Where,
Ra : Armature resistance;
La : Armature inductance;
ia : Armature current;
if : Field current;
ea : Input voltage;
eb : Back electromotive force (EMF);
Tm : Motor torque;
ω: An angular velocity of rotor;
J: Rotating inertial measurement of motor bearing;
Kb : EMF constant;
KT : Torque constant;
B : Friction constant.
Because the back EMF eb is proportional to speed ω directly