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Power and kinetic energy relationship in a small DC motor

  1. Jan 21, 2009 #1
    I am trying to relate two concepts, Power and Kinetic Energy, using a small DC motor.

    Given the no-load max angular velocity w (rads/s) of the motor and the current drawn (A) at a given voltage (V) the power being used (P) in Watts is:

    P = V x A

    Given the no-load max angular velocity w (rads/s) and assuming the armature is a cylinder of uniform density, known radius (r) and known mass (m) the kinetic energy (Joules) of the armature is given by

    KE = 1/2 I w^2

    Where I is the moment of intertia given by

    I = 1/2 m r^2.

    So I have the Watts (P) being "consumed" and KE in Joules. Can I use this to calculate the efficiency of conversion from electrical to mechanical (under the stated conditions)?
    Can I simply express that in 1 second the efficiency is KE / P x 100?

    Basically, I am having trouble reconciling the time dimension of Power (J/s)

    Thanks in advance.
  2. jcsd
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