# Make: Projects — Build a Wind Lantern

On the latest Make: Live, we had Dustyn Roberts, our mechanics mentor for the month, give us a tour of her Wind Lantern project, from her book Making Things Move. She’s also has put up the build on Make: Projects. It’s a fun introduction to vertical-axis wind turbines (VAWT), building a diode bridge rectifier circuit, and working with the basic mechanics of a turbine.

Wind Lantern (Make: Projects)

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Dustyn explaining her Wind Lantern on Make: Live

### 11 Responses to Make: Projects — Build a Wind Lantern

1. “If I had a small gear here, let’s say, the twice the size of this I would get twice the amout the output power” OUCH! A professor saying something like this hurts me…

• If there was a small gear on the main shaft that was 1/2 the size of one attached to the motor (so the one on the motor was twice the size), there would be twice the output power as there is in the existing state. Let me know if that clarifies things if I misspoke. Thanks!

• In general people expect accurate terminology from professors, and professors tend to define “power” in a strict sense: as the energy conversion rate.

Therefore gears can’t add power, only waste some portion of it (depending on efficiency of a particular gear train). They convert speed and torque.

• Dude she wasn’t giving a classroom lecture, you don’t need to nitpick on the difference between output power versus mechanical power as the this is an electromechanical project, and yes a bigger gear in this case increases the output electrical power. Good job and neat project!

2. johngineer on said:

There is an interesting thought experiment here (that could lead to real experiment — the best kind), which I think has been ignored in the comments.

Let’s assume for a moment that we’re not talking about shaft power, but rather electrical output power. Further, not instantaneous power, but rather average power. I could be way off here, but if you follow my argument maybe you’ll see what I’m talking about.

saimhe is correct that gears can only convert speed and torque — mechanically the power remains the same because power = angular velocity (speed) x torque. For a stepper motor, which is really a form of alternator, it’s voltage output varies in proportion to it’s angular velocity. Because electrical power = V^2/R, the power output also increases as the RPM’s increase. But there is a minimum required force in order to make the motor shaft turn at all, owing to mechanical and electromagnetic resistances in the motor and drivetrain.

Wind is a funny thing. Sometimes it blows bloody murder, and sometimes it doesn’t blow at all. Much of the time, it’s just a gentle breeze that is barely noticeable. It’s very possible that with a 1:1 gear ratio in a gentle breeze, the turbine would remain still — it’s too heavily damped by the losses mentioned above. If we do as the author suggested and change the ratio of the gears from 1:1 to 1:2 (wind-driven shaft:stepper shaft), we reduce the mechanical load on the wind-driven shaft. By reducing the mechanical load on the turbine shaft, you lower the threshold for the minimum amount of wind required to move the system.

This means you might have long periods of time where the motor shaft moves (albeit slowly), whereas before it didn’t move at all. Integrating something gives you more than integrating nothing. By changing the gear ratio you may very well increase the total amount of average power transferred by the system. I don’t know if it would increase by a factor of two, but it’s quite possible.

• Great points Johngineer (and thanks Jason!). You’re right that power = angular velocity x torque = V^2/R, so given a constant R across the motor, anything that increases torque or angular velocity increases the electrical output power. Since energy can’t be created or destroyed, it just changes form, whatever mechanical energy we put in = the electrical energy we get out, and power = energy/time.

So, say we want to increase electrical output power by making the motor shaft to spin fast. We could do this by putting a bigger gear on the wind turbine shaft, and a tiny one on the motor, but that puts our wind turbine at a mechanical disadvantage and requires us to have a strong wind. It may work well with bigger sails in a windy place but not so well with this little thing. We could also increase electrical output power by increasing torque, which we could do by putting a smaller gear on the wind turbine shaft and a bigger one on the motor. This would probably work out better given the size of the turbine and low wind conditions. In this case, there is a mechanical advantage.

I’m working on a project now the determines energy consumption of DC motors through current sensing with a constant voltage supply. Maybe I can work on that in reverse for this project so we can measure everything! It would be awesome to make a kit with interchangeable gears and a power output reading so people could learn about the connection.

• johngineer on said:

I just had a neat idea — a servo-controlled CV belt drive between the wind shaft and the motor. In normal operation it runs around 1:3, but as the wind gets more intense it tightens up. This would not only demonstrate the generator principle, but a feedback concept as well. You could probably print the conical pulleys on a makerbot, and use a rubber band as the belt.

3. I favor a “more with less” approach to this whole discussion. The gears are really for “coolness points”, they add nothing to power or efficiency. The stepper motor could be mounted direct to the shaft on the bottom plate, and an efficiency increase be realized, i.e. brighter LED’s, better performance in light winds.

Even the thrust bearing could be eliminated, and the stepper’s bearings taking the endwise load of the vane’s weight. These steppers are way over-engineered, having ball bearings at each end.

The only value I see in having any gear increase or reduction is to match the typical speed of the vane to the ideal speed of the stepper, if it has any such ideal speed. But yes, I agree the gears do demonstrate a principle of mechanical motion. Power is more related to the area (i.e. W x H) that is cut by the vane. Or in other words, how much wind is caught by the vane.

4. Anonymous on said:

Varying amperage fed back to automotive type alternators allow load to be varied, depending on torque available, obviating need for gears and their inherent losses at all? Great hairy rare-earth magnets not needed either, just a rechargeable field battery? Any better? any advantages? Gusts captured by fast acting electronics? Engineers, help! Bigger scale of same wind-generator needed for charging larger battery packs, even for my home-style light-wired D.C. LED lighting system? Plans? For off-gridders? We are people too! Consumers even! Need corporate catering too, but on scale, products, we control! This is a nice little wind turbine! I like it! Shiite on the ground, bury it, save America’s top-soil! Peace, Love, sustainability, to you all!