DC Motor Speed Increases Under Load - Modeling w/ Simulink

[Nick Goodson]

Hi, I am modelling a DC motor using Simulink.
Under no-load conditions, I get a max speed of 180 rads/s, but under load, the speed goes up to 290 rad/s, although takes longer to reach max speed, acceleration decreases.
I have a mechanical background so don't know much about motors, but would expect that, under load, the speed would reduce?
Any ideas?
Many thanks.

 

[Baluncore]

Assuming it is a simple DC motor with brushes.
Torque is proportional to current. Back EMF is proportional to RPM.
Applied voltage – back EMF appears across the rotor resistance and so decides current.
Unloaded speed will be proportional to applied voltage.
Increasing load will slow the motor until current rises to give torque required.

You have a problem with your model.

 

[Nick Goodson]

Ok, thanks Baluncore. I need to review what's happening.

Cheers
Nick