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Rusty Magnets
By Shadow, Section Wind Posted on Mon Sep 04, 2006 at 04:01:30 PM MST
Rust after 7 months

I decided to lower my 14' Wind turbine I built last winter to check out a few things after being up for about 7 months.I have never been satisfied with the 14 inch discs that were on it as I think they are causing balance issues. So I had new discs made and upgraded from 4 bolt hub to 5 bolt.The fibreglass resin was still good, everything nice and secure. But after chipping the old resin off I was surprised to find so much rust already. I had clear coated and painted everything after putting the resin on, so after the resin shrinks over time it lets moisture in all around the magnets.So perhaps covering the magnets completely is the only way to go, providing the ends are totally enclosed too.

Rusty Magnets | 16 comments (16 topical)

Re: Rusty Magnets (3.00 / 0) (#1)
by Wulff on Mon Sep 04, 2006 at 12:42:51 PM MST

I agree, I was planning on encasing my magnet/rotors in resin with a thin coat across the tops.
Just wondering, does it get pretty cold where you live? Here in the AZ desert we have mild winters and very little rust..
Thanks for sharing.

Re: Rusty Magnets (3.00 / 0) (#2)
by wdyasq on Mon Sep 04, 2006 at 03:24:51 PM MST

Polyester resin is not waterproof, it shrinks and does not bond to steel. You are getting results I would expect to see except in all but dry climates. Some of your problems are materials dependent and not climatic problems.

Covering the magnets completly with the proper resin and using a thin layer of glass cloth should work. Proper materials and proper techiniques are necessary for good results. Some conditions allow sloppy, stupid and ignorant material or processes to last. Other conditions try man, nature and workmanship to the hilt.

Ron
Adventure is just bad planning." -- Roald Amundsen

Re: Rusty Magnets (3.00 / 0) (#3)
by vawtman on Mon Sep 04, 2006 at 03:57:02 PM MST

Ron i always wondered about this being a problem with the rotors in cold climates.
The steel expands and contracts but the epoxies and fiberglass dont.Thinkin about building the axial.
Whats your so called proper resin that will flex and keep a tight seal to protect the rust happy steel.

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#10)
by wdyasq on Tue Sep 05, 2006 at 09:57:28 PM MST

I am a big fan of epoxies. But - that axial is "Your cow". Don't let a well reasoned, researched, proven process get in the way of ignorance and stupidity.

From a website I found one time:

Polyester resin is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC's. Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive. It is also not tough and fractures easily. Polyesters tend to end up with micro-cracks and are tough to re-bond and suffer from osmotic blistering when untreated by an epoxy resin barrier to water. This is really cheap stuff.
---

I suspect the problem is the type and formualtion of resin and not heat, cold or dead of night causing the problems. I also suspect a good quanity of glass fiber in any resin would help although I don't recommend ruining good glass cloth, mat or fiber with polyester resin. Then again, I do not have control of others actions.

Epoxy has less shrinkage than vinyl ester resin and vinyl ester less than poly-puke-ester resin. Vinyl ester bonds much better than polyester and epoxy is about 2 quantam leaps in bonding above Vinyl ester. With proper technique the right epoxy will bond to steel, cast iron, aluminum, copper, stainless and bronze. If one really needs to bond they can go to polysulfides or even more exotic goos(techinical term here).

There is a lot of room for improvement in the materials and design now used in axial wind turbines (except for the wooden blades in material only). There are also many research papers available dealing with the problems. One just need to dedicate the time to find and evaluate them. One must also know what is used now, its' strengths, problems and if the improvement really is an improvement or just a change. At some time, all of that has to stop and machines must be built be built before electricity can be produced.

Dorthy Parker said it, "You can lead a whore to culture but you can't make her think."

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#11)
by Flux on Wed Sep 06, 2006 at 01:15:36 AM MST

Ron I agree about polyester, it is reasonable for jobs that are made from glass fibre in its own right and where adhesion to other things is not required.

It also works fairly well for stators considering the extreme temperature some are expecting it to stand. I have not had the slightest trouble with stators but as others have found, even if you cover the magnets completely the resin parts company from the steel plate and rust starts.

You may be right about epoxy but again the snag is finding the right ones,not all epoxies are good for this.

I have recently had to repair an epoxy potted bell hammer coil and expected major difficulty removing it from the cast iron housing, in fact it was brittle and was stuck little better than polyester and presented little trouble so obviously this type of potting epoxy would be useless.

I have used West system with great success on wood but I have only used it to stick windings on to an iron core, I have not used it in bulk as for a magnet rotor. It may be good for this.

It is definitely not capable of standing the temperatures of stators. Even 3M Scotch cast and similar electrical potting resins are only good for class B temperature rise.

They do stick extremely well but the cost would make one think twice about casting magnet rotors with it.

I think that gradually people will try different things and find what works and what doesn't. Data sheets don't always give a good idea, every product is perfect when you read the data sheet. If you use the material for a standard application this is no doubt true but when you use it for something odd then not always so.

For magnets, sticking them on is no problem, weather protection needs thought if you are in a damp climate.

For stators, if you want to push them to class H then vinyl ester is likely to be the best material, but I suspect that it's true rating is about class F.
Flux

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#13)
by wdyasq on Wed Sep 06, 2006 at 05:53:53 AM MST

I concentrated on the rotors and my comments on epoxy are directed mainly at the rotors. You are correct in suggesting Vinylester is probably the better choice for the hoembuilt axial stator.

One of the other other problems is the incorrect use of language, not all the fault of the posters here. Polyester resin is just polyester resin. If one adds glass fiber if may become 'Glass Reinforced Plastic' as the Brittish are known to call it GRP. But any glass reinforced plastic seems to get the same moniker.

We should all remember to look for the negatives in a product as well as the positives. Data sheets seldom ballyhoo the problems a product has. One has to rely on secondary knowledge gained from those who have experienced the problems. One doesn't know if things found on the internet are born of the truth, ignorance or a bad hair day.

Thanks for the clarifications,

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#4)
by tecker on Mon Sep 04, 2006 at 06:57:16 PM MST

I feel certain the end design will incorporate mounts for the magnets and bonding elements to a resin incapsulated rotor. I've used glass to mount the mags ( L shaped around the mags ) All rotors are jigged now small and large . The 14 I have going on now is getting 1/8 X 1/8" sheet metal angle on 3 sides glass around the outer edge.

Re: Rusty Magnets (3.00 / 0) (#5)
by Countryboy on Mon Sep 04, 2006 at 10:12:21 PM MST

Hmm. Anyone ever tried coating their completed rotor with that automotive rubberized undercoating in a spray can? That might be able to keep water out, despite differences in expansion/contraction of metal and resin.

Will a thin layer of rubber affect magnetic flux?

Re: Rusty Magnets (3.00 / 0) (#6)
by Shadow on Mon Sep 04, 2006 at 10:40:36 PM MST

Actually these were coated with the rubberized auto stuff in a can. I didnt coat the face of the magnet though, but it almost seems like the rubber coating shrunk along with the resin to leave a small air space around the magnets.So maybe worth a try covering everything with a rubbberized coating. So far I'm impressed with all the stator bolts and hub bolts etc, after assembly I sprayed them all with undercoat and nothing had loosened or moved.

In an unrelated topic, I'm using 3/8" steel 14 inch discs, with 3 inch long N48? magnets, Would the pull on those magnets facing each other be enough to flex the metal in the discs?Or would the magnets pull each other off long before moving any metal? I'm thinking I should have used 1/2 inch discs but had lots of 3/8 on hand, but we are getting balance issues at slower speeds and its as if our disc have a wobble. Trying a new set now and went from 4 hole to 5 hole.

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#7)
by Dave B on Tue Sep 05, 2006 at 12:17:40 AM MST

Just my thoughts that between the 3/8" disks (this is marginal) N48 magnets (very strong for 14" 3/8 disks if thin stator with tight airgap) and fiberglass resin (this is not an adhesive but looks nice) instead of quality epoxy you have too many variables against you. Your disks may be flexing (maybe only when yawing at higher rpm) and even if not the temperature changes will cause gaps in your resin, water gets in, rusts, possibly freezes etc. and degrades further from there. Enough water trapped behind rubber and resin may even cause balance issues. I have not found a reason to completely embed the magnets, clean the rotor and magnet surfaces spotless, maybe even rough them up a bit and use a thin layer of quality epoxy to "glue" them on then maybe some around the edges, finish with clear coat or paint. 2 years here in the north climate with Wests System Epoxy, no problems. Just my opinions, good luck with the rebuild. Dave B.

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#8)
by Flux on Tue Sep 05, 2006 at 12:39:13 AM MST

I am sure those magnets flex the discs, but only during assembly, it's not a cyclic load and shouldn't matter. The magnets have stayed on so it's not a big issue.

Keeping moisture out is another issue. Completely potting with epoxy or vinyl ester may be effective. If you don't want to cover the magnets, use a glue that you can trust and cover with urethane or something but get a complete film over the magnets.

Only time will tell if any of these methods are foolproof, but they should delay the problem.

Wobble on the plates will not be the cause of your vibration problems, it would have to he terrible to introduce enough couple at that radius to worry you. Check that each magnet rotor is balanced on its own, which it should be if the spacing is right and the potting is uniform. Correct any error by adding resin or sticking lumps of brass on if it is not completely potted. If completely potted you can drill holes in the potting between magnets.

If both discs are balanced the alternator will not be the problem ( you can't balance the complete alternator with both discs on without a machine to resolve the couple.)

Get the prop balanced as well as you can and make the blades track. If it vibrates at all speeds it is an issue. You will almost certainly see some vibration at a resonant speed and this may be caused by very small forces that you needn't worry about. Usually this resonance is at a low speed and once above it all runs smoothly.
Flux

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#9)
by phil b on Tue Sep 05, 2006 at 07:20:20 AM MST

I initially sealed my magnets by pouring the rotors to the top and putting a sheet of fiberglass. After several months, I noticed a hairline crack all the way around between the fiberglass and metal rotor.

After a year, the magnets and rotors rust was visible on the outside of the rotor. The nickle coating on the magnets also loosened and allowed the sintered portion on some magnets to rust.

After a good cleaning, I used a plastic epoxy (Home Depot)to remount and seal the magnets. I painted and balanced the rotors. So far, I've found no more rust and the mill runs much cooler.

Re: Rusty Magnets (3.00 / 0) (#12)
by Hans on Wed Sep 06, 2006 at 05:33:38 AM MST

http://www.huntsman.com/
This site has a wealt of information on epoxy. They produce this stuff, and are not repackers.

Hans

Re: Rusty Magnets (3.00 / 0) (#14)
by Billy Wood on Wed Sep 06, 2006 at 07:06:46 PM MST

I use a few coats of Scotchcoat to keep corrosion out...

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#15)
by Kevin L on Thu Sep 07, 2006 at 11:15:31 AM MST

I have never used this for this application, but it would be interesting to see how well automotive glass urathane works. It is very durable. It resist water, as that is its purpose, bonds extremly well to steel with primer, and is near impossible to remove after it cures. It's messy to work with, rubber gloves are a requirement. Anyway, just my 2 cents.

http://www.ppgautoglass.com/catalog/partspic/essex/expressesx.pdf

Could probably pick up a couple tubes at auto glass repair shop.

Kevin L

[ Parent ]

Re: Rusty Magnets (3.00 / 0) (#16)
by DanB on Fri Sep 08, 2006 at 07:53:00 PM MST

I don't believe it's your magnets rusting - I think its rust from the steel rotors 'staining' the nickel surface of your magnets. I've seen the same thing on my 17' machine before we re-poured the magnet rotors with West System epoxy.

Rusty Magnets | 16 comments (16 topical)

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