On Jan 11, 2006, at 8:27 AM, John Hennessy wrote:

> I have a box full of Hard Disk Drive spindle motors recovered
> from IBM (Sun) 4.2Gig Hard Disk Drives. They have 4 connections.
> My question is: Are they Bipolar Stepper Motors ?
>
They're typically three-phase motors, which operate somewhat
similarly to stepper motors.  The 4th wire could be a common
ground or a hall-effect sensor output for speed feedback.
Unlike stepper motors, they're designed for continuous rotation
rather than stepping; I would expect current consumption needed
get decent "holding torque" to be quite excessive compared with
most steppers, for instance (put another way, I think the windings
of a stepper are designed to be self-current-limiting due to their
inherent resistance, while those of a spindle motor are designed
to be current-limited by their inductance and the expected drive
waveform at the design RPM.  Or something like that.)

The RC people have made a business and art form out of converting
spindle motors (mostly from CDROM drives, but anything goes) into
high power brushless motors for airplane propulsion.  See
http://www.gobrushless.com for basic info.  I don't think I've
seen much on using such motors in general purpose applications
or as stepper replacements...

BillW
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