# Simulating a DC motor with a constant friction torque

I'm trying to simulate a DC motor with simulink. The problem is that I don't know how to take a load torque into account. If the DC motor drives a wheel with constant velocity, there will be a constant frictional force working against the DC motor. How would this be implemented in a DC-motor simulink simulation?

I have tried subtracting a load torque in between the two transferfunction blocks but then the simulated motor speed is always goes negative.

• Constant torque means constant current. I don't see you modelling current yet, it's all hidden in K. Mar 30 '17 at 12:18
• @Janka I believe the current is modeled by the $\frac1{R+Ls}$ part. Mar 30 '17 at 12:54
• "there will be a constant frictional force working against the DC motor" is an error. Retarding force on a motor is load + friction + windage. Friction and windage effectively go to zero at zero speed. (For your purposes). friction + windage go negative at negative speeds. Each follow some curve on ether side..... Mar 30 '17 at 14:37