The speed feed back required for all large (multi HP) motors is nothing more
than a permanent field(magnetic) motor.  If you spin a dc motor you will
receive an arm voltage proportional to motor rpm.  Be sure to buffer tho, as
on a windy day the feedback may be very high.  An average tach could produce
100vdc at a 1000rpm
CSS

                -----Original Message-----
                From:   Adam Bryant [mailto:adamdb@JUNO.COM]
                Sent:   Monday, November 08, 1999 11:17 AM
                To:     PICLIST@MITVMA.MIT.EDU
                Subject:        Re: Spindle motor driver with an SX28 (and a
related PIC question)

                Keith
                Aren't these DC motors?  It sounds like you are trying to
drive them with
                AC?  I have several of these that I scavenged from full
height 5 1/4"
                floppy drives and it was my understanding that of the 4
lines coming out
                of the motor, 2 were used to drive the motor (DC voltage and
ground) and
                2 were used as feedback to the controller board for speed
control.

                I had a related question about these motors: How do you
measure motor
                speed (RPM) via these two extra wires?  From experimenting
(IIRC, it's
                been awhile since I did this) you could drive the motor from
a battery
                pack by applying V+ and ground to either pair of the wires.
That would
                lead me to believe that if you spun the motor, both pairs of
wires would
                generate voltage dependent on how fast the motor was spun.
Read the
                voltage generated with an A/D and you can then figure out
the motor RPM.

                My thought was to use one of these motors to build a wind
speed indicator
                for our R/C flying field (with a PIC in it of course).
Mounted
                vertically with the 4 cup arrangement attached to the shaft,
it should be
                possible to get an accurate enough indication of wind speed.
Any
                thoughts on whether or not this motor would generate enough
juice to
                power this circuit (PIC 16F84, A/D, plus 5 to 8 LEDs for
wind speed
                indication)?

                On Mon, 8 Nov 1999 20:40:01 -0700 Keith Causey
<ffight@GEOCITIES.COM>
                writes:
                > Hello all, I am attempting to drive a spindle motor (the
type used in
                > floppy
                > drives) with 3 duty-cycle generated sine waves 120 degrees
apart.
                > The SX
                > generates these signals from a look-up table. These
logic-level
                > outputs are
                > converted to +- voltages with a comparator. This bi-polar
output is
                > then fed
                > into 3 totem pole amplifiers consisting of 2N3904's and
2N3906's.
                > The
                > resistance of each of each coil is 4.6 ohms. It barely
works.
                > Voltages above
                > +-3 smoke everything and the positive side of the drivers
gets
                > distressingly
                > hot. Also I couldn't find a ground return for the 3 coils
so I just
                > made one
                > (they were connected - just not routed anywhere) What am I
missing
                > about the
                > way these motors are controlled? Thanks - Keith Causey.

                Adam Bryant (age 0x23)
                adamdb@juno.com
                Parker, CO, USA
                Robotics, RC Airplanes, anything using a PIC


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