Another one bites the dust

[TheCasualTraveler]

    Lost my wind generator the other day in some nice winds. I had a pvc threaded coupling as a link between the rotor shaft assembly and the yaw pipe. An accident waiting to happen. It snapped at the threads and the turbine hit the ground breaking one pvc blade. Weird how the alternator part of these things usually survives with just a few scratches.

     OK, so I had a set of newly carved 6' wood blades that were supposed to go on my next machine but I had not tried them except for freewheeling in light winds. Visually comparing them freewheeling side by side with my old blades, the new ones were slow to start but in a light wind would soon be spinning faster than the pvc blades. I had to see what the new blades would do with a load. So I transfered rotors and stator from the alt which had just taken up skydiving on to my more robust machine and added the 6' prop. It had been flying with a 5' 4" pvc prop and the alt is a simple little 8" disc, 12 of ED's wedge magnets and a second blank disc to return flux. Stator, 9 coils, 120 turns #22.

     Now I knew larger blades would spin slower but since I was capturing more swept area and the blades were upgraded from pvc to wood with a expected TSR of 6 I thought I would have a net gain. Not so, only about 1/2 of what it was doing. OK, I suppose I have a classic case of blades not matched to the alternator. Funny how adding only 8" to the blade diameter would have such an effect. I also wonder if the pvc blades might have had a much higher TSR than I thought.

     I can't help thinking that one of the most important things in design is the ratio between the blade rotor and the magnet rotor. My reasoning is that I always see the blade as a second class lever, the rotor shaft as the fulcrum and the magnet placement as the load. Actually it would be the sum total of the blades (levers) and magnets (loads). So I thought the bigger the blade, the less impact the load has on it and the closer it will run as if freewheeling. I expected the larger blades to notice the drag of the alt less and so run faster canceling out the decrease in speed from larger diameter. I would be very interested in any comments on this.

     Now I have to say at this point that one change I made was to tighten up the air gap when I reassembled the machine. The change was only about 1/16" and I expected the new larger carved blades to eat this up and spit out more power. It seems that if I open up the air gap at best it will probably match the old results. Arrrgh !

     So, I'm thinking to get more out of this alt I should go with smaller blades and also, to make the most of light winds, (8 to 12 mph) go with 5 blades instead of 3. I know that in most cases 3 blades is optimum but can't help thinking there must be a happy medium between slow (water pumper) blades and fast 3 blade rotors if I am targeting a specific range of wind speed. More comments welcomed here.

     Now my last question. In another post, Flux recommended that a good match for my new 6' prop might be an alternator with dual rotor, 10" discs, 8 magnets (1" x 2" x 1/2") on each disc, and a 6 coil stator wired star.

               I'm wondering,

     In light of my pondering's on levers and fulcrums, targeting a wind range of 8 to 12 mph as well as the ease of construction, what if I instead increase the discs to 12", put all 16 magnets on one disc, have the second disc blank and use 12 coils wired star. I wonder how the 2 designs would compare as they would use the same size and amount of magnets, I estimate the amount of wire used would be similar and only the size of the discs increases. Construction would be simpler since I only have to pot 1 magnet rotor.

     Well, thanks for reading and I hope you find it interesting enough to comment on.

     Andy

[Flux]

Looking back at your posts you seem to have asked a lot of questions but given little details of what you have done.

" 8" disc, 12 of ED's wedge magnets and a second blank disc to return flux. Stator, 9 coils, 120 turns #22."

This is the most detail I can find. there is no indication of stator thickness or air gap so I can't do much. Don't 12 of those magnets touch on an 8" disc? Wire seems to be the big issue you have used something very thin and although you may cut in ok you may be short of enough speed to overcome the serious resistance.

If you can give the air gap I may be able to make some sort of guess but it will be difficult with those magnets. If you have cut in speed it would help a lot.

You are either not reaching cut in speed with the larger blades or you are hitting stall and at present |I really have no idea where you are.

The 6ft prop should out perform the smaller one by quite a lot but only if it is matched right.

I wouldn't get too excited about thinking of levers, that is not going to help.

You raise an interesting question that I shall have to think about. I have never considered the effect of doubling the number of magnets on one rotor and adding a blank disc compared with a dual rotor. It leads to a thinner and less stable stator and you waste space for mechanical clearance with the smaller air gap. The leakage flux is higher with the blank disc.

You also have half the winding space per coil but have twice as many coils so it will take a bit of thought. Instinct tells me that you get more out but only if you are very good mechanically and can maintain small gaps between rotor and stator safely.

Flux

[scottsAI]

TheCasualTraveler,

You cover a lot of ground in your questions.

Wind generators Blades run at half speed for proper loading, thus when unloaded they spin faster.

Shorter blades spin faster. Higher TSR blades will spin blades faster.

More blades on a turbine will slow down the RPM. 5 blades spin slower than 3 which is slower than 2 which is slower than 1:-) (yep somebody tested (verified) this out couple years ago. Unloaded blades are not making use of the wind, may as will be shorter.

PMA output voltage is proportional to RPM. Once at cut in, higher winds can't spin blades linearly faster. Ex: Cut in is 6MPH at 100RPM, at 12MPH the blades will NOT be spinning at 200RPM. If it does spin at 200RPM then the wind is overpowering the generator, (Assuming 12v battery) which is now outputting 24v, this voltage mismatch has to go some where. The voltage is dissipated in series resistance as heat. This explains why some have discovered with higher winds they can use series resistance for heating without loosing much going into the battery! If no external resistance then the windings in generator are the weak link and fry.

Will stop here, to go further will become long, weather is too nice outside!

Also like Flux said more details are needed.

Have fun,

Scott.

[TheCasualTraveler]

Sorry about missing the stator info.

     First, on my current mill the magnets do not touch, they are however close leaving about a 1/4 inch space between them. The gap is uniform due to their wedge shape.

     The coils were also wound wedge shaped, in fact they are triangular with a somewhat rounded bottom. A cross section of the coil would show the area taken up by wire to be about 1/4" x 1/4". The stator thickness is then about 5/16 to 3/8 for a total air gap of approx. 1/2" to 9/16"

     I suppose my questions are a bit amateurish, however I can't help but wonder about the blades. The 6' carved prop I put on the alt was not intended for it, I only tried it out of curiosity. It seems to me that if by decreasing diameter of the blades you increase speed but at the cost of losing torque, and also that adding more blades will increase torque but at the loss of speed then somewhere in there I think there could be a compromise since, if I remember correctly, more blades effect speed mostly at higher rpms and actually help startup and cut in at lower speeds. Thats why I made mention that the wind speed I am targeting is in the 8 to 12 mph range and that the blades would seldom run at higher rpm's.

     As for the alt with 16 magnets on one side there would of course be less flux density around the coils but more coils and also the magnets and coils would be on a larger diameter which would mean more speed for the magnets.

     Thanks for the comments, looking forward to anything else you might add.

Andy

     

[Flux]

Starting to make sense now.

You have taken a magnet layout that is bad but more suitable to an iron cored layout with slots and tried to adapt it to an air gap design with single layer winding.

You are using incredibly little of the magnets usefully. In fact with the air gap much wider than the magnetic length ( magnet thickness) little of the flux will be returning via the blank disc.

I can't even make a reasonable guess at your useful flux per pole. With the large number of turns your cut in is likely to be reasonably low and probably lower than ideal for even the 6ft prop, but the small winding area has forced you to wind it with wet string and the resistance is going to be incredibly high, probably over 10 ohms.

The thing is going to have to reach something probably in the region of 1200 rpm to produce about 120W and that is way above the speed capability of a 6ft tsr6 prop in less than a gale.

The end result should be that you produce power on virtually all days even in very light winds but you never get anything reasonable as the wind picks up. The smaller prop may be able to run faster so does better and the thing never gets near stalling because of the resistance. On any but the lightest wind days an even smaller and faster prop would do better. You just can't load even the torque of a small blade so the big ones have no chance.

Just forget that alternator, consider it a learning experience and don't combine ideas of air gap and iron cored alternators.

Don't use thin magnets especially if you want to use a single magnet rotor with flux return disc.

I will come back to you about the dual rotor versus single rotor and blank disc with twice as many magnets.

Flux

[Flux]

I have only looked at this very quickly but it does look as though you get a better alternator with all magnets on a large disc and none on the other. There are mechanical issues to bear in mind such as the need for thicker discs for stability, problems of keeping them running true working with thinner stators without warping and also not wasting too much space in mechanical clearance between discs and stator but taking all these into account it look worth a serious thought.

Don't now go and make all these changes to your alternator for the blades you have without coming back. If you use all the magnets this way you may overpower the blades.

Flux

[TheCasualTraveler]

Thanks a bunch Flux,

     I will continue to use the alt I have built as a test alt for playing around with blades and will try smaller 3 and 5 blade props on it. It always was meant as an experiment and I have learned lots from it.

     I'm going to pursue the other idea of the alt with 16 magnets and since it is single magnet rotor it will be easy to do test coils and such with before committing to  the finished design. I'll come back to check my numbers before I pour a stator since I will need to get the # of turns, stator thickness, air gap, and blade diameter all in sync.

     In your dept once again, Thanks,