KidWind generator speed and resistance load experiment

In the process of designing, building and testing a model wind turbine to generate electricity and compete in a KidWind Challenge it can be simple or complex. I believe the time, effort and learning will be proportional to the simplicity or complexity of the design. 

The KW process starts out pretty simple. Make some blades. Attach the blades to a shaft. Gear up the shaft a bit to increase the speed the generator turns at. Connect the gears to a small DC electric motor and you are in business. Fifth graders on up handle these tasks pretty well and have a lot of fun in this learning process. 

If you choose to take it up a notch read on.

In competition about one third of a KW Challenge points is about the electric energy (Joules) your turbine produces. Produce one volt at one amp for one second and you have just made yourself one Joule of electric energy. Who knew? Who cares??

The KW Challenge makes the process a bit more difficult by making you produce your Joules of energy through a load, a 30 ohm resistance to be specific. They give you 30 seconds to make all the Joules you can in a wind tunnel, with a 4 foot square opening, 4 feet deep, that has a wind speed of 4.5 m/s (meters per second) being pulled into it by fan/s.

In the beginning the list of variables the teams come up with will consist of -

Material used for blades

Length and width of each blade

Number of blades in rotor

Pitch of each blade

Simple one step gear ratio between the blades and generator ( 1:2, 1:4 or 1:8 )

As we advance into the Open Generator Class the number of variables increases a lot and so does the learning. 

Design and build airfoil shape blades

Design and build complex 2 step gear ratios ( 1:16, 1:32 and on up )

Design and your own generator or buy one to fit your design.

Design and build a shroud to increase the wind velocity in the wind tunnel

Determine the resistance load you will run with your generator for maximum Joule output.

And here is where we will start the new learning. With this set up in the picture above to determine what combination of generator RPM and Resistance Load the generator we have chosen produces the most Joules. The most Joules, that is, while staying below1 volt and below 1 amp. Exceed these limits for even a nanosecond and you are disqualified! 

For this experiment I have chucked up the input shaft of the 24 volt DC generator to test in the drill press (see picture above)  

The RPM speed that the drill chuck is turned at is determined by a frequency drive controller on the left. 

Here's where the "serious fun" learning starts. We want to find the RPM the generator is turning at. I can tell you that the frequency is 9.8 cycles per second. Multiply that by 60 seconds and you get 588 cps the RPM (one cycle = 1 RPM) of the 3 phase motor running the drill press. Now multiply 588 times .63 the belt pulley ratio on the drill press and you get 370 for the drill press chuck RPM. But wait, there's more. The generator has a planetary gear ratio of 1: 30 to the generator so 370 times 30 = 11,100 generator RPM 

One down one to go?

NEXT. A 30 watt 1 to 100 ohm variable resistor is in the box at the base of the drill press and connected to the generator being tested.

After about an hour of testing the generator at 3.9 cps freq. (turbine blade 150 RPM), 4.6 cps freq. (175 turbine blade RPM) and 5.3 cps freq. (200 turbine blade RPM) and recording the volts and mA outputs when loaded with 10, 15, 20 and 25 ohms. I came to the conclusion written above and needed to secure the generator body from turning so I could use both hands at the same time to change the cps frequency (RPM) and resistance ( ohms ) applied.

A small quick clamp on the generator body did the trick as it was held from turning by the drill press column. This allowed me to dial in the frequency and watch the voltage go up but stay under the 30 volt limit. Then change the resistance ohm load to get the mA output to increase to the maximum but still staying under the 1000 mA limit. After a few cycles I had the design numbers I needed for max Joule output with this particular 24 volt generator with a 1:30 gear ratio.

370 rotor RPM

28 ohm load

Produces 28. 3 Max volts

Produces 965.0 mA Max current

For 753 Joules in 30 seconds. (Note: the Joules value comes out a bit low math because these values are Maximums and averaging)

Just one more thing. How much torque would it take to get the generator shaft turning? A Vernier wireless load sensor c-clamped to the quick clamp would do the trick here. You can see it just below the quick clamp bar in the picture above. I placed it so that as the bar rotated it gave a force reading.

The LabQuest 2 micro processor collects the voltage and mA current produced by the generator it will also  collect the pounds of force generated by the turning of the generator body. As you can see in the reading 2.499 pounds of force. The distance of the force from to the center of the generator base was 5 inches. 2.499 lbs x 5 inches = 25 inch pounds of torque (torque = Force x distance) to get this generator turning at 370 RPM with a 28 ohm resistance load. 

So we need to test a set of blades to see if they will develop enough torque (at least 25 in. lbs. of torque) to turn the 24 volt generator.  You can see the set up in a KW wind tunnel. A loop of string is placed around one of the 6 blades on this small rotor and the wind tunnel turned on. As the blades try to rotate the force is read on the LabQuest as .071 lbs.  x the 12 distance the string is from the center of the hub. That equals .852 inch lbs of torque and we need a torque of 25 in. lb. At .852 in. lbs. this rotor won't do the job. What about longer blades?

OK lets try a 6 blade rotor with longer blades. Set up to work on a rotor that turns CCW the force reading comes in at .166 lbs. x 20 inches = 3.32 in. lbs of torque and we need 25 in. lbs. Well what about more blades? And then will those blades be able to turn at 30 RPM in the 4.5 m/s wind velocity??

Maybe a shroud to up the wind velocity??? And the cycle of learning goes around and around. Pretty cool.