Re: What makes a motor a servo?

Hi Scott,

Believe it or not you are not comparing watts to watts. It is not
real obvious here but knowing Mariss and being used to reading his
posts watts is not watts in this instance. Well watt is it then (pun
intended). Mariss often uses the unit watts as a measure of
mechanical power because 746 watts = 1 hp.

In his chart below he is not showing how much electrical power the
motor draws at a RPM. He is in fact showing how much mechanical
power each motor has at different RPM's. The point he was trying to
make is that a stepper with 133 watts mechanical power has 320 IPM
rapids when a servo with 266 watts mechanical power only has 145 IPM
rapids.

I agree with you that these motors and reduction ratios are poor
choices for a milling machine. But he picked these motors and
reduction ratios to make his point.


BTW, a clue those numbers are not electrical power draw of the motors
is the fact that a stepper draws full power at all speeds. That's
why good stepper controllers have idle current reduction. So the
stepper does not sit there and cook when it is not turning.


Wally







--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@y...> wrote:
>
> > So what happened? How come a stepper having only 1/2 the servo's
> > power (133W vs 266W) gets 320 IPM rapids to the servo's 145 IPM?
> >
> > The difference is the motors' different speed-power curves. The
> > stepper has a flat 133W power vs speed curve past 300 RPM while

the

> > servo power peaks over a narrow speed range. Here's a comparison:
> >
> > 0000-RPM 000W 000W *no RPM, zero power for both
> > 0100-RPM 044W 004W
> > 0200-RPM 088W 009W
> > 0300-RPM 133W 013W *stepper reaches full power
> > 0600-RPM 133W 026W
> > 1000-RPM 127W 039W
> > 1600-RPM 118W 059W *stepper torque = 100 in-oz (max rapid)
> > 2000-RPM 112W 089W
> > 3000-RPM 098W 133W
> > 4000-RPM ---W 178W *stepper torque too small to be useful
> > 5000-RPM ---W 222W
> > 6000-RPM ---W 266W *servo reaches full power
> > 6500-RPM ---W 192W *servo power begins to drop
> > 7000-RPM ---W 096W
> > 7250-RPM ---W 048W *servo/10:1 reduction torque = 100 in-oz
> > 7500-RPM ---W 000W *servo no-load speed, zero power
> > -------------------------------
>
>
> So let's see what is going on here:
> At 300 RPM when the stepper is producing full power of 133W making

the

> deep cut, the servo is making the same cut, at 3000 RPM, and using
> the same 133W.
> Right? they are both delivering 600 oz-in continuous. Sounds like
> the servo is doing just as good as the stepper here.
>
> Now lets move past the cut and rapid back at each motors max rate.
> Now the stepper is rapiding using
> 98W and the servo is using 266W according to the table. Right?
> WRONG. The servo is NOT going to be producing its 600 oz-in
> torque for a rapid. There is no cutting load. This power may be
> available but it does not
> mean it is being used. This table only shows the POTENTIAL for the
> servo to produce large power at a rapid rate. So I don't think the
> statement "stepper having only 1/2 the servo's power (133W vs 266W)
> gets 320 IPM rapids to the servo's 145 IPM?" is a fair comparison.
> You have just shown that this servo can produce twice the power
> that the stepper can, yet produce very reasonable 145 IPM rapids.
> It's extra power is in the motors upper range, but none the less,
> it is there.
> Not only that, the servo has extra punch that it can apply during

hard

> accelerations, etc. The motor manufacturers don't give you the peak
> torque rating because it is off limits. It is there and can be used
> and like anything else, it can also be abused. The stepper has no
> extra punch. WYSIWYG.
>
> The servo is the hands down winner here unless you are crazy and
> want 320 IPM rapids. In that case, just use a larger servo.
> Let's not forget, this only represents only
> one scenario. I don't think you can get 6000 RPM motors and
> reasonably gear them down 10:1 in a mill application. 6000 RPM
> is more like a peak RPM rating, with a continuous RPM rating of
> 2500 to 3000 being the norm. You would use larger servo motors
> here with a 2:1 to 3:1 gearing.
>
>
> Scott