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Angular velocity of a motor

  1. Sep 12, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi all I am doing a school physics project and I am trying to find the maximum angular velocity of a DC motor. I have built a circuit consisting of a charged capacitor (of known voltage) and a motor. I then try to predict the maximum angular velocity attained by the motor.

    2. Relevant equations

    3. The attempt at a solution
    Is it possible to equate the energy of a capacitor and the kinetic energy of the motor like this?
    [tex] \frac{1}{2} C V^2=\frac{1}{2}Iɷ^2 [/tex]
    can i similarly integrate it to find the angular displacement?

    Thanks for the help guys
  2. jcsd
  3. Sep 12, 2015 #2


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    No, you cannot, because there will be conductive losses in the windings of the motor, and the capacitor will not be completely discharged ( due to the back-emf of the motor ).

    You must make a complete model of the capacitor/motor, including:

    - Capacitor voltage (t).
    - Back emf in the motor.
    - Self induction in the motor.
    - Motor inertia.
    - Resistance in motor windings.

    The easiest way is to do this by Laplace transforms.

    You know how to do that ?
  4. Sep 12, 2015 #3
    not really, can you explain it? thanks
  5. Sep 12, 2015 #4


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    Well, I can explain it ( with some diagrams, and so on ).

    But are you familiar with Laplace transforms at all ?

    For example: The impedance of a capacitor, ZC(s) = 1/(sC) ?
    Or when you ( in time-domain ) integrate a signal, you divide by s in the Laplace domain ?

    If you are familiar with that, I can sketch a diagram with an explanation.
  6. Sep 15, 2015 #5
    I have just read a bit on Laplace transforms, I can try to understand it
  7. Sep 16, 2015 #6


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    Do you have some (realistic) values as for the above? ( also the value of the capacitor ).

    I think that an algebraic explanation will be a mess.
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