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Dc motor clarification?

  1. May 25, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    For the textbook diagram of a separately excited dc motor, the voltage equation for the armature circuit is given as

    ##V_t=E_a + I_aR_a##

    But in the class notes, it says ##V_t = V_a##
    Firstly, I don't know if the ##V_a## refers to the induced armature voltage ##E_a## or the ##I_aR_a##.

    But either way, according to the voltage equation, shouldn't the two not be equal?

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  2. jcsd
  3. May 25, 2015 #2


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    From the hand-drawn diagram, it's pretty clear that ##V_a = E_a + I_aR_a##.
  4. May 26, 2015 #3


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    The coils of the armature is represented by a pure inductor and a resistor in the equivalent circuit.
    Going round the circuit we come to the conclusion that the supply voltage Vt is

    Vt = Ea + Ia Ra

    where Ea is the voltage over the pure inductor.
    Next they introduce a "new" variable, the voltage drop over the armature as a whole Va .
    Clearly this has to be equal to the supply voltage Vt .
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