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Forum Index : Windmills : Axial, potted coils
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| kitestrings Senior Member  Joined: 23/04/2014 Location: United StatesPosts: 102 |
Howdy, I’ve been wanting to post on this turbine build here for a while. It’s a HA axial WT that we completed near the end of last year. It is up on a ~25’ guyed stub tower right now, as we get to know one another's patterns a bit better. We hope to have it up on a 100’ tower by the end of the summer. I’ll give a few of the specs and then describe some of things that we did, or did differently, with this one: Upwind, horizontal axis, direct-drive 3-blade, fixed pitch 15’ rotor Furling tail governor Neodymium PM, 16-pole axial-flux, ironless core alternator ~90 RPM cut-in at 58V Dual full-wave stud mount diode bridge rectifiers Charging 48V nominal battery bank, via (2) MPPT controllers RPM gage Local network connection for system monitoring and configuration Web-enabled log history Here are the main departures from some of the more common axial builds: We individually potted stator coils, or sectors using thermally conductive epoxy encapsulant 
The stator sectors are fitted using T &G interlocking joints to the neighboring sector. 
So, the magnet rotors can be assembled on the hub and gapped and then the stator inserted into the alternator. 
The coils have open cores for better cooling. 
Conceivably the stator could be field-removed, and/or the alternator lowered independent of the yaw-head assembly, tail boom and vane. The hub plate (hi-strength, A556) is also the upwind magnet rotor. 
The stator bracket assembly connects the stator to the spindle using a keyless bushing, so it too could be removed if needed. 
More to come... |
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MacGyver Guru  Joined: 12/05/2009 Location: United StatesPosts: 1329 |
kitestrings You appear to be way ahead of the pack; great planning and nice machining! Very impressive. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Very nice work, keep us posted. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Very good let us see some more Bob Foolin Around |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
We used CNC cut SS retainer rings to locate and partially secure the magnets 
The rotors are gapped using SS spacers or stand-offs off the back of the hub casting instead of all-thread.. Matching shim washers allow for fine tuning the gap spacing. 
The tail-boom is aluminum. The inner boom can be adjusted in or out for furling adjustment. The tail-vane was originally SS, but we since have gone to aluminum to reduce the weight. 
The tail hinge has graphite/bronze radial bearings and a needle-roller pin thrust bearing 
The top of the yaw head assembly is extended past the hinge point to form the stop. here is the boom in the unfurled and furled positions 
more soon... |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
What a beautifull piece of machinery and craftsmanship. I hope those 2 little boys inherit your skills. Also the photography is excellant. Ok, that out of the way ,lol, I'm glued to the backshed for following stages. Kitestrings, thanks . Bruce. Bushboy |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Kitestrings Great work and its good to see someone sharing their build here, obviously you have good engineering skills and facilities to utilize, you have done a excellent job. I know all to well the amount of work that would have gone into this build and what appears simple can sometimes take the longest to design. I have built quite a few Axials to date with varying forms of controls to limit the output in strong winds to stop the problems they face with cooking the stator . Last AX Build It will be interesting to see the blades you are going to use and any forms of control methods , I would highly recommend fitting a manual furl to keep everything in control in howling wind , I pretty well fit it as standard to all my designs now. Once again.... Great work and keep the info and pics coming PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
Thanks for the kind words, but I should clarify that I'm neither the machinist nor the welder for this project. The mold was machined by a friend/associate at a shop nearby me. We held him to pretty tight specs particularly on the T&G fit. He did an excellent job. I did the design work for this project with some help from my son. He did all the CAD/(Vectorworks?) drawings except for a few hand drawn bits & pieces. I cut the logs, carved the blades, wound the coils, potted the coils fabricated much of the metal pieces, and did all the electrical, assembly & installation, paint-ready prep and such. With only a few exceptions, the painting was done just down the hill by a neighbor & friend who does it for a living. Bruce_DU, those are two of our 4-boys. All full of independent. PillM, I followed the story you posted closely. Very nice work. I thoroughly enjoyed seeing your progress. It is a bit like time-lapse photography. Our project was the better part of 2 1/2 years. Here is the mold set-up. Breifly, I sprayed the mold with a couple coats of releaser, inserted the coil and bushing, located the terminals in the top of the mold, covered and heated (warmed) all parts. Then I applied a vacuum to the bottom of the mold, while injecting the epoxy into the opposite end. Inverted, topped off, then air cured 24- hours and post cured in an oven. 
Here are the blades. They were cut from solid blanks of local eastern balsam fir. 
Our paint rack 
Hopefully I'm not exceeding limits on photos. More soon... |
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| nshea Newbie  Joined: 14/03/2014 Location: United KingdomPosts: 10 |
This looks excellent; I particularly like the modular stator design, which allows the replacement of burnt out coils without having to recast the entire stator. And clearly you have anticipated any cooling problems with thermally conductive epoxy and hollow coil centers to allow air flow. Smart. Nicholas |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
Thanks Nicholas. Yes, part of the logic was to potentially allow for work on the machine without having to take the whole unit down, say a bearing is to be replaced. With the traditional build you pretty much have to jack apart the alternator, so everything must come to the ground. It doesn't appear that heat in the stator is as big a factor with MPPT, but it makes sense to do what we can to dissipate heat. Here's the tail-boom and vane. Mary, my wife kindly lent some ballast before we got the base welded to the test stand. The boom has three positions. 
This is the stub tower. There is are matching flanges on the tower and head assembly. We used a metal/graphite thrust washer between them. 
Here's the yaw-head. There are no slip-rings on this machine. The strain relief holds an SO cord down the center. Beside this is a pivoting pulley head for manual furling. 
We put it up with a gin pole. I don't know as I've ever had one that I could reach with a ladder. 
back soon to finish... |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
Here's the manual furling winch (vis-à-vis PhillM's preference above). This is really just for manual shut-down - maintenance, icing, forecasted high-wind events. 
Here's the completed machine except for a weather shroud that I'd planned for the alternator and a spinner/nose cone 
You can see the dyneema line that actuates the furling in the next couple of photos. This line runs down the center, then transition to the winch cable at a "sash weight" which just keeps the line taught. 
I'll try to post a bit on the electrical/electronics next... |
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| AB Hammer Newbie Joined: 06/05/2014 Location: United StatesPosts: 3 |
kitestrings Very nice build. I hope it brings you a lot of electricity to your needs. Thanks for sharing. I build wheels in attempts to get energy from gravity. They say it is impossible but after one nearly took my thumb off I started to believe. Alan |
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| nshea Newbie Joined: 14/03/2014 Location: United KingdomPosts: 10 |
I see. I am not familiar with Maximum Power Point Tracking, but read a little about it here: http://www.solar-electric.com/mppt-solar-charge-controllers. html With regard to coil burn out, I was thinking of some unforeseen failure in the furling mechanism, controller electronics, or mechanical strain due to yaw. One such example is shown at the bottom of this page: http://www.scoraigwind.com/nirvana/page2.htm In this instance, your modular stator design would have saved a good deal of money. I love Hugh Piggott's website. It is innovators like Hugh who pave the way for others. I haven't visited in a long while and his site seems to have grown exponentially. So much valuable information - there and here! Thanks for sharing your expert build and all the great photos. I look forward to any details regarding the MPPT and how you set it up. Nicholas |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
Nicholas, The goal of MPPT for wind is probably what's common to both, but how it is accomplished is very different. With solar, the CC can periodically sweep the open circuit voltage of the array and attempt to best match the voltage & current relationship to get the most power. Wind simply changes too quickly for this approach. With MPPT for wind, you basically program into the controller a power curve for the turbine (and it's user adjustable). The CC continually tries to match the available power to the best spot on this curve. As with solar it allows the voltage to rise much higher, and then bucks this down thru a converter to the bank. The challenge is one of load matching. Ideally you'd hold a constant TSR. The blades of a turbine want to rise directly wind speed. With a direct-tied machine you are trying to match up a cube-curve (ideal power curve) to a straight line alternator output, basically have to compromise somewhere. With MPPT you get the speed rise by letting the input voltage to climb. This improves the prop efficiency. This is where the biggest advantage is made. The controller varies the relationship of voltage to current which improves the alternator efficiency - and reduces heat there - and you reduce losses both in the alternator and the transmission to the house by lowering the amperage. You can also cut-in a bit earlier because there's usually a bit of "headroom" above the nominal voltage of the batteries to the first step of the curve in the controller. More modest gain here, but you're at least not starting with a prop that is miserably stalled in light winds. Hope this helps, ~kitestrings |
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| kitestrings Senior Member Joined: 23/04/2014 Location: United StatesPosts: 102 |
Here's the rectifier box that I built. It has two isolated stud-mount diode bridges rectifiers. I built the box so that the front pair of heat sinks hinge open to the sides to allow access. BTW, I don't live in CA, US, this is just the box shell that I started with. 
On the front cover is an RPM gage (0-5V = 0-500 rpm). The small green LED begins to glow when there is anything over 5-10 rpm, and glows brighter as the rpms increase. This enclosure also houses the breakers, hi-limit and dump relays and a hi-limit voltage sensing relay that works independent of the controllers. 
The rectifiers feed into two Midnite Solar Classic MPPT controllers. 
Each of the controllers can handle about 2.6 kW. The first one springs to life when the input voltage hits cut-in (~58-59V), the second one wakes up at about 80-85V, but starts to handle any excess at about 104V. 
This is the "local app" that allows you to see instantaneous output and make configuration changes. There's also a web hosting site that lets you view log data. 
I'm building the resistor load box for it right now. We still have to move it up on the permanent (guyed) tower later this summer. I have to first complete a weather shroud for the alternator and fit-up a proper spinner (to make a truly happy turbine  ). That's pretty much our project to date.
Regards, ~ks |
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| nshea Newbie Joined: 14/03/2014 Location: United KingdomPosts: 10 |
Thank you Kitestrings. That was very interesting. As for alternator regulation, I am only familiar with the more approximate EMF and MMF methods which I explored as a youngster when experimenting with a pull-start stator from a lawn mower... Which reminds me, there is an old book called 'Windmills and Wind Motors' by F.E. Powell, now reprinted in "The Lost Technology Series". Mr Powell's method of maintaining his "dynamo" at the correct speed when the mill ranges too high is a simple system of belts and pulleys. This is too involved to quote at length without infringing copyright, but basically involves a weight, which, when properly adjusted, ensures that no matter how fast the drive belt runs from the mill, the drive belt to the "dynamo" remains at a constant speed. Of course, this was decades before integrated circuits and semi-conductors, and the alternator was at ground level, but one can see that Powell anticipated all the problems that concern power regulation and dissipation in alternators today. His automatic switch is ingenious. When the "dynamo" is still or running too slowly to produce current, the battery energises an electromagnet which raises a see-saw switch out of a pool of mercury. But when the speed of the "dynamo" supplies enough current to begin charging, the polarity of the electromagnet is reversed, so tilting the switch in the other direction, which then makes connection between the "dynamo" and battery, thus commencing charge. I am a newcomer to integrated circuits, so MPPT is all very whizz-bang and impressive. I have designs for a mechanical system of regulation that utilizes a rotary governor – similar to those used on old steam engines. As the speed of the alternator increases, two metal weights (spheres) fly upwards and outwards due to centripetal force. This action raises a carbon brush on the shaft which connects to ring commutators - each commutator selecting a different star/delta configuration for the speed, thus keeping voltage and current within given bounds for given conditions... Although inexperienced compared to most of the people here, with regard to alternators in general I have found some real old gems on the internet archive which are more extensive and thorough in their treatment than any modern textbook of today. The four I refer to most often are: Alternating-Current Machines, (being the second volume of dynamo electric machinery) Its Construction, Design, and Operation by Samuel Sheldon, A.M., PH.D., D.SC. Dynamos, Motors, Alternators, and Rotary Converters by Gisbert Kapp, (Translated from the third German edition by Harold H. Simmons, A.M.I.E.E). Armature Winding and Motor Repair by Daniel H. Braymer, A.B., E.E. The Design of A Polyphase Alternator (a thesis) by Millard Gilmore. I mention these here only because I consider them vital to anyone building a generator from scratch or changing a motor into an alternator. Having only just joined, I will be retiring from this forum for several months whilst I try to save more funds and complete my pedal generator. I'm currently waiting for my N52 magnets to clear customs. I got them from CMS Magnetics, Inc in the USA. http://www.magnet4sale.com/ I’m not affiliated with them in any way – it's just that they have an excellent website that’s definitely worth checking out. Good bye for now, Nicholas |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Kitestrings, My congratulations on a truly excellent build. I am in awe of the standard of the design and construction effort that has gone in to your project. I would like to offer some comments on maximum power point tracking (mppt) of wind turbines, which is a topic that I have been following for some years. The maximum power point at any wind speed occurs at the optimum tip speed ratio (tsr), or ratio between the wind speed and the speed of the turbine blade tip. The optimum tsr is a function of the blade geometry, and, for a 3 bladed horizontal axis turbine, will typically fall in a band somewhere between 4 and 8. If you have a look at Les Bradbury’s excellent Wind Power Program site http://www.wind-power-program.com/ , it will show that the output of a typical wind turbine initially increases with increasing wind speed, then effectively flat-lines (or even reduces) with further wind speed increases. I believe that somewhere just before the maximum power point is the point at which the turbine’s cubic power curve crosses the alternator’s power curve. Before the cross-over point the alternator’s load capability can dominate and cause the turbine to run at speed lower than the speed at which its optimum tip speed ratio can be achieved (ie its maximum power point). As you note, if the alternator’s output voltage can be permitted to increase then the turbine can speed up to its optimum tip speed ratio (and hence its maximum power point). To be able to extract additional energy from the higher alternator voltage, some sort of switchmode converter is required to transform the alternator voltage down to the load voltage in a reasonably efficient manner. At the cross-over point, the turbine is likely running at its maximum power point (ie optimum tip speed ratio) for that wind speed, and permitting the alternator voltage to increase here would not result in additional power being harvested. Beyond the cross-over point, the turbine can produce more power than the alternator can harvest, and so it will run over-speed, at above its optimum tip speed ratio. While the alternator power output may be flat-lined, or even reducing in this region, the efficiency (which is the power extracted compared with the available wind power), falls dramatically as the wind speed increases. So, from this, I believe that the potential for mppt lies only in the lower wind speed region below the maximum output point. Happily, this is where the wind turbine will mostly be operating. As you rightly indicate, to extract the maximum power from a turbine in the lower wind range, you need to try and maintain a constant tip speed ratio. One method, although somewhat difficult to implement physically, is to measure the incoming wind speed somewhere around one or two turbine diameters ahead of the turbine, and at the height of the turbine axis. Knowing the incoming wind speed, it is a relatively simple matter to set the control system to permit the turbine to speed up or slow down to maintain a constant tip speed ratio. The other method of tracking the maximum power point is to set the controller to be continuously making small up or down changes to the alternator voltage set-point. Having made a change, the controller then looks to see if the power output increases or decreases. If it increases, then the controller would make another change in the same direction, while, if it decreases, it will make the next change in the opposite direction. This is known as the “perturb and observe” technique and, while it can be effective, it is inferior to the control achievable when the incoming wind speed is measured. Concerning operation above the cross-over point, as noted above, the alternator can slow, but not limit, the turbine speed . I believe this to be true also for electric braking by shorting the stator output, unless a lucky lull in the wind lets the speed drop below the cross-over point. There is really no alternative to furling or an effective mechanical brake if extreme wind speeds are experienced, unless the turbine stops itself by shedding blades, etc. Anyhow, congratulations again on a truly excellent effort, and we look forward to further posts from you on this project. Regards Don B |
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