Basic Principles Of The Homemade Axial Flux Alternator If you are new to the topic of building Permanent MagnetAlternators, then this is for you! My purpose is to help people learn about the DIY wind turbines that are gaining in popularity. Basic concepts are introduced here, but it’s still up to you to read on; this hobby is very multidisciplinary! For every topic, you can find a wealth of additional information – I hope I can help start you off in the right direction. Those with corrections or suggestions are welcome to contribute. A Successful 17’ Wind Turbine Design by Dan Bartmann (www.otherpower.com) Hugh Piggott’s Popular Design Manual (www.scoraigwind.com) Steven Fahey Version 1 December 18, 2006 Page 1 of 15 The magnetic field intensity is measured in either Teslas (after the inventor Nikola Tesla). Some types of magnets are longer on the polarized axis. or Gauss (after the mathematician). Below are pictures of some common sizes that are used: 2” x 1” x 0. The example magnets shown above are more flat in one direction: the poles are on the broadest faces. Magnetic Field Lines Around a Magnet When magnets are made. its strength will weaken. B. “W” for weight). There is always a North Pole and a South Pole. The intensity. The symbol “B” is used for the field intensity (like “F” for force. and no bigger. Steven Fahey Version 1 December 18. but an axial flux alternator is efficient and lighter when the magnets are just big enough for the job.Basic Principles Of The Homemade Axial Flux Alternator Magnets The Neodymium magnet has been a key technological development that allows practical and efficient alternators to be built. Many sizes now available are perfect for use in the DIY alternators. The magnets we prefer to use have poles on the faces with the most surface area. If a magnet gets too hot. the magnetic poles are “frozen in” with an external electromagnet as the metal cools. The high strength of Neodymium is part of what makes computer hard drives so compact. 2006 Page 2 of 15 .5” 1” Diameter x 0. gets stronger as you get closer to the magnet. Now the material is avaialble commercially for all sorts of purposes.5” Circular Arc “Magnetic field” is the technical term for the lines of force that are often drawn to symbolize the magnetic field around the magnet. since the lines get closer together. holding a magnet beside the plate of iron is like holding a ball above the ground.Basic Principles Of The Homemade Axial Flux Alternator Magnetic Fields A few illustrations will improve the understanding of how magnetic fields are manipulated. the potential energy is lower. and the effect is felt as repulsion. The ball falls due to gravity. The increasing size of the arrows in the diagrams below illustrates this. more lines get closer together. in much the same way that the object itself is drawn to the magnet. In a sense. If their opposite poles are close together. then the lines diverge. the field lines can become very concentrated in the plate. field lines pass through the plate and get stronger. making the field more intense. The magnetic field of the magnets is manipulated in this way. The next illustrations show two magnets that are close together. Same with the piece of iron. then the lines converge (attraction). When magnets are attracted to metallic objects. 2006 Page 3 of 15 . As they get closer. and it comes to rest at a lower potential energy. Through the neodymium magnet itself. The lines are drawn to that object. Magnetic Repulsion Magnetic Attraction Magnets in Proximity (dense field lines) Steven Fahey Version 1 December 18. the attraction can be witnessed by a distortion of the field lines that we saw above. very few lines emerge out the other side. and if the plate is thick enough. MAGNET IRON PLATE Flux Lines Through an Object Attracted to a Magnet Flux Lines Through an Object in Contact With a Magnet When the plate is in contact with the magnet. the magnetic strength doesn’t change much. They concentrate themselves in the plate. As the magnet gets closer to the plate. If similar poles are close together. once it is in contact with the magnet. Basic Principles Of The Homemade Axial Flux Alternator Concentrating Magnetic Energy The magnetic field is manipulated to our advantage. ROTOR PLATE MAGNETS “THE GAP” MAGNETS ROTOR PLATE Flux Lines Traced Through a Dual-Rotor Permanent-Magnet Alternator The path of magnetic flux should be more clear with the diagram above. because the fields have been confined. The magnets are arranged in a N-S-N-S pattern around the circumference of the rotors. By concentrating the magnetic flux between two opposite magnet poles. it is often more practical to use rectangular magnets. when making permanent magnet alternators. Opposite poles face each other. so that the magnetic flux is conducted by them. and the wire coils are more compact. The flux has been concentrated by confining it between the plates. 2006 Page 4 of 15 . and capturing the flux in iron plates that would otherwise be wasted. which are available in larger sizes. A compass outside the plates is weakly affected. Rotors of a Small Permanent-Magnet Alternator Steven Fahey Version 1 December 18. It is important that the rotors be made of steel or iron. but as they get larger. and back across the gap. Coils of wire in the gap capture the magnetic energy in those field lines. The final product usually looks like this: This set of rotors features round magnets. A compass inside this gap as the rotors turn would flip back and forth frantically. The flux also alternates between North and South. If you trace the lines of flux. This is common on smaller axial-flux alternators. we direct as much energy as we can through the gap between the faces. then travel through the steel rotor plate to the next magnet. they travel from one magnet face. straight to the magnet face opposite. In our machine. a voltage potential is produced. coils of wire are held steady.Basic Principles Of The Homemade Axial Flux Alternator Harnessing the Magnetic Energy Now we come to the humble coil of wire. flip over and over. while the magnets spin past on the rotors. and pulse of electricity is produced. The field passing through this area is a “magnetic flux”. They are all the same size. The diameter (or “gauge”) of the wire determines the maximum amount of current it can carry. or more turns of thin wire. Heavier wire can carry more current than thinner wire. This objective conflicts with the objective of allowing more current. that oscillate back and forth. Steven Fahey Version 1 December 18. Hopefully the diagrams at the end of this document will help. and instead flip the coil back and forth. A single loop of wire encloses a certain amount of area. the direction of the field flips each time a magnet goes by. Less turns of heavy wire. If a single loop of wire captures a certain amount of voltage in a changing magnetic field. because heavier wire takes more space. but no bigger. Experienced builders know off-hand how to strike the right balance. Here is a set of 9 coils that were wound for a Permanent Magnet Alternator. but when you put the system in motion. The builder selects a wire size that allows the current required for his design. Wire comes in a variety of sizes. the more voltage is created. or perhaps you don’t move the magnets at all. Not much happens when the surrounding magnetic field is sitting still. The builder wants many turns of wire to capture as much as possible. and have the same number of turns each. a pulse of opposite voltage is created. interesting things happen. Because the magnets were arranged N-S-N-S. but in the presence of magnetic fields. It is more of a mystery to the new recruit. Each coil sees a flipped magnetic field. When the field flips back. It doesn’t do much on its own. then more of those loops will capture more voltage. You may have magnets that get closer together. It doesn’t actually matter how the field is changed for the phenomenon to occur. The more rapidly the magnetic field changes (either greater or lesser). It is measured in Webers. A balance is sought by the builder to meet his needs. 2006 Page 5 of 15 . This coil is now producing alternating voltage. or they have enough wires coming out to allow some external connection changes. but it can be done. but also by having them 3 times more often. they are supported as a unit called a “stator” (it remains “static” while the rotor rotates). Builders still use single phase when it’s convenient. a lot of space would be wasted in plastic jackets. While this is simple. and when it has cured. When coils of enamelled wire are wound. the other two are either dropping or rising to their next peak. Either they selected one particular 3-phase connection arrangement. It is also more complicated to overcome the inefficiency when rectifying that voltage to put DC into a battery. Connecting the coils of wire introduces an important question in the design of the Permanent Magnet Alternator: Will there be 3 separate phases or just 1? Single phase alternators are simple to hook up – all coils are wired in series with each other. When the coils of wire are cast together into one plate. Then they close the mould. the current is also much smoother. the stator comes out as one big disk with the coils encapsulated inside. Steven Fahey Version 1 December 18. 2006 Page 6 of 15 . A solution was found a long time ago. and wire can be bought that is coated in a thin non-conductive enamel.Basic Principles Of The Homemade Axial Flux Alternator Coils of Wire in the Stator If regular jacketed wire was used to wind coils. Builders usually arrange the coils in a star-shaped pattern in a flat mould. A more elegant solution is to wire up the coils for 3-phase operation. They will last a long time if properly selected. and they all work together to make one large pulse at the same time. At any given point. When rectifying the 3-phase power so that a DC battery can be charged. and the maximum compactness occurs. (See Appendix B for how it can be done). the windmill experiences quite an abrupt “bump” for each pulse. and adapt the design to resist the vibration. each loop is isolated from the other. Vibration is reduced not only by having peak currents 1/3 as intense. only one third of the alternator is at peak power. All of the internal electrical connections were made in advance. Into the mould they pour a polyester or epoxy resin. It can hinder windmill performance and cause damaging vibration. The cost of extra rectifiers should not be considered an obstacle. don’t go slicing a magnet in half. and the first-time builder should beware. 2006 Page 7 of 15 . Basically. The size and gauge of wire should be selected to produce the right voltage. Does the windmill run fast or slow? Are there often strong winds that. The charging voltage will rise to a peak level (about 10% above the battery’s standard voltage). the more coils you have. One for each phase. there are 1. we haven’t mentioned the wind mill blades that will ultimately be attached to the alternator. the alternator won’t load the prop efficiently and they will spin too fast. The stator will produce less current. then one group of coils is at peak current while the others are not. either 12V. the more voltage you produce (if all other parameters stay the same). and then all your gains are in the current produced. The batteries are a load on the Alternator. Coils 6 9 12 15 18 Magnets 8 12 16 20 24 # Coils Per Phase 2 3 4 5 6 No. however. here’s the trick: For every coil of wire in the 3-phase stator. Actually. requiring a windmill that gets the most out of a gentle breeze? Once a size and speed range of the windmill has been chosen. So far. When at the point of deciding how to configure the stator. Steven Fahey Version 1 December 18. The absolute minimum number of coils in a 3phase alternator is 3 coils. It takes more work for the alternator to produce more voltage. the size and design of the windmill must also be considered. Instead of figuring out how this is done from scratch. and the rectifiers to the batteries. the builder can proceed with the selection of a stator configuration. If the blades are too big. 24. unless you know how to avoid the pitfalls of making single-phase alternators (but you wouldn’t be a newbie). Here are some typical combinations: Anything with more than 24 magnets is getting complicated. that would be fairly clunky.33 magnets. You would therefore need 4 magnets. higher winds turn the mill faster. but that may be offset by a windmill speed range that captures more energy in the long run. would be of benefit? Are winds usually light. Similarly. If you hook the PMA to the rectifiers. or sometimes 48V. there won’t be enough energy to start the alternator in low wind. If the blades are too small. then you effectively clamp the voltage to a specific value. Usually. the purpose of this machine is to charge batteries. depending on your system. if harnessed. somewhat mystical. Matching the Alternator and Prop The decision on how many coils/magnets to put into the Alternator is somewhat arbitrary.Basic Principles Of The Homemade Axial Flux Alternator Magnet and Coil Matching In an alternator producing 3-phase power. In the windmill situation. and provide more energy to overcome the heavy load. Therefore the magnets align with only one phase at a time. varying the proportion of magnets and coils begs trouble. There are several ways. One Magnet Rotor Mounted to Hub Jacking Screws Extend Up to Mount And Adjust The Opposite Rotor Rotor and Hub Mounted on Tower Head Stator is in Position in Front of The Rotor Steven Fahey Version 1 December 18. in fact. 2006 Page 8 of 15 . but most resemble the hub of an automobile.Basic Principles Of The Homemade Axial Flux Alternator Mounting The Rotor and Stator Together The stator is fixed while the rotor turns. How do people usually put this together? The best way to start is to see how others have done it. automobile hubs are usually used! You can see 12 coils in these stators: There are 16 magnets on the rotors below: Below you can see the process of assembly of the rotors and stator. (Roughly 12 Amps). 2006 Page 9 of 15 . This is actually a fairly large PMA. with a 6-8 foot prop. The Rotors weigh 23 pounds each (13 Pounds of magnets + steel plate X 2) The Stator weighs nearly 20 pounds (16 pounds of copper) At 80 RPM. but at a significantly higher RPM. Dan has recorded 600 Watts at 100 RPM. A more modest size of alternator. this alternator will produce 50 Volts.Basic Principles Of The Homemade Axial Flux Alternator Second (Opposite) Rotor Being Lowered Into Position Second Rotor in Position Technical Data What performance should the Permanent Magnet Alternator have? Dan Bartmann tests his alternators and has collected a lot of data over the years. Smaller props naturally turn faster. could also produce up to 500 Watts. suitable for a windmill size of about 17 feet diameter. At higher wind speeds. so a good match can be made by the beginner who works carefully to select prop and alternator operating ranges that coincide. Steven Fahey Version 1 December 18. unloaded. the prop turns much faster and he can capture upwards of 3 kiloWatts. When connected to a 48V Battery system. and to follow the instructions carefully. weld a lot of tubing. etc. then figure out what you need. The location and the wind you’ll get had better figure into the design early. The electricity may be useful in your house. But you don’t seriously think that DIY is a lot cheaper. and Ed Lenz all have well-designed wind turbine projects that offer the beginner an excellent start. too. Hugh Piggott. It takes twice as long to get anything finished! 4 Do I have to do it myself? There are a lot of companies that build these things for you. Maybe you’re ambitious and can feed it back into the grid. You’re going to draw a lot of drawings. neighbours. 2 How much power do I need? That jumbo windmill could light every light in your house. then the right size is the size you can handle. 2006 Page 10 of 15 . batteries. inverter. mix lots of epoxy. My dumb mistake: building two simultaneously. too. Maybe you want to heat water. One of the best ways to start is to pick something that works. tower. You get the whole thing. 1 What will I do with the power? Often Renewable Energy systems are designed to charge a bank of batteries. and how you can conserve to make the job easier. then you probably want a large windmill that’s producing power even when turning slowly – though it won’t be able to take full advantage of strong winds. “Just build it – then put it in the wind…” but most of us live near trees. (Or maybe you’re looking for the perfect lawn ornament to impress your neighbour). etc.Basic Principles Of The Homemade Axial Flux Alternator Where to Start With Your Wind Turbine There are a lot of questions to be asked at the beginning. 2 Where will the wind come from? Something we often take for granted. drill a lot of holes in steel. do you? Steven Fahey Version 1 December 18. It doesn’t have to be this way. or instead in a remote building with no power at all. apart from what you want. An inverter turns the battery voltage into AC that household appliances and lights can use. buildings. Dan Bartmann. And they’re not the only ones. If you have a purpose in mind. but how can you commit to that scale of project on your first time out? You don’t seriously think you can run your air conditioning system all summer on a puny wind turbine. You may want to put the energy to heating coils in your house’s heating system. and stand around scratching your head in puzzlement quite a lot. wind a lot of wire. cut a lot of wood. If you have light wind most of the time. Go the DIY route if you’re sure you’re going to enjoy the experience and finish the job. 3 How long will this take? This project could take a while. Or… you often get strong winds and want to capture all that energy – but it may not produce anything in lighter breezes. do you? If you’re building it just for the fun of it. that is well documented. com/Windmill Commercial Wind Turbine Manufacturers Bergey Windpower Southwest WindPower Jacobs Windmission Marlec Flowtrac African Windpower AeroMax www.wind-works.ecn.htm www.com/african36/ aeromag.sandia.uk/products/products.net/index.scoraigwind.uiuc.com/ www.allaboutcircuits.nimnet. and then venture out into the enormous volume of information DIY Projects Hugh Piggott Dan Bartmann Ed Lenz The Backshed Scientific Research Sandia National Labs NREL UIUC Airfoil Data ECN (Dutch) Electrical Theory All About Circuits FEMM (Magnet Models) www.windpower.bergey.gov/wind/ www.windenergy.windstuffnow.com/ www.com/ Wind Energy Associations and Watchdogs AWEA (USA) CANWEA (Canada) Danish Wind Insustry Assoc.org www.com www.marlec.ca www.otherpower.asn.ae.scoraigwind.Basic Principles Of The Homemade Axial Flux Alternator To Learn More Below is a brief bibliography of places to go to learn more about building windmills.net/ www.org www. You can start here.au/~kali/ www.org/ Steven Fahey Version 1 December 18.canwea.thebackshed.femm.com www.gov/wind/ www.org/auswea/index.windmission.html www. AusWEA (Australia) Wind-Works by Paul Gipe www. 2006 Page 11 of 15 .windturbine.auswind.com/ www.html www.edu/m-selig/ www.nrel.com www.awea.foster-miller.dk/index.co.nl/en/ www.html www. 40384 9.0641 0.2576 0.0983 0.9 4.016 5.02362 0.2 0.8128 0.295928 1.57404 0.6282 0.3 1.26592 8.37 40.512664 0.361 0.7 3.1019 0.184 4.92 0.049 0.47 0.0226 0.202704 0.56984 8.8288 1.64516 0.0359 0.36 25.0285 0.15062 1.81508 1.40728 33.0808 0.255512 0.9428 26.3 7.17248 16.634096 2.40386 0.36068 Ohms per Ohms per Maximum amps for Maximum amps for 1000 ft km chassis wiring power transmission 0.7 3.82676 5.060496 2.598088 3.9989 1.3648 0.90576 2.1148 3.9 2.2 1.62814 1.8 1.032 0.027624 1.7808 84.3133 0.4096 0.51054 0.7 2.64616 0.292 41.Basic Principles Of The Homemade Axial Flux Alternator Appendix A .2485 0.162 0.Wire Gauge Table AWG gauge OOOO OOO OO 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Diameter Inches 0.1328 5.81 51.66522 3.1563 0.4982 0.67 32.18922 4.1144 0.7 302 239 190 150 119 94 75 60 47 37 30 24 19 15 12 9.46 0.54304 5.064 6. 2006 Page 12 of 15 .7921 0.0618 0.8216 380 328 283 245 211 181 158 135 118 101 89 73 64 55 47 41 35 32 28 22 19 16 14 11 9 7 4.0254 0.0159 0.9392 66.25246 7.15 12.45034 1.29032 1.2294 0.1239 0.525 3.288 Steven Fahey Version 1 December 18.051 10.44352 13.5 2.1976 106.003 2.0907 0.282 10.16072 0.0571 0.588 2.7239 0.1443 0.3951 0.62026 4.3249 0.1736 133.385 8.60992 20.8 16.8568 168.30378 2.2639 2.2893 0.91186 0.4 5.276392 4.34822 6.684 10.729 0.05232 1.14 20.406392 0.45466 0.0779 0.58826 2.1285 0.2043 0.457 0.072 0.0403 0.984 52.0508 0.0453 0.577 0.1819 0.26 1.322424 0.5 1.0142 Diameter mm 11.20864 6.0201 0.0179 0.197 0.7 2. 00787 0.00441 0.43 0. 2006 Page 13 of 15 .8 34 Metric 1.072 0.55 855.4024 338.2 130.1000 0.113 0.0228 0.752 1079.7 0.6 35 Metric 1.28702 0.83 103.27 0.00492 0.200 0.0289 0.26 0.13 0.32004 0.11 0.17 0.0 33 Metric 1.18034 0.12 38 39 40 0.0031 0.056 0.16002 0.005 .16002 0.004 0.2 831.5 260.0137 Metric 1 .008 0.035 0.1 169.Basic Principles Of The Homemade Axial Flux Alternator 28 29 30 31 32 Metric 2.2 0.61 678.9 207.2032 0.0089 0.01 0.00394 Steven Fahey Version 1 December 18.496 426.8 428.125 0.182 0.25 37 Metric 1.1143 0.112 0.00709 0.034 0.8 659.127 0.9 329 339 414.126 0.088 0.1 164.0175 0.752 855.1016 0.163 0.0225 0.12 1114 1360 1404 1715 1750 2163 2198 2728 3440 1.872 268.226 0.20 0.0071 0.07874 64.0277 0.21 0.0889 0.33 0.86 0.4 36 Metric 1.4 1.0063 0.43 0.056 0.0126 0.0113 0.142 0.33 0.0063 0.22606 0.8 1049 212.044 0.091 0.51 0.180 0.728 538.0045 .9 260.072 0.0035 0.39 206.6 670.09 0.53 0.2 523.9 81.00551 0.1 533.632 680.140 0.254 0.248 555.0056 .043 0.14224 . Basic Principles Of The Homemade Axial Flux Alternator Appendix B . 2006 Page 14 of 15 .Neo Magnet Data Steven Fahey Version 1 December 18. 3-Phase Connections The simplest way to connect the coils of a 3-phase alternator is in “Star”. Steven Fahey Version 1 December 18. a “Delta” connection can be made by joining the starts and ends of each successive phase together. Alternatively.Basic Principles Of The Homemade Axial Flux Alternator Appendix C . Join the ending wires together. Each phase comes out of the start wires. 2006 Page 15 of 15 . and the three start wires come out.
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