Importance of balancing a Savonius vawt?

[gotwind2]

I've been experimenting with 6" diameter P.V.C pipe, halved and set up as a basic vawt machine, depending on it's performance I hope to make a single rotor PMA at the base. This is a continuation of last weeks initial diary

http://www.fieldlines.com/story/2007/4/22/152542/564

The white disc is a 12" diameter piece of paper, just for visualization, the height is 20" and the can is my favored beer for scale

I was wondering about the importance of balancing the rotor, e.g if I mounted it horizontally. How important if at all would it be?

Other concerns, It is pretty heavy - might be a non starter, but thought it was worth tossing in to chew over.

Ben.

[thyristor]

The wind must be able to pass through where your axle is at the moment. i.e. it must be open to air flow. It should turn, but with reduced efficiency, I think!

Try two discs, one bottom and one top, each with its own bearing.

Enjoy the testing!

thyristor

[gizmo]

Hi Ben.

With a slow running drag type vertical like your sav, I dont think balance is as important as it would be with say a high speed darius or HAWT. But you do need to be aware of the torque pulses as it goes around. The force against the drive shaft/tower isn't constant, it pulses as the mill rotates, and this is why most verticals have a bearing at top and bottom.

Thyrister is spot on too, you need to have a gap between each shell for the air to pass through, where your shaft is. The gap gives a big gain in performance. You could leave the drive shaft there, but use spacers on your bolts ( longer bolts too ) to provide a space around the drive shaft.

Glenn

[Ungrounded Lightning Rod]

The wind must be able to pass through where your axle is at the moment. i.e. it must be open to air flow. It should turn, but with reduced efficiency, I think!

Yes:  In the original two-half-cylinder savonius design each half-cylinder should have one edge ending at the radial center of the other half-cylinder.

When the blades are opening-upwind and opening-downwind the concave-upwind blade reverses the jet of air that enters one side of its opening (partilly directed by the outer surface of the concave-downwind blade) and reverses its direction - jetting it out the other side of its opening and into one side of the concave-downwind blade, which reverses it again.  Momentum is transferred reasonably efficiently, and the air is decellerated, because both blades are retreating from their respective inbound jets.

If you join the two edges of the blade the concave-upwind is trying to spit the reversed jet back into the incoming air, rather than into the windshadow of the other blade.  No soap.  Instead, the back of the concave-downwind blade directs some of the air that hits it into the concave-upwind blade, which hooks it back and dumps it overboard.  Only one reversal, with lower flow to boot.  You also get a bit of torque from the lower drag of the convex surface of one blade versus the concave surface of the other.  So much less torque than if the air had a clear s-curve flow through the center and along the inner surface of both blades.

[Flux]

It's probably true that the speed may be low enough for balance not to be a big issue.

Also the comments about the air flow through the centre of the rotor are important.

It may be worth some comments about balancing. Discs ( propellers) can be statically balanced near enough. Structures such as this and the Darrius will need to be dynamically balanced to perform satisfactorily. When the device does not approximate to a disc there is the possibility of introducing couples which may not show as a static balance but will cause vibration when running.

There is no easy way to carry out a dynamic balance without special equipment so the only advice I can give is to keep everything symmetrical and accurate. If you find that one side is heavier than the other, find out where the trouble lies, you just can't add a balance weight at random. This will give you a perfect static balance if you get it right but you will almost certainly introduce a couple and it will still vibrate badly and may even be worse.

Flux

[gotwind2]

Thanks guys, also I have realised the importance of keeping everything square from the start - a hole drilled a few degrees off 90 degrees in the steel pipe can cause a lot of hassle.

As Flux suggests dynamically balancing is tricky, so it may be a case of trial and error. I will be fitting 12" diameter plywood discs top and bottom.

I understand the importance of the 'axle spacing' now, and have added 2 extra nuts on either side as spacers.

[thefinis]

The better the balance the less vibration and the easier it is on bearings. The smaller the diameter of the vawt the more important it is due to increased rpms. With proper bearings and balance weight should not cause undo concerns. Starting is not usually a concern for the drag type turbines.

I am still trying to decide how much positive effect the end pieces cause(the circles). On some of my vawts it helps by keeping the air flow in the turbine instead of letting it spill out the ends. On the bigger models it is harder to tell how much effect it is having but this may be the because the ratio of end area to turbine area is lower.

It gets hard to remember all the forces that are acting on the turbine. My last one died due to my miscalculation of forces and their direction. I underestimated the lifting force of the lever action(height) above the bearing.

Finis

[tecker]

That's an easy way to mount the rotor sections no doubt .The problem is two fold . The center shaft has to be plumb as well. I opted to eliminate the shaft and connect the top and bottom with strips ( in my case Flat bar )this gives two distinct balance points . These two dominate the balance as the centrifugal forces come into play and leaves a wide open center .If you put a tell tail at  different points you can see air movement low speed  not much air goes through until high wind rises . I'm now adding sections to improve on the parasitic upwind dynamic .

[tecker]

I used small gage all thread to give sability that also makes an easy plumb adjustment .A top bearing is nessesary without a center shaft but stacking more rotors is the way to get useable torque .  

[Norm]

More like this:

http://www.picoturbine.com/PTDELUXE-cover.jpg

Modified works much much better, I've made one

this size, plan on making a much bigger one,

fits inside ornamental 8 ft. windmill tower,

for now.

Instead of disks use blade supports top and

bottom... bearings from front bicycle hubs.....

readily accessable and free...just ask the

neighbor kid for one from his junk bike

(you know....the one that was out in their front

yard...buried in the snow)

            ( :>) Norm.

[hiker]

nice norm..

whats the output??

hey gotwind2 looks like your on the right track..

[PeterAVT]

I wonder offhand if one could pay the local mechanic a bit to use the tire (tyre?) balancing machine, to good effect.

[Norm]

  I have my present one spins at about 120 rpm

in a 6 mph wind (breeze?) no load....that's the

problem Ghurds rubber tire stepper has some

cogging and I guess a 15mph wind would drive it

at the estimated speed to light up a 2 led lamp

I have that originally takes 4 AA batteries in

series to light so I have to make a larger VAWT.

Goal is to light up that light in a 3 mph wind.

Todays goal is to find how many ft.lbs it takes

to run that lamp and a Prony brake to use on

my present VAWT.

                 ( :>) Norm.