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BUILD YOURSELF A WINDMILL GENERATOR Part 4: the nuts and bolts . . . by Glenn Littleford* In our final article of the series, we look at a couple of propeller options, the mast and further refinements of the alternator. This set of tim ber blades w ere carved by Dennis La th and they are am. Length is 1150mm performing w ell on the F&P windmil l. reduce visual They’re painted blue to impact. siliconchip.com.au March 2005  83 T he propeller is the engine of the windmill, taking the power of the wind and converting it into rotary force to drive the alternator. It gets its power from the wind by effectively changing the wind direction and slowing the wind down as it passes through the propeller. The air behind the windmill has lost most of its forward direction and is instead “swirling” in a spiral, until it regains its forward direction some distance downstream. There is a lot of science and maths involved in this process and I’ll only touch on the basics here – you could write a book on the subject and still not cover everything. Fig.2: a blade can be thought of as a series of “stations”. Note the twist in this blade. The blades Each blade has a flat or concave front surface and a curved rear surface. As the wind passes over the blade it provides Lift, driving the blade forward. Our blade has an angle of attack, calculated to provide the most lift without stalling and is usually around 5-10°. So if we know the blade speed, angle of attack and wind speed we can calculate the best overall angle for our blade to provide the best lift, as well as the chord, or width, of the blade (see Fig.1). We also need to allow for the fact that the blades tips are travelling much faster than the blade root (the point closest to the center), so the tips must have a different angle with respect to the blade root. We call this the twist of the blade. The blade angles are calculated at set points along the blade, called “sta- The propeller blades need to extract as much energy from the wind as possible and provide as must rotational speed as possible. Propellers used in power generation are designed to rotate faster than the wind speed. This is called the TSR, or Tip Speed Ratio. A propeller with a TSR of five means the tips of the propeller are travelling at five times the wind speed, so if the wind speed is 25km/h, the tips are travelling at 125km/h. A good TSR for power generation is between four and seven. A TSR of over eight is achievable but at these speeds the tip velocity is so high that blade wear and noise become a serious problem. Modern blades are designed like aerofoils and need to factor in angle of attack, lift, drag and stalling. Fig.1: modern windmill blades are shaped like the wings of an aircraft and use the same principle of operation. 84  Silicon Chip tions” (see Fig 2). On a 1m long blade you might have 10 stations at 100mm intervals so we need to calculate the angle for each station. Fortunately there are free calculators available on the internet that do all the maths for us – we just type in the basic figures and the calculator will give us the best angles and chord widths for each station. Making it with wood The windmill kit described last month includes an adapter plate to suit a set of timber blades, plus an adapter to allow you to fit commercially available blades. The timber blade adapter was designed to suit blades carved from 140mm wide by 45mm thick timber planks, a common size in treated pine. Why use timber? Carved timber blades can offer excellent performance, as we can achieve a near perfect blade profile and have good strength ( trees are very good at bending in the wind without breaking ). But there is a catch: carving timber blades is a very time consuming process, and you need patience and wood working skills to produce a good set of blades (The first blade is easiest, it’s getting the other two exactly the same as the first that’s hard). If you have the time then I would recommend making a set of timber blades, as their performance is exceptional. But if you are like me and couldn’t cut a straight line if you life depended on it, then factory-made blades are another option. You can purchase high quality extruded plastic blades for about $35 each. The adapter in the siliconchip.com.au of PVC is a bit of an unknown. UV light will weaken PVC and it could shatter, sending sharp splinters in all directions, so a safe operating life of two years or less is expected. Currently experiments are been carried out by windmill hobbyists in UV-protective paints and blade mounting, so time will tell if PVC is a viable alternative to timber as a material for home-made blades. Balancing PVC is another windmill blade option, but the long term reliability is yet unknown and there is the fear of them shattering under load. windmill kit will allow you to fit three or six blades, depending on your own needs and location – six blades for low wind areas and three for high wind or costal windmill sites. At the end of the this article are a few links to websites about carving timber blades and sources for factory made blades. There has been some development in using large-diameter PVC pipe as blade material. By cutting a PVC pipe lengthways and reshaping the leading and trailing edge with a file, you can achieve a near perfect blade profile, and the process is so simple you could make a complete set of blades in a few hours. You would need some large diameter (250mm), 10mm wall thickness pipe. A concern is that the durability Once you have made your blades and mounted them on your windmill you will need to balance them. I can not stress how important balancing is. An unbalanced blade will vibrate at high speed and cause bearing failure or worse, blade breakage. At low speed an unbalanced blade will cause the windmill tower to wobble and strain guy wire supports. Balancing is best done in a windfree workshop with the windmill mounted level, as it would be on top of the mast. To check the balance of your propeller, check for a heavy blade; the blade that always wants to turn down. Add weight to the light-side blade/s until there is no noticeable heavy blade. Once done, give the blade several gentle spins and again see if there is a heavy blade. Add weight to the light sides until balance is best. Weight can be added by drilling and adding lead into the blade tips, the lead glued in place with 24-hour epoxy. To test if the weight is correct The windmill kit includes these adjustable plates, used to get the best propeller balance possible. before drilling, sticky-tape the weights to the blade tips until you have the correct balance, then permanently fit the weights. Make sure the weights are properly glued in - at high speed there will be a lot of centrifugal force on the weights and you don’t want them coming out. The windmill kit described last month includes a set of adjustable weights that are secured to the blades with the boltholes at the blade roots. The weights have slots that make fine adjustment easy. Decogging When you rotate the alternator by hand you will notice a cogging action, or a stiffness in rotation at certain points. This is caused by the interaction of the magnets and the stator Fig.3: decogging involves rounding off the stator poles to make the windmill easier to start in light winds. Far left is the standard stator with square ends, creating a stiff magnetic resistance to overcome. Rounding off the stator, as shown alongside, sacrifices some power but reduces cogging. The photo at right shows a decogged motor. siliconchip.com.au March 2005  85 poles. This can make the windmill hard to start in light winds, as the propeller needs to push the alternator past the first cog. Once started, cogging has little effect on the windmill performance - and in fact the windmill can continue to operate in much slower winds than was needed to get it started. If you live in a high wind area, the cogging effect is not a problem. But in a low wind area, your windmill may spend most of the time just sitting there, doing nothing. You could argue that if there is not enough wind to get the windmill started, then there is not really enough wind to generate any useful power anyway so it’s better to have the windmill stationary to save on wear and tear. But if you do live in a low wind area and want to extract as much power as possible, even if it’s only an amp or so, you might want to consider de-cogging. You can modify the F&P armature to reduce cogging, at the expense of a small amount of output power. Decogging involves reshaping the stator poles with a power file (or hand file if you have the time and strength). From the factory the poles have a square edge and are spaced about 0.5mm from the magnets as they rotate past. This square corner gives a sharp rise in magnet flux through the pole producing more power but also increases cogging. If we round off the corners we introduce the magnetic Folding tower flux slowly into the pole and reduce cogging (Fig 3). But as I said before, this will also reduce output power slightly. While cogging can’t be eliminated completely, we can reduce it to a point where the windmill will start in lighter winds. Another way to reduce cogging is to space the magnet hub out from the stator. You can try this by un-doing the plastic hub retaining nut, effectively sliding the hub off the stator. A more permanent solution would be to fit spacer washers onto the drive shaft before fitting the magnet hub. Again performance is lost, so you need to find a compromise. The Mast Its not much good having a windmill unless you can mount it in on something – the mast. As a rule, the higher, the better. You want to get the windmill up into a clean breeze without any turbulence from trees or buildings. While a 20m mast would be great, it’s just not practical for most of us. We do need to consider two things, maintenance and safety. You will need to get the windmill down from time to time to do maintenance and modification, especially in the early days while you are experimenting. Once you have the windmill sorted and making power, you would only need to take it down ever year or so to apply a little oil, check connections, and remove bugs and frogs. Fig.4: two common mast types are folding towers and tilt towers. Each have their advantages but in all cases, use as many guy wires as practical. Tilt tower 86  Silicon Chip siliconchip.com.au Yeah, frogs! Up here in the tropics I had little green tree frogs climb the mast and crawl into the stator at night, only to get centrifuged the next day when the wind picked up. A little grease smeared around the base of the mast put a stop to that. The other consideration is safety. In good winds the tips of your propeller could be doing over 200km/h and should be considered lethal. Your mast should at LEAST be high enough so it’s not possible for anyone to reach the propeller blade from ground level (even jump up and reach), plus a safety margin – say another metre or so. Other things can and do go wrong: blades can come off and towers can fall over. My first tower fell over after days of heavy rain had soaked the ground around the guy wire supports and then a storm pulled one guy wire support (a star picket embedded in concrete) right out of the ground. So you want to make sure your windmill is placed in a position where such a failure could not do any harm to people or property. As a general rule most towers require council approval and such approval is rarely given if your tower could fall onto your next-door-neighbour’s property. I highly recommend you talk to an structural engineer when designing your windmill mast. Consideration will need to be made of soil type, tower height, weight (about 25kg for a completed windmill) and wind loading, based on the diameter of the windmill blade. I use a folding mast for my own windmill. The mast pivots in the middle and I use a small hand winch to raise or lower the windmill. It takes about three minutes and the design means I can work on the windmill without any assistance. The mast pole is 70mm diameter 5mm wall galvanised pipe, and is 7m high when upright. The base is bolted to a buried concrete block 500mm round x 500mm deep. There are three 8mm guy wires, each attached to concrete blocks 300mm round by 700mm deep. So far this new mast has performed well and survived several storms with no problems. For higher masts you would need to look at a gin bar setup and use more guy wires. The more guy, wires the siliconchip.com.au better – they stop mast wobble and give you peace of mind (see Fig 4). Some Useful Links . . . Battery Charger Hugh Picket at www.scoraigwind. co.uk has detailed instructions on windmill building and in particular timber blade design and carving. Once your windmill is up and going you need some way to regulate the output. Most windmills are used for charging battery banks, so a charge controller must be able to switch the windmill over to a load once the batteries are fully charged or battery damage is likely. A commercial controller available is the Plasmatronic range of solar/wind charge controllers, and feature programmable switch over voltages and logging. Or you could build your own. My own charger is based on a PICAXE chip and uses power mosfets to handle all the heavy current switching. The design is a work in progress but the circuit diagram and program listing is available on my web site. (Also see the note below). I have a couple of 12V car batteries wired in parallel as storage. The system powers a string of garden lights modified to take 5W 12V light bulbs, a 50W 12V bed side lamp and a DC water pressure pump. I also have a small 300W inverter on standby to power the TV when the power goes out. Shutting down the windmill. The windmill kit described last month includes a furling system that will turn the windmill out of the wind safely if the wind speed or alternator load are excessive. But if you’re expecting a storm, or plan to go away for a few days it’s always a good idea to shut down the windmill. Once a windmill is shut down the propeller is stopped or only spinning slowly and therefore presents less area to the wind. Only when a propeller is at operating speed will it reach its maximum wind load and exert the maximum force against the tower (a stationary propeller has a relatively small wind loading). We can shut down the windmill in two ways. If you have a rope attached to the tail you can pull the windmill out of the wind and tie the rope to one of the guy wire anchors. Another option is to short out the windmill by connecting the output leads together. This will usually slow the windmill down to a safe speed unless the wind speed is too high, in which case the windmill will con- Michael at www.ecoinn.co.nz has been using F&P motors as generators for many years and sells F&P parts, water wheels and complete windmills using F&P motors. He also sells high quality blade sets suited to our windmill kit. www.otherpower.com has a collection of windmills made from car parts and scrap materials. OtherPower also hosts the FieldLines message board, a great place to share idea’s and ask questions. Plus my own web site at www.thebackshed.com has more information on the F&P windmill, as well as other windmill creations and ideas. tinue to run and possibly burn out the stator. In finishing, there has been a great deal of interest in home made windmills in the last few years and recently in using the F&P Smartdrive motor as an alternator. Windmills offer a very cheap source of power compared to solar, You can build a 300W windmill for less than $300 with a bit of workshop activity and scrounging around – that’s $1 per watt compared to solar cells at close to $10 per watt. And there is a lot of satisfaction is generating your own power from something you built yourself. On the internet you can find a wealth of knowledge on home made windmills. I’ve included some links which you should find useful but there are a whole lot more on the ’net. SC COMING NEXT MONTH While this practical Windmill series from Glenn Littleford has now concluded, next month we plan to bring you a Wind Turbine Regulator and Dummy Load, developed independently by Oatley Electronics. This design will suit virtually all of the wind generation systems in use today as well as many hydro-electric, solar and other “alternate” energy sources. March 2005  87