to the magnetizing force we used in the first place.
This sets up a problem. We need all the flux from the magnetizing field (magnets) to penetrate the coils for us to get the maximum benefit, but we have opposing fields trying to stop us. The net result is that we cause some of the incoming flux, to NOT link the coils, and find some other more amenable flux path, which does not benefit our generator..... ie we lose some of our flux to some other flux path.
This problem was caused by current flow in the stator coils, so the more current we cause to flow in the coils, the more interference with the magnetizing field we cause.... we can see where this leads, in the extreme, the current flowing in the stator presents enough megnetomotive force (MMF) in opposition to the magnetizing MMF, that we get a stalemate at some point. We see this point when we short a mill, to stop it, only to find it runs away instead.... amp turns.. not watts. It runs away because we are no longer linking all the flux through all the turns. Use less flux, need less torque..... mill runs away...........
With axial flux, the magnets are very strong, and the air gap very big, and no core to focus the field, so it is much harder for the back MMF to influence the magnetizing field... so we see these alternators as resistance limited (how much heat can you get rid of). The inductive reactance is of little consequence as the inductance of the air coils is very small, and so the frequencies needed to cause significant impedance are not practical considerations, and the armature reactance is similarly muted.
With iron cored things, we get more problems from the inductive reactance and the armature reactance.
Because we are using iron, we can use less magnet, and use small airgaps. Smaller magnetizing force channeled better through iron core stators, allow for better performance with less magnet.
Nothing is for nothing, and so we find that the inductance of the stator coils will be bigger, so the effects of inductive reactance will be more pronounced, and can indeed control things at high enough frequencies.... and the armature reactance becomes a capitol problem.
With less magnetizing force, and tight air gap, and a core to focus the back MMF against the magnetizing MMF, we are set for a true showdown.
It comes down to Ampere Turns.. You can use more turns at less amps, or more amps at less turns. It will produce the same magnetizing MMF, and current limiting will set in.
This overrides the resistance limit of the stator in some cases, and salient pole machines with long fingers and ferrite magnets, can run short circuit all day without over heating. The AWP can too.
BUT
If your going to generate such high power, you must be going to put it somewhere. The battery bank would be huge. I'm guessing grid feed??
If so, perhaps just use a big three phase induction motor, and drive the grid direct. (see breezy site for ideas). Your already building the gear box, maybe this could solve the generator problem simply too.
We have a 35 foot induction generator here, and it works particularly well, and has done so for over 25 years. Direct connect to the grid. No pitch control so big direct brakes are necessary. We dont use a motorbrake (like breezy), but brake the prop directly. It does have tip flaps.... I love em.
..................oztules
Must go will edit later perhaps
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