# Modelling a DC motor

I am modellign some examples using the modelling language Modelica. Unfortunately, I am not an electrical engineer, so it is hard to understand the physical behavior of the following example (DC motor):

Where emf "transforms electrical energy into rotational mechanical energy". The model "emf" looks like this:

The input voltage is plotted in the following graphic (top). I dont understand the following things:

1. Why is the current zero in steady state?
2. My interpretation: In steady state, the voltage input is equal to the voltage drop at the emf. The angular velocity is proportional to the voltage drop at emf.
3. What is the "emf model" in a real DC-motor?

Thank you very much for your help

• Note: your model is seriously flawed, with one fixed L. In reality, you'll have multiple coils, which are toggled (and have polarity reversed) sequentially, in relation to $\phi$. That's essentially AC supply to the inductive load, with frequency proportional to the angular velocity and phase offset by $x {2 \pi} \over n$ for x-th out of n coils. – SF. Sep 18 '17 at 7:38

2. Yes, your interpretation is correct. You modelica component explicitly state that with equations kw=v and tau=-ki. As an alternative, different constants such as $k_v$ and $k_i$ can be used. Because of the conservation of energy, you must pay attention to input-ouput energies. In your model $k=1$, so it is ideal energy converter. Input power to EMF component, $P_{in}=v_{emf}\cdot i$ and output power of the EMF component $P_{out}=w\cdot\tau$ is equal (on the EMF component). The heat lost of the motor is represented by resistor.