[TriEmbed] laptop fan motor control question

[Brian]

On 2/3/21 11:37 AM, Pete Soper wrote:

> As I shared with you, the last "weird motor" I encountered was from a 
> server disc drive with just three windings in series.
> 
> I didn't include yesterday that I learned in the hardest way imaginable 
> how difficult it is to control such a motor without arranging to sense 
> the back EMF the motor design requires. The summary is that it is a 
> fool's errand. There are no Hall effect sensor(s) in that motor, in case 
> somebody is tempted to 'splain this.  I spent an embarrassing number of 
> days on that, against the hard stop in Edison mode and wishing I'd spent 
> five minutes in Tesla mode earlier. By the time I realized I had to use 
> a controller my time budget had been exhausted five fold and I never 
> confirmed the Phillip's chip was the right part. Bought a stepper motor 
> and got on with my life. 😀
> 

Steppers and synchronous DC motors are actually quite similar in theory. 
  A synchronous DC motor could be thought of as a stepper with a "step 
angle" of 60 degrees (or 120, depending on how you look at it).

It should be possible to drive a brushless DC motor open-loop, as long 
as the excitation signal is sequenced such that the rotating magnetic 
field doesn't attempt to accelerate the motor beyond its ability to keep 
up.  That said, of course the easiest way to drive one is to hook it up 
to a controller. XD

I have a small pile of HDD spindle motors just waiting for the day I rig 
up a VFD[1] to drive them.

-B

[1] - Variable Frequency Drive (as opposed to Vacuum Fluorescent Display)