As small as machinably possible. You want to have it as close as possible WITH CONSIDERATION for changes in size because of temperature change, and wobble (if its not 100% exactly right) you get exploded bits. Closer magnets and coils = better magnetic flux continuity = better efficiency. If you can machine it to a tolerance of less than a millimeter so much the better, but you'd better be darn sure about it holding up to strain and not contacting. Tips to get it closer: If its a disk-brake style on a flat plate of magnets/coils, get your plates as flat and smooth as possible so they can be close. If it is a drum-brake style 'cage' of magnets around a stator, then use specialty rounded magnets to make a true inside diameter circle. If thats not an option (too expensive) use more, smaller magnets in clusters. This has the same/more effective magnet area, but more sides on the proverbial polygon to make a rounder surface.
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thanks i have got the gap down to 1.5 mm /.5mm now it is a diusk rotor with 12magnets and 12 coils 3 pase in star mode dut ican not get any volts from it the resistance on allthe coils reads the same .i have the magnets N-S what have i done wronge thanks martin
Also, for 3phase star don't you need a different number of coils as magnets?
how many magnets should i have
https://www.instructables.com/id/DIY-1000-watt-wind-turbine/ This setup has 12 magnets and 9 coils.
it looks like i will have to make some new coils then thanks for all your help. PS its 15:23 on the isle of wight sun is out no wind so i will let you get some sleep thanks again MARTIN
Do you have a photograph of the setup? Preferably of the wiring to help take a look... you should 'see' volts at much further than 1.5mm so there is a wiring problem...I haven't read much on the topic in a while - are they all wound in the same direction (clockwise/anticlockwise) such that they don't cancel each other out? https://www.instructables.com/id/DIY-1000-watt-wind-turbine/step2/Build-the-coil-disk/ has a good circuit diagram to hook up the 3 phases through a rectifier to get any current. If you can tap into the common connection you should be able to see a voltage and find out which coil is causing problems.
As Frollard says, as small as practically feasible is best, to reduce the reluctance of the magnetic circuit . That is going to depend on the quality of your bearings, and the precision of your bearings. There are good reasons why motors and generators use good ball races for the rotor. Steve