A simple dynamic model of a controlled DC motor consisting of a variable voltage source, a resistor, an inductor, and an electro-motoric force element representing the coupling between electric energy and mechanical energy provided by the magnetic field in the DC motor. The motor axis is represented by a rotating inertia.

Comparison of step response for the rotational speed of the inertia using the default parameters (DCMotor 1) and with resistance set to 10 Ohm, inductance 0.1 H, and moment of inertia to 0.3 kg m²  (DCMotor 2):

For a step by step tutorial see Multidomain—A Servo Mechanism.