# Generating power with a propeller

#### rkum99

Quite simple of a question:

For the purposes of a competition, I need to design/create a propeller with the greatest capacity for producing power. For the sake of standardization, all propellers must be made from a CD. Of course, since this is a competition I cannot ask for help on the propeller itself.

However, I was wondering what would be the best way to measure the power of these CD-based propellers for testing. What should I take into consideration when purchasing a DC motor for this purpose - would things like torque or rpm have much of an impact? As I understand it, I can just connect a DC motor with a load resistor in a circuit, and use a volt meter to measure the power somewhere in the circuit.

Also, any tips on actually attaching a CD to a DC motor?

Thanks.

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#### Khashishi

You aren't being very clear. Are you trying to design a windmill?

#### rkum99

For this competition, you basically have to create CDs with some sort of fin attachment. This CD will then be mounted on a generator and placed in front of the fan. The goal is to try and design a propeller such that it will generate the greatest amount of power in the presence of the fan's air current.

However, my main question is, what is the easiest way I can construct a testing system to determine the actual power the designs I make produce.

#### CWatters

Homework Helper
Gold Member
Power = torque * angular velocity

It's easy to measure the angular velocity by measuring the RPM and converting to angular velocity. The harder part is measuring torque. One way to do this is to arrange for the prop turn a small DC motor used as a generator. The output of the generator can't be used directly as you don't know how efficient it is so.... Mount the motor so that the casing can rotate slightly and use a weighted arm to stop it rotating. Torque = length of arm * weight. You can connect the motor to a variable resistor to allow you to vary the load and increase the torque. Try varying the load and measuring the torque and rpm. Calculate the power and plot a graph of power against RPM to find the max power point. Repeat for different designs of prop.

#### David Lewis

You can connect a potentiometer across the terminals of the generator (what you call a "motor") and adjust the pot to extract as much power as possible without the generator overheating. Output power would be the product of voltage and current.

If the size and operating voltage of the generator is matched to your turbine (what you call a "propeller") then try different turbine designs and see whether they increase or decrease system output (re-adjusting the pot to keep rotational speed the same for each trial). If they are NOT correctly matched then you can only find the most powerful turbine design for that specific generator at the given wind speed. There is no (fixed pitch) turbine that is optimum for all wind speeds.

I don't understand the usefulness of measuring in absolute terms the power output of the turbine by itself.

Minor nitpick: Angular velocity and RPM are not the same thing -- the former is a physical quantity; the latter a unit of measure.

#### CWatters

Homework Helper
Gold Member
I don't understand the usefulness of measuring in absolute terms the power output of the turbine by itself. .
Because the objective is to optimise the turbine not the turbine and generator combination.

If you just measure the output of the generator you will need to take into account the efficiency curve of the generator. Otherwise you end up optimising the combination rather than the turbine alone. When it comes to the competition they may use a generator that has it's peak efficiency at a different RPM.

The best chance of winning the competition is to find out how the organisers will be measuring the power and use exactly the same technique/generator for your own tests.

#### rkum99

Yes, in an ideal situation I would simply copy the set up that is to be used at the competition, and test my turbine with that. Unfortunately, the event guidelines I was provided with was incredibly vague in describing their setup. I have no idea about the direction of the air flow, or air speed that will be used at the competition. So it seems like my best option is to just try and compare the relative effectiveness of the turbines I construct.

CWatter mentioned that I need to take into account the efficiency curve of the generator - is this why David Lewis mentioned the use of the potentiometer? Since the generator is more efficient at a lower RPM, I turn up the resistance to account for its greater efficiency. Why should does the rotational speed have to stay constant - does this reflect that the system is operating at a similar net efficiency? And how would I measure the rotational speed of the generator?

Thank you for the helpful responses.

#### CWatters

Homework Helper
Gold Member
I suggested measuring the torque in post #4 so you don't have to worry about the efficiency of the generator.

#### David Lewis

Good suggestion. If you measure the torque, you're home free. The only potential problem I see is (from what I've read) it's not that easy to devise and calibrate a torque meter that sensitive.

#### rkum99

I'm not sure if I fully understand the lever arm mechanism. How exactly does the lever arm connect to the DC motor, and how does the motor actually rotate in its casing - and why would it rotate?

I'd like to try and keep the motor in a PVC pipe frame work anyway, so using a lever arm to measure it might be hard. Is there any other way I could effectively measure this purely electronically. If I get a motor that has a very high torque, would the loss of efficiency at higher rpms be negligible?

#### CWatters

Homework Helper
Gold Member
Mount the motor so that the case can also rotate. It only needs to rotate say 20 degrees. Mount a horizontal arm to the case. When the shaft is turned the case will also try to turn lifting the arm. Add just enough weight to the arm or move the weight in/out to counter this.

Some ideas:

#### David Lewis

Correct me if I'm wrong, but measure the speed of the turbine with the generator output terminals not connected to anything (i.e. open circuit). Then attach a pot across the terminals and adjust it until the turbine spins at one half of that no-load speed. That might be approximately the maximum output power operating point of the system (at that wind speed).

Alternatively, test different turbine designs under identical conditions and then assume the turbine that turns the fastest will also generate the most power. This might be the simplest solution when you know neither the generator characteristics nor the wind speed.

If I recall correctly, you want to operate your generator at maximum power, not maximum efficiency, because wind energy is free.

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