# Is there an ideal PWM frequency for DC brush motors?

All QuestionsCategory: Electrical EngineeringIs there an ideal PWM frequency for DC brush motors?
I will be using a microcontroller to generate a PWM signal to control the motor. I understand how PWM and duty cycle work, but I’m not sure about the ideal frequency. I don’t have my own motor yet, so I can’t test it out and find out.

I will not change the voltage, only at the time when it receives a certain voltage. So can I assume a linear response? On 10% running and 24V supply it will run at 15rpm?
If there is a difference, I will include the setting. I run 24 volts directly to the H-bridge that controls the motor. I obviously have PWM pins going from the MCU to the two gates that enable MOSFETS.
Edit: Sorry, the link doesn’t seem to work. I think the firewall at work doesn’t like imgur. The image depicts a graph of RPM versus voltage. It is linear from 50 RPM @ 8 V to 150 RPM @ 24 V.

1 Answers

You have a linear control of the ‘speed’ by applying a pwm signal, now the frequency of that signal must be high enough that the dc motor will only pass the dc component of the pwm signal, which is only the average. Think of the motor as a low-pass filter. If you look at the transfer function or the relationship between angular velocity and voltage, this is what you have:
Is there an ideal PWM frequency for DC brush motors?
This is the first model of a DC motor or simply a cut-off frequency low-pass filter
Is there an ideal PWM frequency for DC brush motors?
where ττ is the motor time constant. As long as your frequency exceeds the maximum, your motor will only see the DC portion or average of the PWM signal and you will have a speed compatible with the PWM working cylinder. Of course, there are some trade-offs to take into account if you go with a high frequency…
long story:
In theory, you would need to know the motor’s time constant in order to choose the “correct” PWM frequency. As you probably know, the time it takes for a motor to reach nearly 100% of its final value is
Is there an ideal PWM frequency for DC brush motors?
Your PWM frequency must be high enough that the motor (essentially a low-pass filter) will produce an average input voltage, which is a square wave. For example, suppose you have a time constant motor τ = 10msτ = 10ms. I will use a first-order model to simulate its response to several PWM periods. This is the DC motor model:
Is there an ideal PWM frequency for DC brush motors?
Enter image description here
But more important are the responses we’re looking for. For this first example, the PWM period is 3τ3τ and the duty cycle is 50%. Here is the engine response:
Enter image description here
The yellow graph is the PWM signal (50% duty cycle and 3τ period = 30ms3τ = 30ms) and the purple graph is the motor speed. As you can see, the motor speed oscillates over a wide range because the PWM frequency is not high enough.
Now let’s increase the PWM frequency. The PWM period is now 0.1τ = 1ms0.1τ = 1ms and the duty cycle is still 50%.
Enter image description here
As you can see, the speed is now largely constant because the high-frequency components of the pwm signal are filtered out. In conclusion, I would choose a frequency of at least fs≥52πτfs≥52πτ.
This is just a theoretical explanation on how to choose the PWM frequency. Hope it helps!

Scroll to Top