2
\$\begingroup\$

I'm planning to pwm-control four of the following motors by a single ArduinoUno: http://uk.rs-online.com/web/p/axial-fans/8179151/

For some reason the nearest pwm frequency I can set is 31kHz.

The other options are: 244, 488, 976, 3096, 7812 and 62500 Hz

In the data-sheet of this motor: http://docs-europe.electrocomponents.com/webdocs/13cc/0900766b813ccfde.pdf it is written "※PWM Frequency:25kHz"

Would that be an issue if I use 31kHz or one of the other frequancies? Which one would work or none?

\$\endgroup\$
  • \$\begingroup\$ The datasheet suggests those fans already have PWM controllers built in. There must be more data on them somewhere - probably for the "9HV type" as a whole. Find and read it. \$\endgroup\$ – Brian Drummond Jun 24 '16 at 11:21
  • \$\begingroup\$ What do you mean? The input I should provide will be pwm or DC? \$\endgroup\$ – user16307 Jun 24 '16 at 11:30
  • \$\begingroup\$ I don't know, that's what the other datasheet will tell you. \$\endgroup\$ – Brian Drummond Jun 24 '16 at 11:36
  • \$\begingroup\$ @BrianDrummond Thanks I found a more detailed datasheet. It seems like the input should be a pwm signal at 25kHz. Olin Lathrop says this is the minimum freq. Here is the datasheet farnell.com/datasheets/san-ace-120.pdf The sink and source currents are 1mA. Would I still need another transistor after Arduino's pins or can I directly hook up to DC motor pwm pin? I just dont wanna ruin it before making a test at 31kHz \$\endgroup\$ – user16307 Jun 24 '16 at 11:55
  • 1
    \$\begingroup\$ The datasheet specifies 25kHz as the PWM frequency it is tested at. 31kHz should be sufficiently close to 25kHz that the only difference will be slightly increased switching losses in the PWM, and slightly increased eddy current losses in the motor, but you'll be able to use slightly smaller filter components. \$\endgroup\$ – Neil_UK Jun 24 '16 at 11:58
7
\$\begingroup\$

31 kHz should work fine. It's not clear why the motor datasheet mentions a specific PWM frequency, but that should be a minimum frequency. Higher frequencies (unless you get much higher) shouldn't matter to the motor.

There are at least two reasons they might specify a minimum PWM frequency:

  1. Audible whine. Individual windings of the coils experience magnetic forces. These will cause some vibration, which can be audible and quite annoying. 25 kHz is often used because that is just above the hearing range of most people. The manufacturer may know that their windings will emit significant audible whine, so tell you to use a frequency you can't hear.

  2. Current smoothness. The current in the windings ramps up and down proportional to the applied voltage times time. The shorter the time, the smaller the variations about the average current. The ideal current is flat, with the actual current being roughly a sawtooth.

    You want the variations to be small compared to the average. Only the average current drives the motor. The high frequency AC component doesn't push the motor forwards but still contributes to I2R heating. The manufacturer knows the inductance of the windings, and may have decided that 25 kHz is necessary to minimize wasted current to meet other specs.

Note that both these reasons set a minimum PWM frequency, but allow it to be higher. The main reason for not going too much higher is that the switching transitions in your circuit take a finite time, and at higher frequency this time is a larger fraction of a whole cycle. That means your circuit will be less efficient as the frequency goes up. However, that's under your control. You should be able to build a power driver that switches at 31 kHz without significant switching losses.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.