Variable reluctance motor drive?

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Hi group-

I've been experimenting with variable 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.  From
my reading of how to drive a VRM, it appears it's driven just like a stepper
motor: energizing one winding at a time in succession.  Basically, I'm
following the method found at .

I have an on/off stepper driver (three switches) which I can switch a
constant current on and off (200 mA) under microcontroller control.  I just
can't seem to get this VRM to turn reliably.  It steps but often steps
backwards and it has a weak holding torque.  I'm having my doubts that I'm
drving it properly.  I'm starting to think that driving a VRM is much more
complicated than the above web site suggests.

I've searched the web for relevent information about VRMs and I can't find
anything that goes into any great detail.  It's either not covered to my
satisfaction on the web or I'm doing something completely wrong.

Can anyone please point me to a resource that I can use that will help me
uderstand the drive requirements of a VRM?

Thanks, JJS

Re: Variable reluctance motor drive?
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 There are two types of steppers. One type has a permanent magnet the
does not.
 This means that one will hold with no power applied while the other
will not.
For the type that has no magnet, turning the power off to a winding
it is safely centered on that pole will cause it to start up in an
erratic direction.
 Does this sound like the problem your having?
 The other thing is that you have to ramp the stepping rate and avoid
staying in the resonant region for that system when changing step

Re: Variable reluctance motor drive?

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(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.

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.

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


Re: Variable reluctance motor drive?
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or both: 1,1+2,2,2+3,3,3+1

how fast are you trying to run it?


Re: Variable reluctance motor drive?

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You're right.  That's what I meant to say but failed.  Both ways still cause
erratic stepping.

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I'm running it very slowly, about 4 steps per second.  I could be fighting
some sort of resonance problem.  I've noticed that stepper motors like a
certain amount of torque load which might be part of the problem.  There's
so little I know about this motor. :(


Re: Variable reluctance motor drive?
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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

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.

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Still sopunds like a BLDC.

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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.

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.

Reversing the sequence gives you the reverse rotation.

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

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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 Carpenter          |
< PC Services
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Re: Variable reluctance motor drive?

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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).

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

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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).


Re: Variable reluctance motor drive?
In comp.arch.embedded,
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Really sounds like a sensorless brushless DC motor, as mentioned by another

Easiest way to drive these is by using e specialized driver IC, the
TDA5140A is one example of such a chip.

Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Early to rise, early to bed, makes a man healthy, wealthy and dead.
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Re: Variable reluctance motor drive?

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I agree - all evidence support that observation.

As a newbie, I don't understand by it's called a brushless DC motor.  The
"brushless" part is obvious.  But the "DC" designation seems wrong.  It
clearly needs an AC drive applied with just the right timing to work

According to Wikipedia, a BLDC motor is the broad class of motors that
includes VRMs and stepper motors.  In conclusion, I believe my motor can
also be called a three phase Y bipolar drive stepper motor which is in the
BLDC class.

Thanks for the help,


Re: Variable reluctance motor drive?
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It's DC because the current is only traveling in one direction.  That is
from the driver through the coils and to the return.  In AC it would be
traveling in both directions, first one then in the opposite, making it

Re: Variable reluctance motor drive?
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The current travels in both directions as the phase rotates (remember
each phase on the Y is driven at different points as +, - and floating).
Each phase sinks or sources current depending on what portion of the
drive cycle it is in. The only difference between a BLDC and a PM AC
motor is the name.  Some make a distinction on the drive waveform but
then those who drive BLDC with sine waves couls claim that they were
converting them into PM AC motors.

BLDC came to be called DC mainly as a result of them being considered DC
motors turned inside out and having the commutation done electronically*
rather than via brushes as near as I can tell.  Also they conventionally
are fed from a DC bus where AC motors are conventionally fed from the AC
line.  Of course with modern drives that has changed but we haven't
started calling AC motors DC simply because we feed them from an

Switched reluctance motors are a different beast.  They are generally
driven in a single direction through the coil (which direction doesn't
matter).  I still don't really consider them a DC motor myself but they
really don't fit the term AC either.


* and thus sometimes they are refered to as ECM
(electronically/externally commutated motors).

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