Re: What makes a motor a servo? - real world applications

--- 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

Mairss did say it was dyno testing and not on a mill.

On a plasma cutter, I have a 250 pound gantry. from zero to 1 inch there is a certain force
required to start to move the gantry. once it is moving at 150 ipm, all the force needed it
to keep it moving.

This fits stepper curves nicely.

So, a decnet 600-800oz-in stepper runs for around $200, plus a Gecko 210 for another
$114. That makes a $314 axis.

A treadmill motor is $29, plus another $65 for an encoder and anther $135 for a
Gecko320 with step multiplier. That makes the axis cost $230.00.

now, that servo is a 115volt motor rated at 1-1/2hp

Sor for approximatly the cost of a power supply, I can get a servo axis. E-bay can trash
that difference.

Also, for servos, there is an optimum speed. As Les Watts pointed out, as you increase the
gearing, you also increase the losses in the power train. for my application, 6:1 was worse
than 5:1. 5:1 being optimum.

But, instead of a 800oz-in motor that is 2:1, I have a 3/4 hp motor that is connected 1:1.5

That offers a step of 0.001".

The servo required machining of the shaft to fit a pulley, or a larger drive pully to fit a 17
mm shaft.


BUT... for a small machine a HobbyCNC 3 axi board is about $80 and e-bay 3 steppers are
$55 each, so $260 and you have three axes of stepper.

There are more things to considder than power alone.

Dave