Suitable Cap for Gecko's was Re: 8-wire 1/2coil stepper?

And Good afternoon to you too!

OK, I think I get it now. Thanks for the further explanations.
Actually not a bad thing, as the inductance effects are not as bad that
way!

I hadn't thought of using my 'scope for inductance measurements. Not
since college!

Alan KM6VV


mariss92705 wrote:

>
> Alan,
>
> Good afternoon! A full step sequence is where A and B are the
> windings, + and - is the "direction" of the current thru the winding.
>
> +A +B, +A -B, -A -B, -A +B
>
> After this, the sequence repeats.
>
> For microstepping, pretend "A" is sine and "B" is cosine magnitude.
> Then "+" and "-" are the "signs" for the sine and cosine functions.
>
> For the full step sequence (nothing more than a single-bit
> approximation of the sine and cosine functions), the motor moves 1.8
> degrees on each "sign" change. Being sine and cosine approximations,
> the sign changes occur every 90 "electrical" degrees. 360 degrees in
> a cycle, therefore 4 full steps per cycle.
>
> You can use a bridge to measure inductance but your reading will be
> high because it is taken at zero DC current. Magnetic saturation
> effects (where you are on the B-H curve) will cause the working
> inductance to be somewhat lower.
>
> I prefer to measure working inductance directly from the V/L slope
> measured with a 'scope. Read the "delta I" over a "delta t" period on
> the 'scope. As an example, say you measure a 300mA change in winding
> current over a 12uS time span and you are using a 72VDC power supply.
>
> The inductance will be 72V times 12uS / 300mA, or 2.88mH if the slope
> was linear.
>
> The cap is fine for power supply voltages up to 50VDC.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., Alan Marconett KM6VV <KM6VV@a...> wrote:
> > Hi Mariss,
> >
> > Very informative! Another "keeper"! Thanks for the
> clarification. Why
> > is it 4 steps/cycle? Seems like something is changing (a current
> > through a winding) EVERY step?
> >
> > I see a "Fl = 2 PI F L" inductive reactance calculation! Can I
> measure
> > the inductance of "unknown" stepper motors with an L/C meter (kit)
> such
> > as:
> >
> > http://www.aade.com/lcmeter.htm
> >
> > I think the operating frequency is 1K. I don't have an HP4275A
> digital
> > L/C meter, or or a B&K 878 digital LCR meter!
> >
> > Also, is this 470uf cap suitable for the Gecko's (only 63v, couldn't
> > find higher)?
> >
> > P10351-ND EEU-FC1J471L CAP 470UF 63V ELECT FC RADIAL Panasonic -
> ECG
> > 470µF 63V ±20%
> >
> > Found in DigiKey's catalog (only a quick online search).
> >
> > Alan KM6VV
> >
> >
> > mariss92705 wrote:
> > >
> > > Hi,
> > >
> > > I think the thing being missed here full-stepping vs.
> microstepping.
> > >
> > > If you have a 5V 1A per phase unipolar rated, 8-wire motor, then
> you
> > > would run it at 1A unipolar (half-winding) and it would dissipate
> 5W
> > > (1A squared times 5 ohms).
> > >
> > > If you connect the windings in parallel, then the winding
> resistance
> > > is 2.5 ohms (1/2 what it was in half-winding). You can now
> increase
> > > the current to 1.41A and still dissipate 5W (1.41A squared times
> 2.5
> > > ohms = 5W).
> > >
> > > What matters here is your torque (from the URL you referenced)
> > > increased 30%. It should have increased 41%; the reason it didn't
> is
> > > because the iron has magnetically saturated.
> > >
> > > If you are full stepping, you don't care. There is no negative
> > > positioning effect from saturating the iron except for a drop in
> > > inductance.
> > >
> > > If you are microstepping you do care because saturated iron means
> a
> > > non-linear motor and the microstep positions will begin to bunch-
> up
> > > at the full step locations.
> > >
> > > An analogy is an audio amplifier. If you overdrive it with a sine
> > > wave, the output will limit or "clip" and produce distortion. If
> all
> > > you are amplifying is a square wave, then who cares if it clips?
> > >
> > > Second point. Take the same motor and drive it at 1.41A half-
> winding.
> > > You will not be able to tell it apart from the parallel connected
> one
> > > at 1.41A torque-wise. The only difference is it will dissipate 10W
> > > (1.41A squared times 5 ohms).
> > >
> > > Third point. This 30% increase in torque occurs at low speeds
> where
> > > step motors usually have surplus torque anyway. It does not get
> you a
> > > single extra oz-in of torque at high speed, where improvement
> would
> > > be appreciated.
> > >
> > > Inductive reactance becomes the bottleneck that limits the winding
> > > current. The drive may be set at 1A or 1.41A, it will make no
> > > difference. 10 mH of inductance will have 62.8 ohms of inductive
> > > reactance at 4,000 full steps per second and will limit winding
> > > current to 0.5A for a power supply voltage of 31.4VDC.
> > >
> > > I = 31.4V / 2 pi times 1kHz times 0.01 Henries
> > > 4 full steps is one electrical cycle, so 4,000 full steps/sec =
> 1kHz
> > >
> > > Mariss
>
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