Tone sound on fan

[DavidDLC]

I recently added a PWM controller to my desk fan in order to be able to modify the speed and therefore the amount of breeze.

I can notice a squeak sound about 1 KHz I will say coming from the motor, and I want to get rid of it, what kind I do ?

Regards.

[dimlow]

Up your PWM frequency, I did that same once, the tone gets real annoying after a time

[Simon]

When you power a motor the rotor will push forward as well as go round. if your using PWM to control it the shaft will oscilate making the buzing sound. I once knocked up a quike PWM controller for an 84W can blower to use at work, I soon installed an optional capacitor to increase the frequency. Lower frequencies got me more power less heating in the control transistor and a sqeek. The high frequency was silent but needed more power for the same speed but was more controllable at low speeds

[Trigger]

Somewhat related.  I remember a talk at Blackhat that used that effect with some modulation to send the users password to a listener with a directional microphone and some signal processing.

http://www.blackhat.com/presentations/bh-usa-08/Filiol/BH_USA_08_Filiol.zip
Here, took me a bit to find it in the archives but it's interesting.

[scrat]

Of course the increasing the PWM frequency will work.
Why not trying with a little L-C filter, to smooth out the PWM carrier?

[Simon]

well thought that PWM gave more torque to the fan at lower speeds than controlling the speed with lower DC voltages which is what an LC filter would do

[Zero999]

Try increasing the frequency.

[NiHaoMike]

Another trick is to try a spread spectrum clock.