A Scheme to Reduce Cogging on Some Mills

[Lurker 417]

I have an idea that might reduce the problem of "cogging" as it applies to low speed operation of a wind generator, by making startup easier in light winds. I have been meaning to post it here, but I haven't even been able to figure out how to post a photo, let alone draw and post a diagram that would illustrate my idea, so I am going to try to describe it with words alone. It's a really simple idea, so the thing just may be within my power to get across this way. I would not be surprised to learn that it was invented in 1912, either, but here goes.

I think the simplest example would be a single-rotor disc alternator. How about allowing the armature shaft to slip "forward and back" so as to vary the airgap? A simple spring would hold the gap at the widest position until there was a sufficient thrust load from the wind pushing on the blades. As the wind speed increased, so would the thrust loading, and of course the airgap would get smaller and increase the output of the alternator. The mechanism for this needn't be complicated or bulky. A coil spring would fit over the shaft, pushing against the inner race of one of the bearings, while a simple set collar or other sleeve arrangement could provide a positive stop at the "maximum flux" position, keeping a safe airgap no matter what the wind conditions might be. It would take some fiddling with spring tension, and some effective way to keep the sliding shaft from getting stuck, but that's minor. Of course one could dream up a system with a servo controlled by an IC device, which could be adjusted by the user, but the simplicity of a spring appeals to me.

I suspect the same idea could be used with a radial flux alternator (converted motor type), but it seems like it would be more complicated and I don't have enough of a handle on all the factors involved to make an intelligent prediction. Of course the airgap would not vary in that situation, only the alignment of the magnets with the coils. It seems that it would be very hard to implement on any machine that used a hub from a car, since those parts are of course designed to keep the car wheel on the straight and narrow. Too bad, too, 'cause those machines are way cool. Anyway, that's my idea, and if I ever move to a place where a windmill is feasible, maybe I'll build a generator and try it out myself. I live in the middle of an old part of town, on a 75 x 125 foot lot. Oh well.

What a great resource this is! I've been around here since the early days of the old board, under various names that have passwords I can never recall, and which are associated with email addresses that are long gone. I have learned a great deal, had many a good chuckle, and have been amazed by the inventiveness of this crowd. Thanks, All!

Zack Reuter

Grand Junction, Colorado

[juiced]

i dont mean to hijack your thread, but i wonder if some sort of gas could be used to manipulate the air gap's properties..?

[finnsawyer]

I've suggested this type of thing in the past, but there doesn't seem to be much interest in it.  I guess the need to machine a step on the shaft stops most people.  Or to put it another way, this would eliminate cogging at start-up, but there's not much energy at low wind speeds anyway.  One would just see the mill free wheel at those wind speeds.

[Lurker 417]

Hmmm. Could be. It would take a powerful spring just to maintain the larger airgap with the magnets trying to pull the rotor toward the laminates, but I think it would be like a car spring; at rest things are in equilibrium, and it doesn't take a great amount of pressure to move the car's suspension. The springs on my truck are pretty hefty, but the truck moves down a surprising amount when my dog climbs in. Yes, he is on a diet, but he is not huge. I have no idea what the thrust load would be on a typical homebuilt  windmill, but I am sure it is considerable. I don't have the math skills to calculate all of this, but I am sure there are several people here who have. Interesting "thought experiment," anyway.

[johnlm]

This approach sounds elegantly simple as long as you don't consider the pull of the magnets toward the stator.  The problem that I think might occur is that the attraction of the magnets in a non linear function - meaning as the magnets get closer to the stator metal (or the opposing magnets on dual rotot) the pull goes up exponentially with decreasing distance.  In general I think springs are linear devices meaning X number of lbs will compress it say 1 inch and 2X number of pounds will compress it 2 inches.  With that in mind it would be very difficult to find the point where the blade thrust on the shaft would be able to push the magnet plate back to the maximum power position without the magnet pull capturing it and not letting it slide away when the wind force decreased.  Or the other extream is if the spring was strong enough to overcome the maximum power position of the plates magnetic attraction it would take some extreamly high wind speed (thrust) to push the magnet plate into the maximum power position and as soon as that high gust of wind went away the plate would be pushed back out of the maximum power position by the spring.  In summary, I think it would be very difficult to find this equilibrium.

Just my thoughts

Johnlm

[Flux]

I agree, the magnet pull and prop thrust are pretty much square law, I doubt if you could balance these against a linear spring force.

Also the problems of a disc that can slide freely when under torque without skewing or sticking rusting up or fretting apart in a harsh environment makes it a difficult mechanical proposition. I am sure it could be made to work but how well or for how long would be a different matter. It seems to fit into the make it as difficult as possible category.

The radial equivalent would be the conical rotor and stator which has been used for specialist applications but has never commercially justified itself except in the Demag crane motor brake.

Flux

[thunderhead]

Another alternative if all you are trying to do is generate DC is to have one more (or less) coil than you have magnets.  Of course that way you don't get three-phase electricity, you get as many phases as coils - but you're going to rectify each coil anyway.

So 8 magnets and 9 coils might do what I am describing, with 9 bridge-rectifiers to produce a pretty smooth DC even for low RPM.  Instead of getting 12Hz at 60RPM you now get 36Hz.

For really low RPM - say for something like a Savonius - I guess you'll want many more magnets and coils.  In this high-torque case cogging is even more important, so the advantages are greater.

But no, there doesn't seem to be much interest - I guess because HAWT windmills only generate a reasonable amount of power when they're turning pretty fast.  So there isn't much to be gained at those slow speeds.