Was looking at doing this , but in reverse for a starter motor.
It's quite do-able - small A/C compressors pull about 3kW at approx 3000rpm. And lets not forget the shock load when they engage. Might be a bit of a concern with the inertia of a generator though - I'd probably recommend a york compressor (the 2 cylinder version) clutch, they're pretty solid.
You can get new automotive clutches fairly cheap, and your local auto electrician / auto AC guy will have a pile of dead compressors about 3 foot high that you can snitch a clutch from.
Things to note:
- You need to do a bit of machining. The idler pulley normally runs on something like a 1.5" or 2" dia cylinder that is part of the front of the compressor, so you need to whip up one of those to suit your clutch and to sit on the front of the thing that you're driving.
- The coil sits underneath the idler. Best way would be to weld that cylinder above to a small plate then fix the coil to plate.
- Maybe you should just hacksaw off the front half of a compressor, it might be easier.
- The clutch plate is splined to the compressor shaft it's normally 10mm or so for small compressors. It's shimmed in and out at that point to get the proper clutch clearance (that is, minimal). You'll need to adapt that spline to drive whatever it is you want to drive.
- They've all got to have the same centres with a fairly close tolerance - idler/clutch/coil are all pretty tightly packed. There's a bit of give in the clutch plate , but too far off-centre and you'll scuff the friction plates.
- Make sure the switching voltage is either on or off via a relay. No gradual ramp up/down - that slips the clutch and burns it out. Ditto for DC fans on the same circuit (eg condensor fans) - as they spin down, their back emf makes the clutch disengage too slowly and burns it out pretty quick.