DC Motor Speed Increases Under Load - Modeling w/ Simulink

In summary, the conversation discusses the behavior of a DC motor under different conditions. Under no-load, the maximum speed is 180 rads/s, but under load, the speed increases to 290 rad/s, with a longer time to reach maximum speed and a decrease in acceleration. The speaker, who has a mechanical background, is surprised by this and asks for ideas. Another person mentions that torque is proportional to current and back EMF is proportional to RPM in a simple DC motor with brushes. The unloaded speed is determined by the applied voltage, and increasing load will slow the motor until the current rises to provide the required torque. The speaker acknowledges the issue with their model and thanks the other person for their input.
  • #1
Nick Goodson
7
2
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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  • #2
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.
 
  • Like
Likes jim hardy and cnh1995
  • #3
Ok, thanks Baluncore. I need to review what's happening.

Cheers
Nick
 

1. How does a DC motor speed increase under load?

Under load, the DC motor's speed increases due to the increased torque required to overcome the load. As the motor works harder to maintain a consistent speed, the speed increases in order to compensate for the added resistance.

2. What causes the speed of a DC motor to increase under load?

The speed of a DC motor increases under load due to the back electromotive force (EMF) generated by the motor. This back EMF opposes the applied voltage and causes the motor to increase its speed in order to maintain a constant torque.

3. How can I model a DC motor speed increase under load with Simulink?

To model a DC motor speed increase under load with Simulink, you can use the Simulink Library Browser to access the Motor Control Blockset. This library contains blocks that allow you to simulate and analyze the behavior of DC motors under different load conditions.

4. Is it possible to control the speed of a DC motor under load?

Yes, it is possible to control the speed of a DC motor under load by adjusting the applied voltage or by using a motor controller. By varying the voltage, you can control the torque and speed of the motor, allowing you to maintain a constant speed even under changing load conditions.

5. What are some real-life applications of DC motors that demonstrate speed increase under load?

One common application of DC motors where speed increases under load is in electric vehicles. As the vehicle faces varying terrain and resistance, the motor increases its speed to maintain a consistent torque and power output. Other examples include conveyor belts, compressors, and industrial machinery.

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