10' Turbine speed

[Janne]

Hello!

A few days ago i finished mounting my windcharger into a short test tower. I calculated  the tail angle so, that it would start furling at windspeeds 8m/s or more. But now i'm concerned that the blades might not be able to take so much revs. They're made of  finnish spruce, with TSR of 7, using mr. Hugh Pigott's blade spreadsheets. Each blade is 1,6  metres (about 5 feet) long. What would you reckon would be the maxium safe speed to use with that rotor? The generator kicks in (12 volts ) at about 120rpm, and i'd hoped i'd be able to run it at least 300 RPM maxium.

here's some pictures of the machine.. http://koti.phnet.fi/~hmgba/gallery/gallery.html

[DanB]

As we build them they cutin at 140rpm and they run up to about 400 rpm - they may reach 500 on occasion for short bits of time.  

[DanB]

I've been looking at the pictures.

Looks like you chose to build a single phase machine - I wonder why?

I also worry a bit about the stator bracket.  It looks like a single rotor machine, and it looks as thoug you have steel brackets in contact with the back of the stator.  It must cog quite a bit and have trouble starting...  and I expect you will have severe eddy current losses.  Perhaps you could reconfigure it so the magnet rotor is behind the stator to keep those brackets away from the magnetic fields.

[DanB]

Other trouble I see is all the loose wires around the stator - those will rub together and short out - and/or - break over time.  I would try to tuck them in neatly and secure them with epoxy or something.  We used to run lots of wire around the outside of the stator on machines - most of them failed.

[Janne]

Thanks for the comments Dan, I appreciate your advise.

About the generator design, well.. at the time I started building it i looked for a most simple way of doing it, and also at the time i didn't know about the drawbacks the single phase design had. I roughly followed the "volvo brake disk alternator"-design  shown on your website, and improwised with the materials i had at hand. The stator is actually divided in 2 parts, both having 7 coils connected in series. The idea behind that was that i could connect the 2 parts in series or in paraller, depending of the wind. Behind the coils i put some rusty 10mm "guy wire", to act as a laminate, like the bandsaw blade in the "volvo alternator" design. Obviously, that was a mistake, since the wire is twisted it must have some serious currents flowing in it. Also, in the middle of the coils i casted some resin mixed with steel shavings collected under the bandsaw at work. It actually increased the voltage about 2 times of what it was before, but added a horrible cogging effect, which of course i didn't thought about when pouring the mix. With these flaws combined with the stator bracket mishap, that i realized now as you mentioned it, the machine needs about 4 m/s of wind to get over the cogging effect and also it vibrates some when running at low speeds. I guess the easiest fix to the stator bracket problem would be to cut it away and weld a new bracket further back. After that i could add a spacer in between the bracket and the stator, made say of a 2" plywood. The overall power is also a bit dissapointing, i think it's mainly because the rather high resistance in the windigs, which are made of 1mm diameter magnet wiring.. it measures a little over an ohm when measured with a multimeter.

The wiring indeed needs to be fixed. I've already broke it several times when fiddling around the machine.. I'll try to glue them tight with epoxy as you suggested.

[Flux]

I share all Dan's concerns and I also suspect that the alternator will not be powerful enough to hold the blades down to a reasonable speed. You will be relying quite a lot on early furling.  I have severe doubts about the accuracy of calculations to determine furling wind speed.

I don't think you have a chance of stopping it in a decent wind with a brake switch.

Flux

[Flux]

I hadn't realised that you had a core behind the stator, that will make it more powerful and may help hold it down but it still seems too small for the size of blades,

If that wire is 10mm diameter for the core you will have terrible iron eddy losses, to be effective at reducing eddy currents it needs to be no more than 1mm. Hysteresis will be dreadful and there is little you can do about that with poor core material. With a bit of luck these inefficiencies may cause enough drag to hold the speed to safe limits. If you have a good wind area it may do what you want, but it will be very poor in low winds.

Flux

[Janne]

Flux, you're right with your comment. It's not powerful enough to hold the rotor down in high wind, even with the stator shorted out. I'm thinkin of adding a mechanical brake to stop the machine completely if a storm is predicted. Also i need to make the tail lighter and shorter to allow more easy furling.

In the long term thought, the best option propably is to start a new alternator from scratch, likely with bigger magnets, dual rotors and 3 phase windings configuration.

[Janne]

The wire's total thicness is 10mm, but i forgot to mention that it's made from dozens of strands of about 0.5 mm thick. It's also rusted completely to the core, which i'd hope would help to separate the strands from each other.

[DanB]

I think you might do well to keep your magnet rotor (it looks nice) - rebuild your stator with no cores, and 3 phase (12 coils) - and run a blank rotor (no magnets on it) behind the stator.  That might be a decent match for your 10' blades.

[Janne]

I thought about the idea of adding a second blank rotor and making new coils to the stator.

However, because of the construction, adding a second rotor would mean almost starting from scratch. What would you think, how much i would lose efficiency if instead of the second rotor i'd build a good iron core behind the windings, likely from thin steel straps. New coils would of course be without any individual iron cores.

The other thing that was bothering me, would it make a big difference if instead of the 12 coils i'd make 15 a little bit smaller ones?

thanks in advance.

Janne

[Flux]

The steel strap makes a pretty bad core, but in this case I think it will be far better than what you have at the moment.

With your 16 poles I would stick to 12 coils and make it 3 phase.

15 coils will be a total mess and you will need to rectify each coil individually and use loads of rectifiers.

Flux

[Janne]

A little update on the project.

I studied a on the 3 phase basics, and realized that in order for me to make 3 phase windings i'd need a total of 21 coils(difficult to fit them in the small space), because my magnet rotor has 14 magnets. If I were to make 12 coils, I'd have needed to make a 6 phase system! I don't even know how to calculate voltages in that kind of a system.

So instead i decided to stic with an 1 / 2 phase system. I studied also the possible core materials and realized that steel strips were indeed far from perfect. Luckily i got my hands on some old transformer eel, just the right width to make my new core.

Now the new (hopefully much better) stator is curing, and i expect to mount it tomorrow to the test rig. Then i get to see if it's any better than the old one.

Janne