Keith Causey wrote:

> 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.

> What am I missing about the way these motors are controlled?

  Let me run a few ideas past you.

  My understanding of these motors is that the assembly = PCB with some
sort of flat coils on it and facing or sandwiched between the magnet
disc (and a field plate), is the *whole* subsystem containing the driver
IC including the speed control as well as the tachometer drive which is
three Hall-effect sensors (look like surface-mount transistors).

  The four wires to the assembly are usually +12V, Gnd, Motor On and
Speed Select (as these drives used different spindle speeds for DD and
HD modes).  They do not go to the coils directly.

  The coils will be connected in a "star" configuration with a single
common which does *not* connect to the drivers.  Drivers are a three-
leg "H" bridge.  The star configuration maximises the effective number
of turns in the windings - in effect there are always two windings in
series.  This is a standard point of motor design - don't use any more
wire than necessary.

  As the motors are electronically commutated, they will not generate
a voltage output when spun.  To do so, and they are certainly *not*
designed to do so unlike some traction motors, would require a second
set of FET switches and control circuitry.

  As originally designed, for their intended loading, the coils are
simply commutated, not driven with sinewaves.  The reason for using
three phase windings is to provide fairly smooth torque, with residual
fluctuations smoothed out by a heavy flywheel (compared to the available
torque).  The later versions with quite small flywheels *might* use some
sort of PWM.

  In general, you have a motor optimally designed with a driver circuit
built-in.  I don't know whether many are actually digitally speed
controlled with a resonator, or whether they use a temperature corrected
R-C timebase (like a 12C508!).  The inbuilt driver is fine unless you
want them to go particularly slowly, or beyond the design torque, which
is I suppose exactly what you are doing!

  I trust that you have already *disconnected* the built-in driver,
otherwise I would take a fair guess at what is smoking!
--
  Cheers,
        Paul B.