Generators #2: Stepper motors suck as generators

I decided to go buy a motor so I went to Glenwood Sales (549 Hague St.) and browsed for a while before eventually settled on a 30 V stepper motor with a 48 ohm coil. It apparently has 4 phases, and each phase would ordinarily consume 0.625 A or 18.75 watts.

The stepper motor could generate as much as 50 volts open-circuit (it's wired with two coils center-tapped.) Through a 51-ohm load I found it could get as high as 5 volts, and through a 280-ohm load, 20 volts. The good news is it's pretty linear from about 1,000 RPM through 3,000. The bad news is those two ratings yield only 0.5 watts and 1.4 watts respectively.

Using a full-wave bridge on each side and tying the outputs together and running at 3,100 RPM, I could get 9 volts into 51 ohms and 34 volts into 280 ohms: 1.6 watts and 4.1 watts respectively. Open, it would generate 90 volts. Indeed, none of the motors would source more than a half a volt at 3,100 RPM; one that I hadn't tested would generate 0.5 V and 1.5 amps, so at least that's promising.

Starting from the original 3-pole rotor.

The third phase is connected to a wire ...

... that is soldered to the bushing, electrically tying it to the motor case.

Method of mounting the rotor bearings; Generators #1: These generators suck

I found that I have some 1" thin-walled tubing from an exercise machine that has a nearly 22mm diameter — the outside diameter of the bearings I bought. I figure I can weld a piece of tubing to the ends of the windmill frame then cut a slot and use a seat-post clamp to grip the bearing.

I also tested the motors I had selected as candidates for a 10-watt generator. None of them performed better than the motor I had originally selected.

Based on my former calculations, the rotors would only run at 300 RPM in a 10 MPH wind, so a 7x gear-up would only yield 2,100 RPM which wasn't enough to generate any appreciable power. To get any power out, I had to run the baby carriage wheel in the drill press at 620 RPM with a 0.33" gear, so the ratio to the 7 3/4" wheel (23.5x) means I ran the motor at 14,500 RPM. If this were 20 mile-per-hour wind, then a 70 mile-per-hour wind would overcrank the motor to over 50,000 RPM and probably blow it up. I'll have to see how slow I can get the motor to run and still get decent power. Well, I know that 3,100 RPM is not enough: the motor, when hitched directly to the drill press, would only generate about 3.4 volts open and 1.9 volts into 3 ohms (only about 1 watt.)

Ok, so let's say I limit the motor to 15,000 RPM at maximum wind speed of 70 MPH, or 2,100 RPM. The ratio, therefore, is 7:1 … bummer. I think I'm going to need to get a different motor. If I use 21,000 RPM as the cap, I can get to a ratio of 10:1, so in a 10 MPH wind, the rotors would only run at 300 RPM driving the motor at 3,000 RPM which isn't enough.