Re: A little experiment on demagnetization

krnldmp,

I'll insert my comments in the body of your message:

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "krnldmp <krnldmp@y...>"
<krnldmp@y...> wrote:

> I'd like to add a couple small points here, and thank you Mariss for
> taking this out of the purely academic realm for us.
>
> I think one of the things that the PM field does in the hybrid

stepper

> motor is drop the nominal inductance at the windings (for any given
> configuration) so that higher pulse rates are practical with good
> output torque. Your test was run at 1kHz, relatively slow for some

of

> the smaller motors that typically concern the group. The idea that

the

> motor output is fully characterized at this frequency and that the
> percentage loss arrived upon is sublte may be a bit hopeful.

My dyno setup sweeps the test motor from 0 to 50 revs per second
(10,000 full steps) and collects data every 100 fs/sec in a single
pass. The motor load is a closed loop that servos the test motor
shaft to a constant 3.2 degrees lag (about 95% load) regardless of
speed. The torque necessary to maintain that lag angle is logged by
an embedded MCU as 100 points of data over this range.

The "53 in-oz" and "82 in-oz" motors were deficient over the entire
range from holding torque to 50 revs per sec relative to the third
motor by the same respective and constant percentages.

I gave a single test point because it was unremarkable and
representative of the motors' deficit over the entire range.


Since the

> saturation level of the core material is allowed to fall lower with
> less bias the permeability of that material rises and therefore

causes

> the inductance to rise, making it difficult to get magnetizing

current

> through the windings at higher frequencies.

There was no discernable inductance rise. In all 3 cases it measured
2.1mH. I prefer to use the di/dt method of measuring inductance, di
was 0.24A while dt was 20uS, yielding 12kA/sec for V/L. V was 25.1VDC
which gives 2.083 mH.

>
> So, power being torque times speed, you might get yourself down to

25%

> of the 'unmolested" motor or even a smaller percentage, depending on
> how the motor is used in the application. I don't think holding

torque

> is affected at all by demagnetization, because once the rotor is
> stalled and in step with the enrgized phase at max rated DC current,
> the magnets really aren't part of the picture.

If that were true then an unmagnetized iron rotor would have holding
torque. It doesn't.


This may be the thing

> that makes it all seem subtle. But, once you try to Drive the motor,
> faster and faster, it becomes quite apparent that the thing has been
> molested er something.
>
>
> Precisely because of the dropping of winding inductance in eight

wire

> parallel configuration, the advantage to be had from converting a 6
> wire unipolar over is much higher that 1%, and so long as the rotor

is

> not axially displaced, demagnetization from the operation is not an
> issue.

Not true. Half-winding operation gives exactly the same torque as a
parallel connected 8-wire motor at the same current. The ampere-turns
stay constant; the difference is the effective wire gage is bigger
because the winding is bi-filar then. Only I^2*R losses change.

This loss becomes negligable when the motor's inductive reactance
limits current and not the drive.

>
> The idea of sleeving a stepper motor rotor, being much different

than

> a PMDC motor, is freaked out on account of the mere couple few
> thousandths gap that there is to work with.... Where a PMDC motor

with

> alnico magnets (WHERE can you find THAT these days?!!!) might have
> tens of thousanths.
>
> Ps... I love motors.
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
> <mariss92705@y...>" <mariss92705@y...> wrote:
> > Jeff,
> >
> > 1) Yes. Do it in a clean area so stuff doesn't get dragged into

the

> > motor.
> >
> > 2) Yes, but what's the point? Weigh the 1% or so performance gain
> > against the likelyhood of an accidental "oops" situation. It

doesn't

> > make any sense to me.
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp

<xylotex@h...>"

> > <xylotex@h...> wrote:
> > > Hi Mariss,
> > >
> > > OK, no conclusions this time, only questions:
> > >
> > > 1) Can you safely open a stepper motor (without removing the
> rotor)
> > > and not loose magnetization? If so what precautions should be
> > taken?
> > >
> > > 2) Can a 6-wire motor be opened and rewired to make it 8-wire,

and

> > > maintain full magnetization, if necessary precautions are taken?
> > >
> > > Thanks,
> > >
> > > Jeff
> > >
> > >
> > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
> > > <mariss92705@y...>" <mariss92705@y...> wrote:
> > > > Jeff,
> > > >
> > > > That's jumping to conclusions. I only know for sure about the

53

> > in-
> > > > oz motor because I monkeyed with it. It's my conjecture the 82
> in-
> > > oz
> > > > motor "may" have been disassembled. How much, how far, I have

no

> > > clue
> > > > because I wasn't there. The evidence is circumstantial.
> > > >
> > > > Just removing the end cap should have no negative effect as

long

> > as
> > > > the rotor stays entirely within the stator. I can't say what
> > effect
> > > > partially removing it would have but it certainly can't help.
> > > > Touching the stator isn't the problem, opening the air-gap

is.

> > You
> > > > can have the rotor touch the stator all day.
> > > >
> > > > I'll stand behind the 33% number though I have reason to

suspect

> > > it's
> > > > more like 50%. The consequence is either bad or worse; it's a
> > > > distinction without a difference.
> > > >
> > > > Mariss
> > > >
> > > > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jeffalanp
> > <xylotex@h...>"
> > > > <xylotex@h...> wrote:
> > > > > Hi Mariss,
> > > > >
> > > > > It looks like your emperical evidece:
> > > > >
> > > > > 106 in-oz Unmolested Motor
> > > > > 82 in-oz Opened (but perhaps rotor not removed? <NEAT>)
> > > > > 53 in-oz Opended (rotor removed)
> > > > >
> > > > > suggests that
> > > > > 1) Just openeing the motor to look at/play with the wires
> > > (leaving
> > > > > rotor remaining inside?) can cause a significant torque

loss.

> > > > > Question: Is this possibly becasue the rotor was allowed to
> > touch
> > > > the
> > > > > stator? If the the rotor were somehow kept from comming in
> > > contact
> > > > > with the stator when opened, might it retain full
> > magnetization?
> > > > >
> > > > > 2) Disassembling it essentailly can cut torque in half.

Again

> > the
> > > > > question: if the rotor were to be removed without touching

the

> > > > > stator, would is loose as much torque.
> > > > >
> > > > > Any comments on the above?
> > > > >
> > > > > Thanks,
> > > > >
> > > > > Jeff
> > > > >