Axial flux alternator windmill plans 8 foot and 4 foot diameter machines © Hugh Piggott -May 2003
How to build a wind generator - the axial flux alternator windmill plans - May 2003 version © Hugh Piggott
Blades These plans describe how to build two sizes of machine. The diameter of the larger wind-rotor is 8 feet [2.4 m]. The smallermachine has 4' diameter [1.2 m]. The diameter is the width of the circular area swept by the blades. The energy produced by wind turbines depends on the swept area more than it does on the alternator maximum output. Alternator The plans describe how to build a permanent magnet alternator. The alternator can be wired for 12, 24 or 48-volt battery charging. Essentially this choice only affects thesize of wire and the number of turns per coil. But the tower wiring for the 12-volt version will be much heavier than the others. And the stator for the small machine is different in thickness. The alternator design is integrated into a simple tower-top mounting arrangement (called a 'yaw bearing'). A tail vane faces the turbine into the wind. A built in rectifier converts the electrical output toDC, ready to connect to a battery. Small wind turbines need low speed alternators. Low speed usually also means low power. The large machine alternator is exceptionally powerful because it contains 24 large neodymium magnets. The power/speed curve for a very similar design is shown below. Maximum output is about 500 watts under normal circumstances, but it is capable of more than 1000 watts forshort periods. The starting torque (force required to get it moving) is very low because there are no gears, nor are there any laminations in the alternator to produce magnetic drag. This means that the wind turbine can start in very low winds and produce useful power. Power losses are low in low winds so the best possible battery charge is available. In higher winds the alternator holds down thespeed of the blades, so the machine is quiet in operation, and the blades do not wear out. You can easily stop the wind turbine by short-circuiting the output with a 'brake switch'. These features make the wind turbine pleasant to live with. Blades The blades are carved from wood with hand tools. You can also use power tools if you prefer. Carved blades are good for homebuilders because the process ispleasant and
the results are quick for a one-off product. Moulded fibreglass blades are usually better for batch production. Wooden blades will last for many years. Furling system The plans include a description of how to construct a furling tail for the larger machine. This tail prevents overload in high winds. This type of furling system has been in use on Scoraig for decades andhas passed the test of time. Units This document caters for both American readers and European/UK readers, so the dimensions are in both inches and millimetres. The mm figures are in brackets [like this]. In some of the theory sections I use metric alone, because it makes the mathematics so much easier. In some cases, the metric dimensions will be direct conversions of the English dimensions, butnot always. The reasons are that different size magnets are used for the metric design, metric wire sizes are different from AWG, and some important physical dimensions are rounded off to make more sense in mm. The US version typically uses a standard GM hub (Citation, Cavalier, etc) with five studs and a bearing at the back. The bearing housing needs a large circular hole in the mounting at theback. I suggest you use only one system of measurement, either metric or 'English' and stick to that system. Your best choice of measurement system will depend on the magnet size you choose. Tolerances Most of the dimensions given are nominal - the accuracy is not critical, so you need to not follow the drawings slavishly. The shapes of the blades are important near the tip but much less so near...