Re: Variable reluctance motor drive?


"Paul Carpenter" <paul@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
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On 31 Jul., 17:57, "John Speth" <johnsp...@xxxxxxxxx> wrote:
<dkel...@xxxxxxxxxxx> wrote in message

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On Jul 30, 8:36 am, "John Speth" <johnsp...@xxxxxxxxx> wrote:
Hi group-

I've been experimenting with various stepper motor samples for
personal
education. One of my motor samples is what I'm pretty sure is a
variable
reluctance motor (VRM) salvaged from an old PC tape drive. It has
three
windings each connected at one end with a common high side
connection.

Sounds like it could be a Brushless DC Motor (BLDC) and the common is the
star common point of a 3 phase BLDC, number of poles would have to be
determined from the analysis of steps and steps required for one
revolution.

BLDC are quite common in disk and tape drives, this could be a sensorless
motor that relies on driving two coils and measuring the third in various
ways to determine position/speed/torque/load.

......
(I'm the OP)

I'm now having my doubts I am using a VRM. The motor has definite cog
stops
which I believe would indicate it's *not* a VRM.

Still sopunds like a BLDC.

The motor is clearly an OEM model. It has 36 cog stops. It has three
windings with a common point verified with an ohm-meter. I can see
nine
winding "lobes" through holes in the rotor. The windings are
stationary and
are mounted on a PCB. Each lobe's winding axis is radially oriented
from
the rotation axle and distributed evenly (360/9 = 40 deg separation).
The
rotor is cup shaped hiding a clear view of the internal parts. It
appears
to have some sort of black material ringing the inside periphery of the
cup
sides. I assume that is a permanent (or a group of permanent) magnets.

Sounds like a BLDC, look at Maxxon motors data sheets for a comparison.

This layout is typical, the nine lobes are the three windings split to
give an 'interleaved' windings on a PCB so the motive force is generated
at three points around the axis, possibly giving smoother and drive and
less vibration.

Have you checked the impedance of each winding and compared impedances?
If the impedance is doubled when measuring across two windings, this
MIGHT confirm the star configuration.

I think you're right about it being a BLDC motor. An ohm-meter does confirm
the star configuration (1.9 ohms at DC across one winding and double that
across two, very easy to confirm).

Unit is likely to be 12V drive (could be 5V), and can be driven with
3 phase sinusoid or trapezoid waveforms, varying frequency changes speed
of rotation, changing voltage drive gives changes in torque. By using
PWM it is possible to simulate the average drive voltage level to give
varying torque.

Basically you drive one winding to VCC (or PWM modulated) and another
winding to GND (or -ve rail), then step through a sequence to get
rotation. The star point is often used to measure the effects of third
winding being undriven becoming a generator.

I think that can be described as bipolar drive for a three phase Y motor.

Reversing the sequence gives you the reverse rotation.

This is documentated in lots of places and lots of website tutorials
exist.

I wonder if this motor is a special type of three winding stepper motor
that
would require microstepping of some kind to spin it up and down
smoothly.
Its former function (IIRC) was a direct drive capstan motor for a PC
tape
drive. Without microstepping, there's only two ways to drive it: turn
on
one winding at a time in succession or or two windings at a time in
succession.

Capstan motors are/were often BLDC motors for simplicity of driving,
cheap to make, efficiency and controllability.

If you are getting any rotation currently by only driving one winding
at a time it is inefficient.

I have driven BLDC from a simple controller and FET drives from 0-1000's
RPM. Get it working in open-loop mode first then start adding feedback
to get closed loop control.

Paul (and others who responded via usenet and email), thanks for the
excellent advice and pointers!

In summary: It's most likely a BLDC and certainly not a VRM as I originally
thought. My homemade stepper driver is built for one way current switching
(unipolar) so major changes would be needed to achieve bipolar current
switching (IOW, some sort of H bridge driver).

JJS


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