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

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