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Assuming I have a brushless DC motor which externally exposes only a + and - terminal. (I.e. I do not have access to the windings and hall sensor directly.)

Is there a way to control the speed? With a brushed DC motor one could just reduce the supply voltage.

Am I correct in assuming that this is not really an option for a brushless DC motor that will give a lot of range? I am thinking that most likely the internal commutation control circuit is powered using a voltage divider: if one reduced the supply voltage, the internal control circuit would experience low voltage dropout, right?

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It depends on the motor. Here are three cases I've seen.

(1) Generate a rather slow PWM signal. I used 100Hz. It was a small fan and it varied speed rather nicely.

(2) Fans with lots of smarts inside (I tore one apart and there was a Microchip PIC in there). It had a power-up delay of 100-200 milliseconds. That means that when your operating frequency approaches 5-10 Hz, the fan is off 100% percent of the time (it never completes the power-on). You can run pulses around 5 Hz or less, but it sounds ridiculous and you don't get much control.

(3) I put a big capacitor across the output in an attempt to convert the output pulses to an average dc level. Fan had an internal commutation circuit alright, and when I reduced the voltage, it would draw more current in an attempt to keep going the same speed! So you couldn't really control it that way, either.

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  • \$\begingroup\$ Thanks for this. For case (1), how noticeable was the PWM acoustically? \$\endgroup\$
    – ARF
    Jul 10, 2012 at 9:24
  • \$\begingroup\$ As I recall, there was no noticeable 100Hz noise. It was a low power fan, though. Expect results to vary with fan type. \$\endgroup\$
    – gbarry
    Aug 6, 2012 at 19:55

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