Thomas,

Like everyone says, freq is not critical, but I have observed the
following. Low freq (100's of Hz) sounds growly and feels more
torquey than high freq (kHz). Can be better at getting a motor
started at low speed as the current pulses are somewhat "lumpy"
and get's the bearings over their static friction "stickiness".

High freq is good if you can't hear it, or can be annoying to some
when it can be heard. High freq gives smoother motor operation,
but a more logarithmic response to the PWM ratio. This shows up
as apparent poor torque at low PWM settings. This is because the
armature current at a given PWM ratio is lower for a high freq pulse
than a low freq pulse,  because the inductance of the armature
means current reduces as freq increases. You still have the same
range of control of 0 - 100%, but it's not so linear in response to
PWM ratio anymore.

Hope that doesn't complicate things too much - I meant to explain
things! In reality I would choose PWM freq based on tests on the
actual equipment it will be operating. Aim for a sound, feel and
accelerator response that is acceptable. Failing that, anything
works!

> Hi everyone,
> I am using PWM to control the speed of a 4.5V DC motor.  Would you
>please
> tell me at what frequency I should set the PWM to run at?  What
> are the advantages/disadvantages for low and high PWM frequency?
> Thank you very much!
> Thomas


Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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