Messing about with slip rings

[Capt Slog]

  This is a story about three phase and slip rings and how it didn't work.

I run a small turbine which I built from scratch.  It's the usual 12 pole 9 coil, two sets of 1" round neos facing each other.  I had the rectifiers built on to the back of the mill, and brought down the DC through two slip rings.  This worked well, although the slipring/brushes made a lot of squeaking on yaw, and were exposed to the elements.

I got the idea that I could perhaps get a bit more power if brought three phase down the  tower and rectified it near the batteries.  As the sliprings I had, were three conductor, it didn't seem too much of a task to build a three-phase set-up to use them.


There used to be a pair of pinch type brushes on the top and bottom ring, but I didn't have a thrid one and didn't like the spacing very much either; a little too close.
So I brought some brushes which should fit in a washing machine, and used a piece of 4" plastic drain pie to make this.....



You're looking at it from the bottom, you can see all the brushes fully extended on their springs.  There are three brushes per phase, nicely spread out so that they can't touch and give a nice even force on the rings.



this is what the look like on the tower and over the rings.  The brushes are glued into their slots, but I put some tape around them just to make sure they wouldn't shift.




Here you can see the cover over the brushes, yes it's a paint pot, but it kept everything dry inside.  It was up for several month and was bone dry inside.


Well it all looked ok, if not very elegant, but it didn't work.  I measure the resistance of each phase, and it would vary from around 2 - 12 ohms!  Not acceptable.  I gave it a good chance to bed in, but to no avail.  I was at a loose end this last weekend and was watching the mill in very strong winds and seeing nothing coming out in terms of power.  I also realised that it was not furling (yes thanks I know why), when I thought it should.  I was getting a bit worried as to overspeed, and so in a lull in the wind I shorted the three phases at the tower base.  This slowed it down for a while, but it then went just a fast as it had before when the rings found a high resistance spot. Ouch.


So, the pictures above were taken as it all came off.  The mill is no running 3 phase with a downlead, no brushes at all.  I'm already seeing the difference on the meter at the battery end, I'm getting output in very low winds.



By the way.  The blades came from the USA, they are Raptors, which I imported.  They look nice blades and seem to work well now that they don't have to contend with poor electrical transmission (I put the raptors on at the same time as the slipring change).
http://www.ebay.co.uk/itm/wind-turbine-blades-3-Raptor-Generation-4-bladesand-hub-/290595649256?pt=LH_DefaultDomain_0&hash=item43a8d9b6e8#ht_13016wt_802

Thanks for reading.  Hopefully this will help someone else not follow this route.

[fabricator]

For every one mill that has successful slip rings there are about 100 that don't work, including commercially built mills.

[DanG]

The spring rate changes with brush position, how much brush was allowed outside the holder-guide?

There are conductive greases that might help but it appears those brushes are high-carbon and meant for high voltage... and conductive paste amplifies Murphy's Law anytime its used by 833%.

Did you know that motor brushes are designed to ride on a layer of plasma, the brush material and ring material gets ionized so is almost zero-resistance!

A brush with a high bronze content, heavy and cold to the touch, with plenty of graphite in it is where you need to go...

[Capt Slog]

Yes Dan, they would have been high voltage jobs, the were for 240v AC motors.

They are shown in the pics at their max extension, when over the sliprings they would have been almost all the way into the holders.  The holders had a catch mechanism that held the brushes in place and then I operated a release once they were over the ring.

I did look for conductive grease, but realised that it wasn't going to be worth the trouble.

[Flux]

The electrical industry has spent many years perfecting brushes for various applications but nearly all are for continuous rotation. This is a worse case than any rotating machine so your choice is even more limited.

Slip rings can work for this application and can be trouble free but a lot of brush boxes and brushes from conventional rotating machines will give trouble.

You need metal graphite brushes similar to those used in starter motors and you need satisfactory brush boxes, those with brushes mounted on arms at about 60 deg with a tension spring between them seem to work best. Those sliding in boxes with spring pressure at the back of the brush don't like reciprocating motion.

High voltage carbon or electro graphite brushes will be useless for this application, they are designed to have a resistive contact to aid commutation on motors and this is a property you can't use here. The other factor that determines success or failure is the local air conditions and you will really struggle in a hostile environment unless you keep the weather out. If you live in a desert then any suitable grade of brush will likely work but in wet or maritime climates you will struggle. There are always issues with electrical machines used in some places such as chemical plant and anywhere chlorine is present, commutators and slip rings normally develop a skin that aids conduction and low friction and if this skin doesn't form then brush wear may be very high.

Flux

[bj]

  As Flux mentioned them, why not starter brushes?  Right material, good easy to adapt mounts, and some come with good
high amp flexible leads.  Toyed with the idea about three years ago, but Ghurd pointed out the real problem, and I didn't need them.
  Didn't mean to highjack, but at least it was on topic.

[prasadbodas2000]

I feel that the axial flux machine with all its features, offers a very robust and low-maintenance option - no open conductors/wires, protected magnets, compact design, no iron stampings to rust, no field windings, so why add the complexity of a set of brushes?

all - please correct me if I am wrong, but in my opinion, a cable running down the tower through the tower pipe/structure and a quick engage/disengage terminal block to free the twist (that may develop say one turn a day) is very easy option.

[clockmanFRA]

I have to partly agree with prasadbodas.

I have 3 Hugh piggott's designs 3.7m (12footers) and in the past years every time one is down, every 6 months or so, for paint, grease or general maintenance ,i have not noticed any winding up of my 3 trirated cables that come down the inside of the tower. I un plug shove the short plug in, check for twisting and re-plug again.

And i have never seen the wind turbines achieving a full 360 degree turn over and over.

However, other owners have reported some slight twisting cables on windy turbulent sites.

[TomW]

I have to chime in here with experience using pendant cables hung from Kellum Grips on 3 turbines on a fairly turbulent site.

I get occasional twisting that I undo a couple times a year by a couple turns unwinding.  I believe the random nature of the yawing tends to even out the twists over time.

Slip rings just seem to be a complicated solution for a problem that simply does not exist. No reason not to experiment but not really a worthy direction for "improvements".

Juyst my real world experience so far.

Tom

[bj]

   Agreed.  Just so I am not misunderstood, I only meant to suggest a material for those who might, for some reason, need slip rings.
  I had fairly severe twisting, due to an out of plumb tower.  At Ghurd's suggestion, I fixed the tower, and the twisting
went away.  That's why I never went any further with the slip rings.

[SnowGhost]

If you want slip rings it may be better to buy a proper set like http://www.mercotac.com/html/products.html  Of course I have no idea how much they are.  They are also no good for very cold conditions.

[Seekscore]

I thought about slip rings but really didn't want something at the top of the tower to fail. I found a "rotary connector" that I thought about using at the bottom of the tower cable. This isn't the one I considered, but gives an idea. http://www.rmhoffman.com/distribution_products/slip_rings/mercotac.html. I found one rated at 30 Amp per connection, 6 connections that would allow for parallel redundancy. It was, if memory serves me right, rated at 1800 RPM. If it was mounted at the base of the tower, the only time it would rotate is when the cable needed it too. Also, at the base of the tower, there would be no weight on it from the cable...

Just a thought anyway...

Seekscore

[B529]

Strongly disagree with a blanket statement suggesting slip rings are not reliable. If done properly, slip rings are very reliable. I base that on my experience with my turbine and others I've installed. There certainly is a need for slip rings. With that said, a drop cord is probably a much easier/cheaper options for most home-brewers.

I'm going to be building a home brew machine for a friend, it will have this slip ring assembly.
 


 

[Dave B]

Blanket statements like that are just indications of how far a builder wants to take his project. I have nearly the exact slip ring and brush gear assembly running going on 3 years. If you want to invest in slip rings it still takes a system to make it work reliably but the hardware is the key. Make sure you have a tight yaw assembly with little slop, I highly suggest investing in a robust bearing arrangement for the yaw to hold the slip rings and brush gear in alignment throughout 360 degrees as well as handling the downward thrust load of the machine. Some type of keeper to keep the machine from raising up as well and to secure it from slipping off the end or moving in the least if you have a tilt tower and you jockey it around while working on it at ground level and especially if you tilt past horizontal. After raising it is no time to realize you botched the brushes and or shorted out the rings because the assembly slipped. Obviously a weather tight enclosure is a must also.

 Ignore the negative about slip rings, this just comes from frustration of limited resources and or finances to do things right and the simplicity of a drop cord is a good argument for not taking it to the next level. Just one (or 3) arguments for is the fact that I have an 85' tower. The weight, expense and longevity of a decent drop cord made the investment for properly installed slip rings a no brainer for my application. It's just all where you draw the line for your own set up, you can't "sort of" do slip rings without becoming another non supporter.  Dave B. 

Strongly disagree with a blanket statement suggesting slip rings are not reliable. If done properly, slip rings are very reliable. I base that on my experience with my turbine and others I've installed. There certainly is a need for slip rings. With that said, a drop cord is probably a much easier/cheaper options for most home-brewers.

I'm going to be building a home brew machine for a friend, it will have this slip ring assembly.
 


 

[Flux]

That is the type of rings and brushgear that works, if mounted on decent bearings, running true and you keep the weather out it will work. With crappy bearings and no weather protection it won't.

Some things work if you do it properly and engineer it. Few sites really need slip rings, occasionally you find a site that does. I also agree that tower height is a considerable factor when the weight of the wires becomes excessive.

It really is down to individual preference in the same way as using engineered yaw bearings over pipe on pipe. Both ways have advantages. I came  at all this from an engineering background but sometimes I see that the simple crude ways do work very well.

For those who can't or won't engineer things and won't understand the basics I would keep clear of slip rings. If you can engineer things and know what you are doing then there are advantages

Flux

[zvizdic]

On my turbulent site I experienced severe twisting so I installed slip ring at a bottom of a tower to minimise spinning and stay simple .
It is 3 rings of brass and a brass fingers spring loaded to maintain contact ,enclosed in a nylon housing with bearings (water tight) .
One year no problem, once in a wile it turns a one turn specially in a this time of a year with leafs on trees .

[Capt Slog]

Thanks for the replies.

I've been running for a few days now with the cabled system, and I have to say that everyday I have to go and sort out the wires which are wrapped around the tower.  This isn't good; I like something that I can leave alone to get on with it. 

Although TomW points out that the problem doesn't exist, it clearly does for me.  You have to realise that this mill is only about 18 feet off the ground, and the bit of pole which supports the mill is less than half of that. It doesn't take many turns of wire before the whole thing is bound up tight.  I can see that if you have a high tower you can wrap a lot of cable up before it gets to be a problem.

My 'tower' certainly looks vertical, if it's out of plumb, it's not showing, I think it's just a very turbulent site.

I used to use the brush set and rings pictured by B529 above, but I only have two of the brushes which is why I couldn't go down that route.


I never realised that there was a massive difference in the brush material, and how this would affect things.  It's obvious now of course, hindsight is always in 20/20 vision.  Despite what Flux said about the sliding design not being the best because of the motion, and I can see that,  I'm considering sticking some brass or phosphor bronze rods in there to replace the carbon.  As mentioned in the OP, it has stayed nice and dry in the brush box, it appears that what was designed to keep paint in certainly keeps water out!

[fabricator]

Your wires are outside your tower?

[Capt Slog]

Your wires are outside your tower?

Yes. 

Ahh.  I see the confusion now, I didn't realise that 'down the centre' was the norm, it's something that I've always thought would be nice to do.  When people have mentioned twists in the wire I always thought they were talking about twists around the tower.

Not easy to change this I'm afraid.  I have two sets of roller bearings in a steel tube which forms the 'tower'.  The shaft in these then goes to the mill which is clamped to it.  This shaft is a 1/2" or so steel bar.

Perhaps my best route would be to redesign/remake this section, i'll see what I can do.

[windy]

Capt Slog,

 I have the wire running down the center of the tower and I have yet to unwind the cable. My tower is 80 feet high and am using a 3 conducter, 6 gauge cord. I think I have had it running 2 years now and no problems with the cord twisting. I sometime think that if the cord does wind up ,it unwinds when the wind drops to 0 MPH.
 I used a timken bearing on the top of the yaw tube and the whole assembly will swing with very little wind. No sliprings for me!

windy

[shawn valpy]

Hi just some pictures to share of my homemade slip rings around a car hub for a yaw bering as well brushes should be good for around 200 amps.

[cdog]

I used brushes from a 6-71 starter motor (6), and home built rings, it is the only part of the mill that has been untouched in 3 years of use......