You could try the alternator off an industrial engine.  I have a 3-Cylinder Kubota with a small single-phase alternator and (I presume) permanent magnets.  Some small cars have 20 or 30 Amp alternators too.  Depending on how long-term your requirement is, you could replace the rotor coil with permanent magnets, but that would also require replacement of at least part of the shaft with a non-ferrous one.  A few alternators have a stationary field coil which would make the conversion easier, but it would still need the non ferrous bit. Check Ebay for "permanent magnet alternator" with your price limit.  There are also PM rotors that could be fitted to regular alternators.

I'm afraid they cannot be had as small and lightweight as required.

An off the shelf product with the given requirements will most likely come at a premium.

The cross-section of the stator core (as with a transformer) determines how much power you can get from the thing.  If you push it too far, the core will saturate with magnetic flux.  If you rewind by hand with a slightly heavier wire, you will be able to squeeze a little more out of the things.  Is it an option to stack two units concentrically ?

Hi Shane, thanks for the tips. Like helge said it, I need sth of less power output and a diametre of 60mm would be nice. My main problem is to get the output at low rpm (1W @560rpm), or to say it in a different way, the low voltage at low rpm. I need an alternator that gives me at least 3V at 500rpm. The other necessary characteristics are lightweight, simplicity, working in hot and cold temperatures...

However, what you wrote is already in my mind for bigger turbines ;)

If you are limited in diameter, consider two concentric motors.  If you want a significant voltage at low speed, pick a higher voltage motor (some voltage at low revs, and minimise the loss of the rectifier).  A DC-DC converter would probably be useful too.  Another option is to find a BLDC motor of the power that you need, then rewind the coils to suit the speed/voltage characteristics that you want.  A high-speed motor will have a few heavy turns, a low speed one, a lot of thinner ones.  For what seems to be required here, the high end of NdFeB magnets would seem to be essential.  If you buy a motor, make sure that it has permanent magnets.  A lot of brushless motors depend on induced magnetism in the rotor.

I suggest a permanent-magnet brushless DC motor.  In theory, the driving voltage (no load) is the same as the output voltage at the same speed.

There are some specialist motors with low rpm/volt. This one is expensive but has a decent winding resistance. https://hobbyking.com/en_us/turnigy-9014-105kv-brushless-multi-rotor-motor-bldc.html This one is cheap has very low rpm/v though has a significant resistance. https://hobbyking.com/en_us/ax-gm2212-72kv-gimbal-motor-for-200-500g-camera-s.html . Though you would get some decent voltage out! 24ish volts at the top of your range. 

hey check these out http://goped.8m.com/Liquimatic.html

thanks for the tip, but not an option for this turbine. hydraulic converter would be too heavy and terrible at low temperatures.

then specify the temperature range

it should work between -40 C and +40 C

I can only suggest you custom generator like this http://www.thingiverse.com/thing:1416740

that's an option. I never thought about making also the generator myself. maybe the best choice at the end.. Thanks 

Try 1Nm NEMA23 stepper motors with 400 steps/rev, they are both ok as an alternator and have significantly less cogging torque when compared to 1.8° steppers.

You'll find the term "hybrid stepper motor" which means that where the torque is enhanced in the motor application as the rotor magnetisation is forced, it will be somewhat less effective as a generator.

The major down side of stepper motors is the sheer mass of iron laminate you'll have to carry around. 

*now thinks about a 3d printed hydrauilic torque converter*

yes exactly, the mass is a problem. The cogging torque usually makes it impossible to harvest energy at low wind speed.. I'll try to get one of those and test it. Thanks

Faulhaber makes some nice bell rotor motors that are free from cogging torque. They are very nice brushed motors.

follow-up on those bell rotor motors. For cogging torque free operation, check out these

http://www.thingap.com/ironless-composite-stator/

it's also possible to manufacture these with planar PCB coils but that geometry blocks a lot of the rotor cross section.

May I also ask if 0.5 m rotor diameter implies that the rotor be fully assembled? If the blades were removable or mounted so they can stack you could go beyond that without increasing the envelope of the transportable state.

Quite an exciting endeavor.

on second throught - these stepper motors are not really suited for high rpm and would only make sense with a larger diameter rotor. Acceptable?

Try looking for RV wind generators. These come up occasionally on eBay or craigslist, or maybe contact the local used RV place. You can also place an ad in the "wanted" section of craigslist, someone might sell you a generator off of an older junked RV.

ok I needed half an hour to find out what RV means (I'm not from USA). Well I guess theese are whole turbines - still interesting to see how they did it. I guess they produce or order specially produced alternators. Thanks for the answer!