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How do I know how much this motor is putting out?

  1. Nov 4, 2015 #1
    Hi Just wondering if someone could help me! I need to know how much torque and RPM a motor will output for a given load (0.75kg), the only information I've been given is how the RPM and current vary with time for 4 different flywheels of masses 118g 166g 208g and 337g! the dyno test datasheet for which can be found in excel format here!! the voltage was always 8 volts during the dyno testing
    The motor will be used to power a model car down an 18m track, and inorder to work out the speed and acceleration of the car i need to know the torque and RPM the motor will supply it! any tips on how to do this are welcome!!

    Thanks People
    (hope this is in the right place BTW)
    Last edited: Nov 4, 2015
  2. jcsd
  3. Nov 4, 2015 #2


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    The dropbox files are csv, not excel files (.xlxs).
    You will probably receive better help if you upload screenshots of your data and analysis as many people, myself included, are hesitant to download files from unknown sources for security reasons.
    From your data, it sounds like you can find electrical power input, mechanical power output, torque output and efficiency all vs. rpm for the motor.
    These are the equation you'll use:

    P=IV (where 'I' is current)
    T=Iα (Where 'I' is moment of inertia, you'll need to find it for each flywheel.)
    η = Pout / Pin

    Compare your results for each flywheel, they should be the same/similar.

    You can then apply those specs to your model car and find acceleration & velocity over the 18m, this could be done in excel or MATLAB etc.
    the difficult part is estimating the frictional losses of your car (rolling resistance, air resistance, transmission losses etc). You may also need to consider the current output of the batteries you plan on using.
    If you plan on building this thing I suggest making you gear ratio adjustable so peak performance can be found via testing as it's unlikely your loss estimations will be accurate.
  4. Nov 4, 2015 #3
    Ive took a screenshot of as much of the data as i can, but the rest can be grasped from the graphs below!
    Thanks the aim is just to estimate and get our predicted time as real to the actual time as possible :)
    But for some reason were not supposed to actually test any equipment only using the data they provided us with.. And we've to pick a gear ratio from a list that they'll give us next week.
    From what you have told me i am able to find all that Thanks dude!!!

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  5. Nov 4, 2015 #4
    So just to be sure, im to take the average torque from the flywheels and apply that to my car since the load on the motor is different?
  6. Nov 4, 2015 #5


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    Not the average torque. The torque the motor applies to the flywheel varies with RPM, the same applies to the torque the motor applies to the wheels of your car.

    What you want is a torque vs RPM scatter plot for each experiment then use excel to apply a trend line to find the best fit equation for that plot. Compare each equation, they should be similar. Maybe find the average of the 4 equations. They should be straight lines that vary from stall torque to no load speed.
    The equation can be used in the model car calculation to find the torque output of the motor for any given RPM.
    This model car calcs can be done via an iterative method like so:

    At t1 = 0, and v1=0, calculate a1 from torque output at RPM=0 (this will be stall torque)
    at t2 = 0.01 find v2 assuming a1 was constant between t1 and t2, calc a2 from v2
    at t3 = 0.02 find v3 assuming a2 was constant between t2 and t3, calc a3 from v3

    repeat until 18 metres have been covered
    You could probably get away with a larger time step than 0.01 seconds

    This is a basic numerical method for an ODE

    I did a very similar project a couple years ago, I had much better results finding and using the manufacturers motor specs rather than using the experimental specs we were given, so that's something you could look into.
  7. Nov 5, 2015 #6


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    I am confused by the flywheel rpm and current plots. Admittedly the time to speed will vary because the rotational inertia is different for each weight, but once the flywheels are to speed, which is indicated on each graph by the horizontal lines, there should be no effect on the motor by the flywheels' weight and the terminal rpm and current should be the same for all of the flywheels; which, clearly is not what is indicated on the graphs. So, there must be some external forces involved like bearing friction variations due to the lateral flywheel weight or air friction variations due to increasing flywheel diameters with weight. As a result, I don't see how these graphs can be of any help in determining the acceleration of the model car based upon their data.
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