DC Motor : Torque relationship to speed
Torque is proportional to acceleration and the constant of proportionality is Inertia

\$T = J\dot{\omega}\$ 

What this model is doing is producing some frictional torque (usually denoted by B) that can be proportional to velocity.

This frictional torque appears as load torque hence the feedback. The example model really should make this clear by stating "load torque is proportional to velocity"

If you view the DC machine model there is a TL input &amp; this is "The block input is the mechanical load torque, in N.m. "

The m (measurement) port outputs:


Speed wm
Armature current ia
Field current if
Electrical torque Te


Thus, the speed \$\omega_m\$ is passed through a gain block (that would have units: \$\frac{Nm}{\omega}\$) to produce a torque, which is fed into the model LoadTorque port