DC Motor Speed Increases Under Load - Modeling w/ Simulink

AI Thread Summary
Modeling a DC motor in Simulink shows unexpected behavior, with speed increasing from 180 rads/s under no-load to 290 rads/s under load, despite longer acceleration times. The user, with a mechanical background, questions why speed increases under load, expecting a decrease instead. Key concepts discussed include that torque is proportional to current and back EMF is related to RPM, indicating that applied voltage influences unloaded speed. The conversation suggests a potential issue with the user's model that needs review. Understanding these principles is crucial for accurate motor modeling.
Nick Goodson
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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.
 
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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.
 
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Likes jim hardy and cnh1995
Ok, thanks Baluncore. I need to review what's happening.

Cheers
Nick
 

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