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Homework Help: DC motor question

  1. May 25, 2015 #1
    1. The problem statement, all variables and given/known data
    A machine, with an armature resistance of 0.16 ohms and K##\phi## is to drive a load that requires a torque that is proportional to its speed. One point on the mechanical torque-speed relation is 16 Nm at 400 r/min. If the armature terminal voltage is 50 V, at what steady-state speed will the load be driven?

    2. Relevant equations

    ##T=K\phi i_a##, for a constant machine speed

    The torque speed relation is
    ##w = \frac{V_t}{K\phi} - \frac{R_a}{(K\phi)^2}T##

    3. The attempt at a solution
    the only thing missing for me to use the torque speed relation is the load torque, but I don't understand what they did here to get it. I don't know what A is.

    Attached Files:

  2. jcsd
  3. May 26, 2015 #2


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    Homework Helper

    The problem states that the torque, T, is proportional to the speed, ω.
    A is the proportional constant which they calculate as 0.382 Ns.
    This is similar to a y=mx relation and they are calculating m from one point on the graph:
    m = y/x or in this case A=T
    All that needs to be done is to convert the 400 rev/min to rad/s.
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