>Andrew Warren wrote:-
>>    Most old 5.25 drives run at around 300 RPM,....
>Indeed they do,
>>    If you're aware of this and trying to double the speed for a
>>    reason (or if your drive is really supposed to run at 750 RPM)
>>    you can certainly make it run at whatever speed you like.
>Yep, It would seem that it's a three wire stepper motor, but the controller
>is a closed loop type, I have tried systematically halving resistor values,
>doubling cap values etc. to find ANYTHING which altered the speed in some
>way. The closed loop system employed gives me a gradual startup, which is
>good for slowly accelerating to speed. Normal stepper motor circuits, have a
>problem of "just sittin thar a jitterin" :-)
>My application must be low cost, so these 5.25" drives are good. any one
>know a way to get that feedback working on a three wire?

[...]
>Damien Wiles (quartz@pavilion.co.uk)
>
>P.S. there is also a circular board under the flywheel with some kind of
>feed back component, but nothing I've ever seen b4 :-0
>also what look likes three hall sensors under there.


What you are describing is most likely a 3-phase brushless DC motor of the
"pancake" type which drives the spindle directly.  The three Hall sensors
detect the position of the shaft and select which winding is driven.  This
is done only with regard to the instantaneous position, not speed.  This
way, the Hall sensors route power to the winding that will generate maximum
torque in the correct direction.  Unlike a stepper motor, the brushless DC
is always trying to turn in the right direction and it doesn't get stuck.
The drive to the motor is DC, which is converted to AC as needed by the Hall
sensor feedback.

Speed feedback is often from a printed tachometer winding around the rim of
the flywheel.  This is simply a zig-zag trace on the circuit board that
interacts with a multipole magnet on the flywheel.  It generates a frequency
proportional to the speed of several KHz.  It is also possible to get
feedback from the Hall sensors, but this is not as smooth as they only make
a few cycles per revolution.

The speed control schemes I have seen are referenced from a ceramic
resonator and have no adjustments at all, although 1.2MB drives have a
digital selection of 300 or 360 rpm (high-capacity disks are recorded at the
higher speed).  Changing the resonator frequency or supplying an external
clock of a different frequency directly influence the speed, of course.
This is the only way to change it unless the division ratio inside the IC
can be changed, or you modify the feedback generator.  The IC divides the
resonator frequency down and compares it to the FG (Frequency Generator)
frequency using a digital phase/frequency comparator.  The output of this
comparator goes to an analog filter, then to the motor drive section to
control either linearly or with PWM the power applied to the motor windings.

It may be hard to get the motor to run at more than twice its design speed
without increasing the voltage applied to it, particulary if the application
requires any mechanical power out of it.

--Mike
SOMETHING IS WRONG HERE / YOU WON'T FIND IT ON A CHART
YOU'RE WELL PAID, YOU'RE WELL TRAINED / YOU'RE TIED TO A RACK
(Dead Kennedys, "Well-Paid Scientist")