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Homework Help: DC motor problem

  1. Jan 13, 2014 #1
    People of physics forum
    i have come here to ask for help.
    i am trying to model a armature controlled dc motor with no load but and opposite force in the form of frictional damping

    the expression i have raised and am assuming it to be right is

    ωout/Vin = k/(L(s)+R)(I(s)+C)+ kt.k

    where K,kt are constants
    L inductance
    I inertia

    R resistance armature circuit
    C damping

    the reference voltage is 25 volts. i really need i think a partial fraction expansion of this to allow me to do the inverse laplace transform

    any body out there can you help me please?

    thanks people. :smile:
  2. jcsd
  3. Jan 13, 2014 #2

    rude man

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    Gold Member

    What does "kt.k" mean? Is it kt multiplied by k which you can write as (kt)(k)?

    Ω(s)/V(s) = k/[(sL+R)(sI+C)+ (kt)(k)] ?

    The denominator is a second-order response. I would put in numbers to determine if this is an over- or under-damped system , then use Laplace tables. If you use partial fraction expansion that is OK but you'd better know how to handle the complex fractions.
    Last edited: Jan 13, 2014
  4. Jan 13, 2014 #3
    Hey rude man, thanks for getting back

    Yes you are correct k and Kt and yes they can be expressed in the form you have shown. They relate to the properties and generated emf. I think but am not sure that they are possible the time constant for the circuit the "Tau" value any ideas?
  5. Jan 13, 2014 #4

    rude man

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    k*Kt would have to have dimension = T (time). But if your denominator is consistent, k*Kt has the dimensions RIT-1 = RMLT-1.

    Anyway, if you believe your transfer function is correct, why are you speculating about a time constant? Do you have numbers for K and Kt including their units?
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