# Dc motor clarification?

## 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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vela
Staff Emeritus
Homework Helper
From the hand-drawn diagram, it's pretty clear that ##V_a = E_a + I_aR_a##.

andrevdh
Homework Helper
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 .