Check out the new dead-serious promotional video we made for our research group! It features us and our dear old heavy-as-frak generator! You can even see me there in the very beginning, close to the end, and here-and-there in the middle as well!
Wait, what?
As you can see, we a producing electricity with a lever-operated generator.
As you can see, electricity is actually really freaking cheap.
Despite breaking our backs for about five minutes in total, we were only able to produce less than 0.5 cents worth of energy. Obviously, even the food needed to offset our efforts cost significantly more than that.
Indeed, a well-trained human cyclist can typically sustain something like 400 Watts of mean output power for an hour or more. For mere mortals, the values are typically much lower, something on the order of 100 W or so. Obviously, peak powers can be much higher. I peaked at 1300 Watts on the Deutsches Museum cyclometer in Munich last Autumn, and even on the video you can see each of us getting close to 300 W momentarily. Were the machine pedal-operated, we all would have probably reached at least 800 or so. I can only guess what top-level olympic lifters might be cabable of (about 4 kW according to the brosphere).
Nevertheless, even under most optimal conditions the amount of electrical energy produced accounts for exactly frak-all. Electricity typically sells at around 10 cents per kilowatt-hour. For the unaware, that means going on at 1000 W output for one hour continuously. Thus, even that top cyclist would earn something like 4 cents per hour – talk about minimum wage. We were exhausted in less than 5 minutes, and would thus have netted practically nothing.
But how does it work?
The machine you see in the video is an interesting piece of technology as well. It was obviously custom-built for demonstrations such as this, given that it is operated by a crank and all (although I do remember hearing that it was once connected to bicycles to heat up a sauna…). It’s ancient and bulky, but it does fulfil its purpose nicely.
Despite the machine being very disc-like, it is actually a normal radial flux machine. The rotor has a large number of surface-mounted permanent magnets, alternatively magnetized in the positive and negative radial directions. As usually, one magnet corresponds to one pole then (didn’t count them, sorry). The magnets have probably been sintered from sone of the early Neodymium alloys, but I’m not completely certain. Due to the high number of poles, the machine can quite easily produce a 50 Hz voltage even when operated by hand.
The iron core of the stator is fairly typical as well, with rectangular slots, stacked together from standard electrical steel sheets. The covered-in-white coils are somewhat special, though: the machine has so-called tooth-wound coils. They are exactly what they sound like: each coil is wound around one tooth. In other words, any coil having its positive side in any particular slot will have its negative side (also called return path) in the next adjacent slot. For the ease of construction (and electrical design considerations of course), only alternate teeth are wound. Thus, each slot has one coil-side and one coil-side only.
Also used in wind-power
Indeed, this kind of winding is quite common in permanent magnet machines with a high number of poles. They suit well to that kind of applications, have a very short end-winding, are easy to manufacture, and probably have lots of other attractive qualities that I really can’t remember right now. Oh yeah! The exactly same winding can often be used with several different rotors with a different number of poles, and still get balanced three-phase voltages out! Pretty nice, right?
On the other hand, this kind of highly concentrated winding is reaaaaaally somewhat difficult to analyze without at least a pen and paper (and definitely the number of poles and teeth in the machine, which I don’t have), so it’s hard to give you any specifics yet. Indeed, I think systematic design approaches have only been published relatively recently. More is probably to come, due to the ever-spreading use of PM generators in wind turbines and such. Maybe I will write something about them later.
This is all for now. I’m still working on calculating Fourier series expansions of matrices in an efficient fashion, and hope to have something usable (and printable) within a week or so. Until then!
-Antti
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