Wear and tear after almost 30 years

[windy]

After almost 30 years I finally need to rebuild the stator and replace the magnets on the outer rotor. Some of the magnets cracked on the rotor and rubbed against the stator, cutting into the windings. It is a 16 foot diameter windmill built using Dan Bartman's book, Homebrew Wind Power. Attached are three pictures of the inner rotor, the stator and the outer rotor.


As the picture shows on the outer rotor, the upper magnet broke and jumped to the next magnet, causing the stator damage. After cleaning the loose coating and rust from the other magnets, I noticed that six other magnets have hairline cracks in them. So I plan on replacing all the magnets on the outer rotor. The magnet show swelling from the rain and snow.

The stator will also need to be rebuilt.

One interesting thing I noticed on the inner rotor is that there is no indications of the coating breakdown or rusting of the magnets, just some paint that flaked off over the years. The magnets were covered with grease and oil from the inner bearing that leaked out of the bearing seal. I am thinking that is what protected the magnet from rusting. I greased the bearing ever year. I plan on spraying some type of spray lubricant on the outer rotor magnets to see if that keeps the magnets from rusting. That will be part of my annual maintenance.

Another question I have is do I need to use the same N rating on both rotors. I presumed that I used N42 magnets as that what was used in the book to build the rotors. If I could find N50 or N52 magnets would that work? I use the mill for direct ac heating so cut in voltage isn't an issue. The magnets are 3 inches long x 1.5 inches wide x 3/4 inch thick. I haven't found any place to order this size. Can be special ordered, but I hate to see the cost. Anyone know of any other companies that I could look at?

I am almost 70 years old and if I can get another 5 or 10 years out of it before it breaks down again, I'll be fine with that. The blades are starting to show some wear but I hope they hold up for some more years. The gusting winds are steadily increasing ever year in this part of the country.

I never thought that it would hold up as good as it did. This is the first time I had any kind of failure. People comment that it is always turning and now they wonder what happened to it.

windy

[Adriaan Kragten]

Your magnets are very big, have a central hole and have inch sizes. I doubt if those magntes can be found somewhere these days. I mostly use magnets of the Polish company Enes Magnets but they supply only metric sizes. I have looked at their website and the magnet closest to the one you use has dimensions 80 * 40 * 15 mm. The quality is N38 and the price including VAT is € 17.93 if you order at least 8 magnets. So replacement of the 16 magnets on one disk is rather expensive. On the website of another company Supermagnete there is a page at which you can find the remanence as a function of the quality. It is about 1.24 T for N38, 1.31 T for N42 and 1.43 T for N50 and 1.45 T for N52. The higher the quality the higher the price but mostly the price increases stronger than the remanence.

The remanence is the flux density if there is no air gap. The real flux density in the air gap depends on the thickness of the magnets and the thickness of the air gaps. The magnetic resistance of the magnet is about the same as for air. You can calculate the flux density in the air gap using the same formula as used for the Ohmic resistance. So the flux density is half the remanence if the total air gap is the same as the total magnet thickness in a magnetic loop.

It might be possible to use the original magnets. First remove all loose parts and then cover everything with some layers of epoxy paint as epoxy is very water tight. The fact that the stator has touched the rotor may also be caused by increased clearance of the bearings.

[MagnetJuice]

Hi windy,

It looks like the stator got scratched by the piece of the magnet that broke.

If it didn't penetrate the copper in the coil, the stator might be OK. The worst areas that I can see are the areas that I highlighted in this picture.



Do a resistance and continuity test on the 3-phases and see what you get. If the resistance is good and you want to use the same stator, clean it good and give it a good coat of epoxy with a brush.

I have bought thousands of magnets from this place.
https://www.magnet4sale.com/



These magnets have 2 holes and yours have 1, no problem with that.

If your present magnets are N42, replace them with the same strength. If you go with N45 or higher, your cut-in speed will be lower, and the blades might stall unless you increase the air gap a bit.

I think that the reason those magnets cracked and broke is because they were not firmly attached to the steel plate or maybe the bolts were overtight.

Some people use superglue to attach the magnets to the steel plate, bad idea. They should be attached with epoxy adhesive and then the entire face of the magnet rotor and magnets should be coated with epoxy with a brush for protection.

Also, a thick coating of epoxy should be placed in between the magnets, that helps to lock them in place.

Ed

[DanG]

It'd be interesting to see behind the outer rotors cracked magnets - was there a a void that liquid water could've wicked into and set up a freeze/thaw heave that eventually cracked the neo block, did the natural blade disc always settle in the same spot so the cracked neos were the ones most exposed to sun baking on bright windless days so the fasteners did not expand as much as the neo block composition... and on and on - anyhow that is about the most handsome kit I've seen in a long time  : )

[MattM]

I wonder if the resonance of the stator played any part.  I know metal can fatigue in strange ways under resonance generated by wind alone, hence the specifations for mounting exterior sheet products tend to be spaced close enough to prevent this effect even when it more than meets holding strength of fasteners.  When guys say stupid things like 30 inches apart are fine you already know who you are dealing with.  Regardless of the source its quite a return on your investment!

[kitestrings]

Well, first off congratulations on a 30-year run.  That's a considerable feat in and of itself.

I don't know if tariffs have affected availability, or cost, but I was going to suggest the the same source as MagnetJuice.

[mbouwer]

@Windy,

In 2011 you had these Clarc-Y profile blades. Polyester covering was the plan.
Are these the blades you still have running?

[windy]

Thanks for all the comments,

I tried to remove the broken magnets, but after tapping on the rotor I seen that a few more magnets had cracks in them. 8 magnets I could see that had hairline cracks in them so I made the decision to replace all 16 magnets on the outer rotor. Thanks MagnetJuice for the link. There was no rusting under the magnets, so they didn't come loose or move so I can't see why they cracked. Could it be that the rotor plate flexed enough that it cracked the coating on the magnets, then the moisture got into them and made them crack? They all cracked outward from the center hole. I still find it odd that the inner rotor magnets shows no sign of magnet failure but that rotor is attached  to the wheel hub so it may not flex like the outer rotor.

As far as the stator damage ,I checked between all phases and the resistance was .8 ohms between each phase. It wasn't shorted or open but the winding are almost cut half way through, so I am going to replace the stator, also.

mbouwer,

Yes, those are the blades I am still running. They were coated with two layers of fiberglass using epoxy and vacuum bagged. The fiberglass is starting to crack, but every year when I bring it down, I seal up any cracks with RTV silicone. I used edge tape on the leading edge, but the blade tips are starting to show signs of tip erosion.

windy

[Adriaan Kragten]

It might be that the cracks are just caused by the centre hole. I suppose that the original magnets have a tapered central hole in the same way as for the magnets with two holes as given on the previous photo. If you tighten a screw with a tapered head, the pressure will have a outwards facing cone shape. The magnets are sintered, so contain small parts which are bound together by heat and pressure. It might be that this bound is lost after being under pressure for enough long time. So if you use new magnets, you should tighten the screws only a little and trust on the glue to keep everything at place. If you calculate the stress in the glue at the maximum rotational speed, you will find that it is very low. I made stress calculations for the glue in chapter 5 of report KD 718.

[windy]

Adriaan,

Thanks for the comment.
The magnets that I will be using are the two hole type.
Will use thread locker to keep the screws from working loose and will use epoxy under the magnets  to seal and hold magnets in place.
I was always concerned that the magnets could turn on a single screw if they broke loose but that never happened. They just broke.

Still, what puzzles me is that the inner rotor magnets shows no sign of damage. I checked again using a magnifying glass and saw no cracks. All of the coatings on the inner rotor magnets are intact. The outer rotor magnets had all of the coating flaked off, even the ones that were not cracked.

windy

[Adriaan Kragten]

As you already mentioned yourself, the grease may have protected the magnets on the inner sheet. But if all magnets would have been protected by a layer of epoxy thirty years ago, the effect of some grease can't be that large if compared to the effect of epoxy. But if you didn't use epoxy but another paint like polyurotane, this type of paint has very little holes in it because the solvent has to escape during drying. Then some grease can have a more important effect.

I expect that the wind turbine rotor is connected to the front plate. This plate will be bent due to the gyroscopic moment and the direction of this moment with respect to the plate varies every revolution. This may have resulted in cracks in the paint if the paint isn't enough elastic. As you will get millions of changes in the direction of the load during thirty years, the stress in the paint will be subject to fatigue. Steel is very good for a fatigue load but paint may be very bad.

The front sheet will also be bent due to the rotor thrust but the bending moment caused by the rotor thrust isn't rotating with respect to the sheet and this bending moment will therefore give no fatigue load on the sheet and the paint.