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

"Kevin P. Martin" wrote:

> >-----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.

no, probably not. It depends on the mechanisms the drivers use to recycle
current in the windings, and the supply voltage. But, at idle, the DC supply
current will normally be very much less than the motor winding current,
due to the switching nature of the driver. As the motor speeds up, and
is delivering energy to the load, then the DC supply current goes up.
In some cases the supply current may reach the full motor current, but
that is usually a transient thing while accelerating, only.

Generally, unregulated DC supplies can withstand short surges above the
rated load without any harm.

> 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, because of the switching action, the drive NEVER comes close to drawing
motor current at any stationary position. I keep trying to tell these people
that they don't need 15 - 75 Amp power supplies to run 3 small motors on a
Sherline!

> 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.

Most stepper motors are RATED to take full rated current on BOTH windings
simultaneously.

Jon