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B How to measure the output of a generator

  1. Apr 17, 2017 #1
    I am doing a math project. Our goal is to use Mathematica to design some small wind turbine blades, 3D print them, and then attaching it to a small dc motor. I did something very similar for science Olympiad, where we had been measuring the "power" by measuring the voltage between the two wires coming out of the dc motor.
    All we cared about was increasing the power output, so it didn't matter that we didn't know the resistance in the multimeter.
    I may need some actual numbers for the project, so in this case, do I put a resistor between the two wires, measure the voltage drop, and use V^2/R?

  2. jcsd
  3. Apr 17, 2017 #2
    That is pretty much the way we did it when I worked for a manufacturer of standby diesel gensets. We shorted the ouput of the generator through barrels of salt water (which boiled furiously!) to load the genset. Seemed to work OK.
  4. Apr 18, 2017 #3
    The power output will depend on the load resistance. You seem to have just measured the open circuit voltage.

    You need a variable resistor and measure the voltage and current to get the power.
  5. Apr 18, 2017 #4


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    DC motors are readily available, which makes them attractive but no one in real life generates electrical power using DC generators. Alternators are so much more efficient. Mathematica will do the calculations for you but it won't give you the knowhow about building Wind Generators. There is a load of published information about amateur projects and available commercial systems at all power levels. Get researching and find how it's done in practice. Bicycle dynamos (always AC I think) are readily available and not too expensive at all. One of those would give you comparatively stunning results.
  6. Apr 18, 2017 #5

    Are you serious, you can still even buy a bicycle dynamo, this I will investigate.

    LED lights and baterries are so lightweight and last so long without draining energy from the cyclist I thought dynamo were extinct.

    Brb searching dynamo sales.
  7. Apr 18, 2017 #6


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    Tell us how you get on. I am talking a bit historical. But there must be hundreds of them, sitting on shelves in peoples' garages - for free!!
  8. Apr 18, 2017 #7
    Done, they are still being produced and are very popular. Come with USB ports as well.

    I be damned. I totally wasn't expecting that.

    Ridiculous concept I think to cater for the cafe latte crowd.

    Still I am buying one, can get full antique replicas.
  9. Apr 18, 2017 #8


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    Same can be said about a lot of 'green' projects. Payback time for a cycle dynamo vs batteries will be several times the life of the bike, no doubt.
    There could be a problem using one for a wind turbine and that would be the initial starting torque. It could require a large turbine diameter. I speak from experience of a wind generator on a boat which needed a finite wind speed to persuade it to start turning - and that was a purpose designed multipole job with a very fancy pancake stator winding.
    I wonder if the OP would consider a Vertical Axis Turbine????? A lot to be said for a small one - no problems of pointing.
  10. Apr 18, 2017 #9
    What's wrong with just a cheap hobby toy motor. There must be a restriction on the physical size of this project, I think the idea is to max the power output not build capacity.
  11. Apr 18, 2017 #10
    My reading of the statement from the OP was that he would measure the voltage drop across the resistor. Where did you find open circuit in his statement?
  12. Apr 18, 2017 #11


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    It's true you can get volts to appear at the output of a permanent magnet DC motor but it isn't very representative of a real generator and, if they're planning to go to the trouble of designing and producing turbine blades, it would be nice if they could actually be of some use. Using Mathematica, a complicated cad package and a 3D printer is about as up to date as you can get. A simple model motor hardly does the project justice. There have been dozens of threads about this sort of project in the past and no one has ever (in my memory) has ever got back to PF to tell us they were really chuffed with the results from a free motor they got out of a cupboard. A bike dynamo is hardly out of this world.
    You could be right about the size of the project - actually getting enough 'Blow' to turn a turbine in the lab can be a problem and, outside, it's always flat calm when you want to show it off.
    Now is the time to be planning for a project and not at the last minute when just one unexpected thing happens to let you down. Student-led projects can often be disappointing then the teacher hasn't pointed them in the right direction. The OP really must nag the tutor for help and direction with this at an early stage.
  13. Apr 18, 2017 #12


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    Yes - that's what to do. (Terminology: 'Voltage across' rather than "voltage Drop' which tells you the internal resistance of the generator.)
  14. Apr 18, 2017 #13
    Some clarifications:​

    We are using a box fan as the wind source.

    The blades will only sweep out a circle with a 15 inch diameter. (it was even smaller in science olympiad, so this feels rather liberating)

    Yes, we won't be trying to use this to produce usable electricity. We are varying the camber, Tip Speed Ratio, and Blade size and seeing how far we can improve from our initial design. (Starting at the ideal TSR of 7 seen in most commercial wind turbines, and a moderate camber). That said, after we finish putting together the project, we may try to make one in a larger scale if we are still interested (doubtful, I start strong and ebb off pretty quickly)

    I will check out the bike dynamo, but I do think that the starting torque would be too big for me to achieve will just a box fan and my blade radius (only three bladed, can't achieve as much torque as I would be able to with more blades)

    Vertical Axis wind turbines seems like an amusing project to do, I'll print a few after the project if I happen to have more time.

    The only requirement for the math project was that we use Mathematica. The class isn't really much of a math class. After we finish linear algebra and multivariable calculus, there is a class called "Advanced Topics", where we pretty much just learn the basic functions in Mathematica. This year, our school got two 3D printers so several students in the class were using Mathematica to design things to print. With this in mind, I don't think the teacher would be much help, since the students are really the ones that are experts in their subject of interest, whether it's CAD, 3D printing, physics, or wind turbines. We are all still in high school.

    I'm using a set of parametric equations I found online in a wikipedia page called Kootz Airfoils, It allows a small amount of freedom in adding camber. (I'm actually pretty sure it's used as a sort of primitive example when introducing people to airfoils, so I doubt the shape is very efficient) This is the parametric equation I used to design my blade. (probably pretty inferior compared to the shape they used in industry), if any of you have mathematica, or know how to read the code and translate it to another language, Here is the code:

    ParametricPlot3D[RotationTransform[ArcTan[2.89/(v + 2)], {0, 0, 1}]@{
    1/5 3 (1 - 2 ArcTan[v/2]/Pi) (-Cos)
    + (1/4 - v/100) 2 ArcTan[v/2]/Pi ( u^2),
    1/5 3 (1 - 2 ArcTan[v/2]/Pi) (Sin - 1)
    + (1/4 - v/100) 2 ArcTan[v/2]/
    Pi (Sin thick + c3 ((u/Pi)^2 - (u/Pi)^4)),
    v}, {u, -Pi, Pi}, {v, 0, l}, PlotRange -> {{-3, 3}, {-2, 2}},
    ImageSize -> 400, BoxRatios -> Automatic],
    {{c3, 2.5, "camber"}, 0, 5},
    {{thick, 1,"blade width"}, 0, 2},
    {{l, 20, "Length"}, 0, 50, ControlType -> InputField}

    The Arc Tan is there to transform the airfoil into a Circle near the hub. We are still making the hub. ​
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