How to build a
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
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.
These plans describe how to build two sizes of machine.
The diameter of the larger wind-rotor is 8 feet [2.4 m].
The smaller machine has 4' diameter [1.2 m].
The diameter is the width of the circular
area swept by the blades.
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 and has passed the test
The energy produced by wind turbines
depends on the swept area more than it
does on the alternator maximum output.
The plans describe how to build a permanent magnet
The alternator can be wired for 12, 24 or 48-volt battery
charging. Essentially this choice only affects the size 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 to DC, ready to connect to a
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 for short 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 the speed 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
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, but not 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 the back.
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.
Most of the dimensions given are nominal - the accuracy is
not critical, so you need to not follow the drawings
The shapes of the blades are important near the tip but
much less so near to the root (the larger, inner end of the
The alternator parts must be constructed...
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