# DC Motor model question

Gold Member
Hello. Just started learning about how to model a DC motor. A simple model consists of the input Voltage in series with an inductor, a resistor and a back emf. I have two questions.

Why have both the inductor and the back emf (voltage source)?

If you know that at a voltage V and a load L1, the motor will have a steady speed of w1.
Now say we remove the initial load and attach another load L2 keeping the input voltage at V.
Will the motor still be able to reach a steady speed (i know it will not be w1)? In other words will it accelerate/decelerate or have a constant velocity?

EDIT:
So now I just realized it has to accelerate/decelerate in order to reach a steady state, I also figured out that it cannot keep accelerating (that would defy the law of conservation of energy). So now I have another question, does the back emf change accordingly to adjust the current which in turn adjusts the torque (is this the "intrinsic feedback of the motor")? Is it this process that controls the acceleration?

Last edited:

meBigGuy
Gold Member
The back emf adjusts with speed thereby controlling the current through the resistor. The torque is proportional to the current through the resistor. The power is the angular velocity times the torque and is also the current times the back emf. The inductance is there because it affects the overall ideal transfer function.

The following is a nice explanation:
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling

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