Re: Variable reluctance motor drive?
- From: "John Speth" <johnspeth@xxxxxxxxx>
- Date: Thu, 31 Jul 2008 08:57:48 -0700
<dkelvey@xxxxxxxxxxx> wrote in message
news:74a922b3-8485-4569-a99b-81517d94de63@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.
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
.
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