5 Feb
2011
5 Feb
'11
7:46 a.m.
[Spindle motor problems]
I'm sure the problem is not with the light beam
detection as the stepper
always moves. Somewhere between the switch and the motor itself the signal
seems to get lost, or at least isn't strong enough to start the flywheel
off. It interesting that the problem has lessened with use. I thought
maybe the flywheel was a bit sticky but it turns very freely. I've oiled
the spindle also.
These motors are so-called 'electornically commutated'. Rather than
having a mechncial commutator and brushgear like the mtoros you see in
many books, they have (normally) 3 sets of windigns that are
electronically controleld and sequenced in the right order nad at the
right freqency to get the motor to turn. The position of the
rotor/flywheel is detected by hall effect devies (magnetic sensors) on
the PCB.
If one hall-effect sensor or the drive to one set of windings is faulty,
then the motor may well stop in a place where it can'r restart (in that
there will be no drive to any winding when you try to start it). Of course
once the motor has got going, the intertial of the flywheel keeps it
going past the 'bad bit'.
The motor is probsbly cvntorlled by a semi-custom IC. If you are lucky it
will only be semi-cucstom, certainly in some of the floppy drives I've
repaired, I've mamanged to get the motor cotnrol ICs as spare parts.
Apparently they were used in VCRs for motor control too [1].
You migth try entering the numbers of an ICs near the motor widings and
hall effect devices into datasheetarchive or similar to see if you can
get data sheets,. If you find a 'burshless motor control IC' or simialr,
it's a fair bet you've found th espindle motor driver chip. Then you can
use a 'scope to see if it's getting inputs from the hall-eddect sensors
and driving the coils properly.
[1] Not quite the same thing, but the motor speed control ICs in my Canon
CX printer have a pin labelled '33/45' in the data sheet. They were
obviosuly designed for use in record player turntables...
-tony