Hey
My brother and I have been working on building a propeller clock and have salvaged two old harddiskdrive motors for the project. Our knowledge with these BLDC motors is very limited though and we are not sure how to build our own driver circuit.
The first motor has 3 leads and while the PCB from that harddisk is fried now, I did get the opportunity to take some measurements. Between one of the leads and the two others I measured a 7V and a 12V squarewave at 3.4kHz.
I don't really understand how that would be able to drive a BLDC motor, but maybe there are some internal electronics in the motor or something? If anyone has a suggestion for something to test with the 3pin motor it would be nice, but we are mainly concentrating on the second motor:
The second motor has 4 leads and from my research that means 3 inductors connected to one common point (the black one on the picture). Between any of the inductors I measured 3.2 Ω and between any of the inductors and the common pin I measured 1.7 Ω.
Here is a picture of the motor: http://wuhtzu.dk/random/hddmotor/mot...on_closeup.jpg
As I understand it the inductors are supposed to be driven in a sequence and since there is no feedback from a hallsensor, my guess is that the feedback comes from the read/write arm and the platters.
Therefore we took some measurements of the inductors (between any of the 3 leads and the common lead), but we didn't see what we were expecting. It was extremely hard to make anything out on the oscilloscope, but we managed to take some snapshots as you can see below.
http://wuhtzu.dk/random/hddmotor/oscil0.png
http://wuhtzu.dk/random/hddmotor/oscil1.png
http://wuhtzu.dk/random/hddmotor/oscil2.png
http://wuhtzu.dk/random/hddmotor/oscil3.png
http://wuhtzu.dk/random/hddmotor/oscil4.png
I'm not sure if those oscilloscope measurements are worth anything, but you can see some squarewaves that are directly on top of each other. What we expected were 3 squarewaves shifted 2/3 pi or something. In hindsight I guess we should also have taken some measurements between the inductors and the molex ground, but we didn't get around to that.
While it would be cool to use the PCB from the HDD to drive the motor this is unfortunately not possible since the motor just goes through a start-up phase where it spins up and down a couple of times and then turns off. Unless there is a way to disable that start-up sequence we need to build our own motor driver circuit.
I realize I haven't asked any real question yet but any input regarding these HDD motors will be very welcome.
If you don't want to read through everything I guess I can sum up some questions:
Are we correct or not in assuming the 4pin motor can be driven with a sequence of pulses to the 3 coils.
How does the 3pin motor work? (how many coils, internal electronics etc.)
Is there any way to circumvent the start-up sequence from the HDD PCB?
Perhaps this linky will help
http://www.epanorama.net/circuits/diskstepper.html
I personally know nothing about HDD motors but this page looks like it knows sumthin 
A quick scan through JAX’s link
shows that you may have problems driving the spindle motors properly w/o some kind of external ‘hall sensor’ input. Are you dead set on using HDD motors or is your construction flexible?
Hack A Day posted this build. They use a PC fan for the motor.