Dc motor clarification?

  • Thread starter theone
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  • #1
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Homework Statement




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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
vela
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From the hand-drawn diagram, it's pretty clear that ##V_a = E_a + I_aR_a##.
 
  • #3
andrevdh
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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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