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Gotwind is a Ben Jandrell's (Shropshire, England) DIY site for renewable energy projects. The site has a number of cool projects and technical info and tutorials. Here's an excerpt from his page on the Sturmey Archer GH6 Dynohub:
The Sturmey Archer GH6 Dynohub was designed as a means of generating electrical lighting power for bicycles during the second world war. It seemed to me as the perfect 'off the shelf' small scale experimental alternator.
It was in production for almost four decades, first making an appearance in 1943.It was a 6 volt 3 Watt unit incorporating a 20 pole ring magnet with a stator having a continuous winding.
The GH6 was withdrawn from production in 1984, but can still be found at scrap yards, established bike shops or eBay.
The main feature of the dynohub was that it reached it's rated output at only 12 mph (with a 26" wheel) this equates to a mere 60 rpm, as compared to other alternators requiring 200 + rpm to get any where near there rated output.
This low speed design was ideal for use in small scale wind production, along with it's excellent bearings, it could be mounted easily on one side of the unit.
The name dynohub is a little misleading suggesting that the unit is a dynamo - it is actually a multi pole alternator that would produce alternating current, fine for lighting bulbs, but not so for battery charging. A bridge rectifier would be needed to convert the alternating current (AC) to a more useful Direct current (DC).
He also has a page on servicing the Dynohub, useful if you buy one on eBay that needs some refurbishment.
It's called a hub dynamo and is build by several manufacturers like Shimano.
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