This is a brief look at a low speed DC motor I've been testing as a generator for the last few months. It is available used at Surplus Center in Nebraska for about $50(US) plus shipping.
This motor has a face plate rating of 24 volts at 194 rpm and 13 amps. It is rated as a 1/4hp motor. It comes in 3/4" and 5/8" shaft sizes. The shell is formed from 1/8" sheet steel and contains four large ceramic magnets. There are four small brushes contacting the commutator, although not at 90 degree angles as one might suspect. The coils appear to be awg 14 wire. without pulling things apart I can't tell what the winding pattern is.
I purchased a few of these and have been working on finding the issues one might encounter attempting to use these in wind applications. In the tests refered to below, I used the motors as they arrived, without refurbishing or replacing parts. I expected (correctly it turns out) that I would destroy some of these in tests.
Matching a rotor to your generator is important in getting satisfactory results. I tested this motor as a generator in a 24v system with two different sets of blades; a 3 blade, 7.5' diameter rotor of carved wood, and a 2 blade 8' dimameter cast nylon rotor. In both cases I observed voltage and current during low (under 20mph) winds and high (over 35mph) wind events. In both cases I used high capacity batteries as the load.
For 12 volt systems the 3 blade rotor is a good fit in low winds, charging as soon as the rotor spun up. Start up speeds seemed a little high, but motors of this type (dynamos) tend to have a fairly high resistance to moving due to heavy armatures and pressure from brushes. A four blade rotor may be a better 12 volt fit as the cut-in speed is a low 100rpm and four blades should provide more low end torque.
For 24 volt systems the 3 blade rotor is disappointing in lower wind regimes. I watched for quite a while as the output voltage bounced between 15 and 20 volts. In a similar wind the output of the 2 blade rotor was much more satisfying, charging up to 5 amps in the same conditions. Startup with 2 blades is lousy for all the reasons given above. Even in a 20mph wind startup is slow.
In stronger winds both rotors provided good results, although the 2 blade rotor gave consistantly higher amperage as long as it was already spinning.
Not knowing the actual limitations of the motor (as generator) I set the furl very stiff with the 2 blade rotor and watched to see how high I could get the current before things got nasty. As happens in Colorado in the fall, nature was eager to grant my desire for high winds. In winds over 40 mph I watched this motor put out over 500 watts before the fiery end.
The weakest link appears to be the brushes, which burned up within a few hours at 20 amps (500 + watts). The brush pigtails, a strong smell and black dust were all that remained. There was evidence of high heat and melted wire insulation.
Higher current replacement brushes might help boost the durability of these units somewhat. But that may not take care of overheating issues. These motors, like most, are totally sealed. They have a removeable band for accessing and servicing the brush connections. Leaving off the service band or allowing airflow through the generator in some creative way that keeps water out could help aleviate heat issues and gain a few amps.
Conclusion:
For 24 volt systems these motors can be used to produce 300 watts and for 12 volt systems, 150 watts. If you decide to use one of these motors in a wind generating application, adjust the tail to furl at the rated 13 amps. Remember that this is a cheap and quick way to get a few watts out of the wind. Design to account for it's limitations and you can get good use of your investment.
If you want more, you'll need to address the heat issue. Don't skimp on replacing the bearings if you intend on leaving it up for a while.
Chuck
