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Hi,
I need to implement a system including a generator with 12V output at 15W. I prefer to use an AC generator and rectify the output to get 12VDC. I have two constrains; 1) speed of water that turn the shaft (1m/s), 2) size. I system should fit in a 20x15x15 cm box.

I have done lot of search but the problem is that I can’t find any small permanent magnet generator that can be used.

I appreciate your suggestions.

Thanks

How about a small permanent magnet servo motor? If it doesn't drive the voltage with the speed you want you can use a small transformer (rated for max load) to boost the voltage. A servo will drive pretty high currents at low speed, but you may not have the open circuit voltage to do your 12V (I assume it's a battery you are driving, and therefore need 12V+ ).

KEJR

Google is your friend yet again. Check out what the hobby windmill folks are using. Their task is very similar to yours and there are some pretty good solutions out there. I have a surplus treadmill motor that would probably do the deed but it is a brushed unit. It wouldn't be very difficult to fit slip rings if you must have an AC unit. An inside out unit with a PM rotor would be easier. You might check out steppers as well to see if there are units with a PM rotor. Alternators are outrageously simple, but small units aren't very common.

Regards

cww

It seems like you need to run at low RPM so you will need a multi-pole alternator unless you gear up. Here's a couple of suggestions:
Alternator from motorcycle or scooter.
Front hub type from pushbike (do they still make those?).

You may need to rewind in order to get 12 V but the winding is usually quite simple.

A lot of the home-built power plants use "Fisher & Paykel" washing machine motors (28 pole) but they are larger than you require.

Regards,
Roy