Why on earth would anyone use Simulink for motor modeling? This is a conserved mixed-technology system; current, voltage, thermal, magnetics, etc. And after you have the motor... what next? What loads will you attached? And what design questions are you trying to ask? (e.g. What happens when the shaft experiences a large inertia change due to the load?) Much better to use a lumped-sum simulator like Saber. Simulink excels however for non-conserved, signal-flow (control) systems.
That said... Anyway since a DC motor is linear (and I assume you mean a DCPM motor), is typically a 2-pole system, and trivial to write out a level-1 Laplace transform, but needs a couple pieces (again need to apply feedback for conservation of energy). If you have done this correctly you likely would not be asking this question, because the terms for armature resistance, inductance, inertia, damping, kt, and ke would be staring at you. These are the typical parameters found on any DCPM motor spec sheet.
If you care to share your derived expression (or model somehow), I'm sure I or someone can tell you what's wrong. Since Simulink models are so verbose, I cannot post my DCPM motor version here (yeah despite my earlier complaint, I did construct one for fun - It works, but not anything I would make design decisions from).
Hope this helps!
