Generator design.

I'm thinking of building a generator, probably to bolt a fan on for wind-power.
There are various examples on t'internet, but rather than blindly following their designs I'd like to understand what's what more and maybe try a few ideas.
I've got the Piggott book, but he just says things like fat generators are more efficient because the magnets move faster.
And I can see why that would give a higher voltage, but I don't see why it would be more efficient.

And does anyone know how to calculate, or measure the magnetic flux from a pair of magnets?
Or any other clever suggestions on how best to choose what magnets to buy... they seem to be the most expensive component.

Umm

Give me a bit of time to revisit ( dredge up from memory) all the stuff I did on Magnets as a Materials Science graduate ( I majored in high tech magnets and ceramics) and whilst working in that field ( High Tc superconductors) .

I do still know it all but it has been buried a bit by hens , compost, chutney making and other more "now" stuff.

Failing that, come and talk to CM and I this weekend, between us we can usually come up with answers to most Physics/Materials related stuff as that is what we did for many years!

Hairyloon wrote:I'm thinking of building a generator, probably to bolt a fan on for wind-power

The first step in selecting the design is to take into account what the generator must be able to do.

a) In this application (wind turbine) the turbine will be turning proportional to the speed of the wind and the power output proportional to the cube of the speed of the wind. The generator pkus the circuit to which it is connected must be able to match that proportionality.

b) The amount of power produced by the generator at all speeds in the operating range must be that produced by the turbine at that speed (must be able to adjust to that).

c) Notice that this is very different from the application where a generator is used in a hydropower application. In hydropower the speed is fixed by the head. If the turbine can adjust for diffferent flows at this speed the power will be proportional to the flow and the generator must be able to adjust to produce various amounts of power according to the flow but at some constant speed.

So if using fixed magnet generators easier to design for wind than water as stand alone systems.
If grid tied the water will be easier than the wind (usually done by using a "synchronous" being driven to a higher speed than the synchronous speed and most sorts of water turbines can tolerate a small deviation from ideal speed without too much loss of efficiency -- the change in speed of the wind turbine is huge, not small).

Constant speed wind turbines aren't impossible but both costly and not great efficiency off some ideal wind speed. Consider only for sites where this is the wind speed almost always present.

I know one of the problems with wind power is that if the wind blows too hard, then the fan spins too fast.
If it blows too much too hard, then it simply breaks.

Most turbines get around this by furling or feathering, but I am wondering how big is that window between the extremes?

Obviously, when the wind is blowing hard, that is when there is most energy available, but you're losing a lot of it if your fan is furled.
However, if your generator is taking enough power to slow the fan in higher winds, then it won't turn at all in lower winds.
But is that perhaps sometimes better?
Obviously not if you want consistent power, but maybe if you want maximum power. :?