RE: [CAD_CAM_EDM_DRO] Stepper current in partial-step positions

>-----Original Message-----
>From: elson [mailto:elson]On Behalf Of Jon Elson
>> However, if the driver does this, each winding is dissipating
>> 0.707*Imax*Rwinding, for a total power (heat) production of

1.414*Imax*Rwinding,

>> which exceeds the motor's rated ability to shed heat.
>Not quite right! Power is I ^ 2 * R (I squared times R). Note that .707 ^ 2

is

>.5 ! So, it does NOT overheat the motor. Knowledge of RMS relationships would
>tell you that right away, though. So, 2 windings at .707 A each will dissipate

the

>...
>If you don't understand the squared bit, it is because power is Volts x Amps,

and

>...
>All based on the wrong calculation of power, therefore, wrong.

oops... Yeah, I understand P=RI^2 all right, it was just hiding in some sleepy
hollow of my brain, and I coughed up a distorted version of P = I*E, instead.
Running the windings at 70% in the half-step positions is just fine for the
motor, but it may draw more power supply current than one would expect at first
glance. 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.

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.

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