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### Topic: determining pwm frequency for a dc motor (Read 1 time)previous topic - next topic

#### ozanenginoglu

##### Mar 23, 2010, 12:32 am
Hello,

I will be using two exactly same dc motors for my projects and for speed control i will use pwm. The problem is that i don't know the best frequency for these motors. How do i calculate it?

#### windhamrules

#1
##### Mar 23, 2010, 12:57 am
Im a bit confused as to the question maybe, but I will try to offer up the answer that I feel is what you are looking for...

There isnt a specific PWM frequency for a specific motor. If you are looking to control the speed (and torque I might add) of the motor then you can pulse the power to the motor (Pulse width modulation). For example if you have a motor that turns at 2000 RPM @ 5VDC, that is at a PWM of 100%. If you supply a 50% PWM you would see roughly 1000 RPM @ 5VDC. The motor would be getting power 50% of the time, thus roughly half the speed (and torque). At 0% PWM the motor wont move. There is also typically a MINIMUM PWM to allow the motor to overcome its own resistance. With the motors I work with its roughly 10% PWM. At 10% PWM the motor barely moves.

I hope this helps you in some way.

Thanks,
Chris
Chris
Its hard to put the smoke back in the wires

#### ozanenginoglu

#2
##### Mar 23, 2010, 01:48 am
Thank you Chris for your reply. I understand that with that logic you change the duty cycle of the pwm and manage to drive motors with the desired speed but the signaly you are applying to set pwm is a square wave and it has to have a frequency right?
In the lab of my uni. i tried that with a function generator and whenever i changed the frequency, the motor's response has changed. There should be an optimum frequency but i really don't know how to calculate it.

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