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 
http://www.cs.uiowa.edu/~jones/step/types.html.

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

On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> wrote:
> 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 athttp://www.cs.uiowa.edu/~jones/step/types.html.
>
> 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


Hi
 There are two types of steppers. One type has a permanent magnet the
other
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
before
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
rates.
Dwight

<d...@hotmail.com> wrote in message 
news:7...@34g2000hsf.googlegroups.com...
> On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> 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. 
>> 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 
>> athttp://www.cs.uiowa.edu/~jones/step/types.html.
>>
>> 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?

> There are two types of steppers. One type has a permanent magnet the
> other
> 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
> before
> 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
> rates.

(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 
succession.

JJS

On 31 Jul., 17:57, "John Speth" <johnsp...@yahoo.com> wrote:
> <dkel...@hotmail.com> wrote in message
>
> news:7...@34g2000hsf.googlegroups.com...
>
>
>
> > On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> 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.
> >> 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
> >> athttp://www.cs.uiowa.edu/~jones/step/types.html.
>
> >> 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?
> > There are two types of steppers. One type has a permanent magnet the
> > other
> > 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
> > before
> > 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
> > rates.
>
> (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
> succession.
>
> JJS

or both: 1,1+2,2,2+3,3,3+1

how fast are you trying to run it?

-Lasse

<l...@fonz.dk> wrote in message 
news:8...@a70g2000hsh.googlegroups.com...
> On 31 Jul., 17:57, "John Speth" <johnsp...@yahoo.com> wrote:
>> <dkel...@hotmail.com> wrote in message
>>
>> news:7...@34g2000hsf.googlegroups.com...
>>
>>
>>
>> > On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> 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.
>> >> 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
>> >> athttp://www.cs.uiowa.edu/~jones/step/types.html.
>>
>> >> 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?
>> > There are two types of steppers. One type has a permanent magnet the
>> > other
>> > 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
>> > before
>> > 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
>> > rates.
>>
>> (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
>> succession.
>>
>> JJS
>
> or both: 1,1+2,2,2+3,3,3+1

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

> how fast are you trying to run it?

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. :(

JJS

> On 31 Jul., 17:57, "John Speth" <johnsp...@yahoo.com> wrote:
> > <dkel...@hotmail.com> wrote in message
> >
> > news:7...@34g2000hsf.googlegroups.com...
> >
> >
> >
> > > On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> 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.

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 
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 Carpenter          | p...@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>;    PC Services
<http://www.pcserviceselectronics.co.uk/fonts/>; Timing Diagram Font
<http://www.gnuh8.org.uk/>;  GNU H8 - compiler & Renesas H8/H8S/H8 Tiny
<http://www.badweb.org.uk/>; For those web sites you hate

In comp.arch.embedded,
John Speth <j...@yahoo.com> wrote:
>
> 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.

Really sounds like a sensorless brushless DC motor, as mentioned by another
poster.

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.
		-- Terry Pratchett, "The Light Fantastic"

"Paul Carpenter" <p...@pcserviceselectronics.co.uk> wrote in message 
news:MPG.22fc4bea9f75346b98969f@172.16.0.1...
>> On 31 Jul., 17:57, "John Speth" <johnsp...@yahoo.com> wrote:
>> > <dkel...@hotmail.com> wrote in message
>> >
>> > news:7...@34g2000hsf.googlegroups.com...
>> >
>> >
>> >
>> > > On Jul 30, 8:36 am, "John Speth" <johnsp...@yahoo.com> 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

"Stef" <s...@yahooI-N-V-A-L-I-D.com.invalid> wrote in message 
news:21ddc$4892c11d$54f63171$7...@publishnet.news-service.com...
> In comp.arch.embedded,
> John Speth <j...@yahoo.com> wrote:
>>
>> 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.
>
> Really sounds like a sensorless brushless DC motor, as mentioned by 
> another
> poster.

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 
properly.

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,

JJS

 John Speth <j...@yahoo.com> wrote:
>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 
>properly.
>
>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,
>
>JJS

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
alternating.