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Energy Exchange Analysis

  1. Jan 6, 2010 #1
    I have a motor I wish to evaluate.

    I wish to measure the Joules input and output.

    I believe I am making several errors.

    Right now I measure the voltage and amperage, calculate watts, and assume that for every second this wattage stays the same. So V*A*seconds = Joules input

    To calculate heat output in Joules I use current squared times resistance.

    Another energy output is RKE, rotational kinetic energy. I measure RPM, convert to radians per second, determine moment of inertia of the rotating mass, and then use:

    RKE = 1/2 * MOI * rps^2

    Again, units are Joules.

    This is a start. But are there errors above?
  2. jcsd
  3. Jan 7, 2010 #2
    "To calculate heat output in Joules I use current squared times resistance."

    I wouldn't do that. The formula you are using is for a resistor which loses energy by purely thermal means. A motor dissipates energy by doing work and through thermal means. So as long as the motor is spinning, there is some energy being put out as work, and the rest is heat. It won't all go to heat.
  4. Jan 7, 2010 #3


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    Staff: Mentor

    ...most motors operate pretty efficiently, so you shouldn't expect more than 10-20% to be dissipated as heat. The rest is dissipated as mechanical work. To find the heat output, I'd just subtract the mechanical work output from the input.
  5. Jan 8, 2010 #4
    In this motor the current is decoupled from the shaft load. It appears that the energy exchange is different from conventional DC motors.

    I would like to measure the work done by the rotor. I believe there is a method involving wind downs. You spin up the rotor using an air line and then plot the rpm per unit of time as the rotor slows.

    This is one test I need to learn about.
  6. Jan 8, 2010 #5
    If I do that, Russ, I have not measured directly, which is my preference. What you propose is an option. However. Since the motor is unconventional I would like to measure several ways if possible.

    Are you familiar with determining work from wind down data, Russ?

    I believe my Moment of Inertia calculation is correct and have calculated RKE. What is needed, I think, is to calculate the work done by the rotor at various speeds.

    In one embodiment I have measured voltage to be 1.27 and average current to be 195 mA. With this input I realize a maximum rpm of 784. The motor windings see a square wave. Duty cycle is adjustable and numbers above are from one set up. Rotating mass is 171 grams.
  7. Jan 10, 2010 #6
    Slight adjustments to system yields over 2600 rpm. Still 1.27 volts. Resistance under 0.5 ohms. Duty cycle roughly 50%. Current source is AA battery. Rotating mass still 171 grams.
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