Re: Stepper current in partial-step positions

--- In CAD_CAM_EDM_DRO@y..., "Kevin P. Martin" <kpmartin@t...> wrote:
<snip>
The too-quick analysis is 3 drivers set to 5A current limit means you
> need 15A power supply capacity. But if each of the motors is in a
> half-step position you might really need 22A. This is the worst
> case; at any other partial step position the total current draw is
> reduced, until it equals the limit current when at a full-step
> position.

Maybe with old unipolar drives! My "real world" experience is that
the power supplies don't need to be anywhere near these sizes...
Especially with chopper style drives. If yo have an app where the
motors are moving VERY slowly(or stopped) you wil need more power
capacity.

> This is assuming the driver uses reasonably accurate sine/cosine
> factors to set the two winding currents. A microstepping driving
> has to get these ratios close to get evenly-spaced step positions.

To get EVENLY spaced positions, it has to be better than close, it
has to be matched to the motor! It is a fallacy to think that you can
hook up a microstep drive to an arbitrarily chosen stepper and get
accurate intermediate step positions! You can have the
sine/cosine "spot on" and the motor will still likely not be where it
is "supposed" to be...


>But for the half-step positions, just having equal currents in both
>windings will give the half-step position; it is possible to cut
>corners and make a half-step driver just ram the full limit current
>through both windings at the half-step positions. This is just part
>of the driver design; in this case if your motor spends a lot of
>time dwelling on half-step positions, it could well overheat.
> -Kevin

I prefer to boost the "other" half step positions (single coil on)
and use auto-idle to protect the circuitry/motor. Nowadays, it is
just too simple to control the winding current of the half steps not
to, IMO.

Hope this helps.

Ballendo