Notes from Windward: #70


Axial-Flux Alternator

Oana updates the work

[Walt:     Oana is an Assistant Steward who's currently away from Windward working on becoming accredited as an out-door science teacher. Previously, one of the projects Oana worked on was the construction of an axial flux alternator. This summer, Oana returned for a month-long visit, and was able to move that project along.]

Oana with the day's harvest


     During my short stay at Windward this summer, I focused on pushing the alternator closer to completion. I had, and still have, mixed feelings about this project. I will speak about the pros and cons of this undertaking after I describe the progress made:

     When I returned to Windward, I found most of the various pieces in good shape, except for one rotor, which had suffered from rust caused by humidity during winter and spring since it was not stored inside. This gave me motivation to work on casting the rotors in resin so that they would be protected from the elements. This would also provide mechanical support to the magnets on the rotors and the coils that make up the stator. A mold had to be made. I dredged up the old drilling, sawing, and filing skills I learned the previous year and went to work.

     I used melamine boards 3/4" thick to make the molds. They consisted of a base, a form, and a lid. I made one mold for both the rotors and one for the stator.



     And the finished products:
the rotor mold bolted shut

the stator mold opened up

     I then went to work on building the stator. The coils were ready; I had made them last year. First, since the coils were made of two parallel wires of 30 turns each, I soldered those together to make a continuous loop of 60 turns. Then I made a jig to hold the coils in place and arranged them appropriately, labeling the end for a "delta" wiring configuration. I left the rest of the soldering work for next time.

     Why choose one configuration over the other, especially since they both produce the same power?

     Well, the star configuration generates a higher voltage than delta at "low" rpm (rotations-per-minute), which is usually described as below 500 rpm, and can start charging the battery bank sooner. For us, however, the steam engine turning the alternator rotates at 500-700 rpm, so we don't need to start slow, in the star configuration. Although the alternator wired in delta requires more effort to get going, once it gets up to speed, it generates a good current at lower voltage.

     When an alternator is used in a wind turbine, speed is dependent on the wind. Using only the delta configuration in a low-speed situation may result in stalling of the alternator, from what I have researched. Often, alternators built for a wind turbine will have a star configuration or are built to transition from star to delta when the rotations-per-minute are high enough, to prevent stalling. In our case, we can very quickly get to 500 rpm or more, making the delta configuration ideal for our electrical generation needs.

     One last thing...

     Why make an alternator on such a small scale? The main reason is research. It is clear that this alternator, which will hopefully put out about 700 watts, will not even remotely be able to satisfy our power needs. Seven hundred watts powers seven 100-watt light bulbs. The point is to find out what is feasible to create for a small community. Several more alternators will have to be built if we plan on using them as a constant source of electricity.

     Using alternators would also involve storing electricity in batteries, which is another can o'worms. Alternators, once built, have a lifespan that certainly exceeds the human lifetime. Rechargeable batteries need to be replaced as they lose their storage capacity. The harsh acids and heavy metals in batteries cannot be incorporated into our current local ecosystem, and must be shipped off to be recycled and reprocessed. We would be dependent on the economy's ability to consistently provide new batteries at a reasonable price.

     So although it has certainly been fun learning to work with wood, metal, and electricity as part of building the alternator, I wonder what we are willing to put in to get the result we need. A lot of energy, both mechanical and human, has been used to create a machine that may not ever pay back its energy debt.

     The question, as always, remains: How much energy and how many resources can we afford to invest into the future we dream of?

Notes From Windward - Index - Vol. 70