Wind Turbine Experiments

Here are a few notes on my first attempt at making a small wind-turbine. The main motivation behind this is 'just because I can' and it seemed a good opportunity to learn a few things.

Before embarking on such a task, it is prudent to check a few basics, such as average wind-speed in your area, where you can put the finished machine and try and get an idea of the size of machine you want to build.

A bit of initial research uncovered that the 'average' windspeed in the UK is about 15km/h but this is somewhat skewed by rural remote areas and if in a residential/urban area, you are more likely to get about 7km/h, and that will tend to be turbulent air as it gets swirled around houses and trees etc. Also, to get clean air, you need to place the turbine at least 10 metres above the largest surounding tree/ building, a house-roof wont cut it. Having several 20metre+ trees in the area, this means errecting a 30m + mast in my back garden. Not only is that fairly impractical, there are guy wires to trip over, potential for a nasty accident and the neighbours would not be too happy either! So, I live, like the majority of people in the uk, in a place that is totally unsuitable for siting a wind-turbine. But that isn't really the point, it is just necessry to temper expectations of the amount of power that can be generated.

Many of the ideas here are based on the designs found at http://otherpower.com which have been suitably modified for my purposes. This is an excellent site written by people who are off-grid and set about building turbines as a necessity, in order to generate electricity. Other useful sites are http://www.scoraigwind.com and http://www.windstuffnow.com. I would recommend visiting these useful resources as they give much of the background I am only skimming here.

In the following pages are the details of 2 different wind-turbines. After playing around with a prototype, I built a small turbine whose main aim was to trickle-charge a car battery and generate only a few wats of power. This was used as the testbed to tweak a few parameters and with the lessons learned, I then set about building a larger machine that could produce several hundred watts of power. Details of each of these can be found by following the links below.

When starting out, you need to consider how much power the machine is likely to produce. Reading around, several hundred watts seems a common number but these are usually quite large machines so for my first machine I set myself a more modest goal:

The Goal
12V, 1 Amp (12watts).

This should provide a respectable trickle charge for a car battery. Details of this design can be found here and contains a lot of the background used in the other designs.

Design Selection

The goal is to make a machine that will generate power at low RPMs ( < 500) which precludes using a car alternator as they are designed to work at several thousand RPM. To generate electricity using wind, we need to somehow move magnets past coils or vica-versa. A 'standard' motor/generator uses a radial design which involves some oddly shaped coils and a difficult to make rotor. An axial design where flat disks spin past each other is more realistic to make at home. Twice the power can be generated by sandwiching coils between two spinning rotors, and with the ready availability of neodymium magnets, I reasoned it should be possible to make a small machine pretty cheaply. As it turns out, going small is not the best option as you are limited by:

• magnet size
• coil size
• rate of change of flux (the outside of a large disk travels further than a small disk).

All three of these factor affect the output generated - more on that later. However, I was limited by the tools at my disposal as as I had a 100mm steel holesaw, it seemed that a disk rotor diameter of 100mm was a good place to start!

The Prototype

I made a simple bearing housing from wood and used a length of 10mm allthread as a spindle, threading it through some mini-moto bearings bought off ebay. The disks that were to be rotors were cut from the side of an old PC case and everything mounted using nuts. I wound 6 small coils and used six magnets per rotor.

My first attempt was to say the least, somewhat disapointing. Firstly the mechanics of the machine were sadly lacking due to a poor fit of the allthread within the bearings. The steel that made the disks was too thin so tended to warp when drilled. And finally, I had wired the coils as three-phase before setting it all in resin and found I had wired it incorrectly so that the voltage generated by the coils cancelled out, resulting in me only getting tens of milliVolts even when spinning at 500 rpm. This was just prior to launching magnets at high speed around the garage.

With the leasons learned from the prototype, it was time to come up with a better design that I could make from readily available parts. Although wood had proved useful for prototyping, I could see that I needed to move to a steel mechanical structure and it dawned on me - 'there is a time in every man's life where he needs to learn to arc-weld'. Now was my time. It was also time to find some engineering works in the area to get hold of some steel and various other parts to produce a more robust structure to house the generator.

10-Jul-2007