Re: What makes a motor a servo?

A Watt is a unit of power. It can take many forms.

1)100W of heat can give you a painful burn; grab a lit 100W light
bulb for a demonstration.

2) 100W of mechanical power can lift a 531 lb weight at 100 IPM.

3) 100W can launch a radio signal from a space probe way beyond Pluto
srong enough to be detected on earth.

4) 100W of infrared light from a CO2 laser can burn impressive holes
in many things.

Of course only (1) and (2) matter here. Motors produce power and heat
so Watts are a convenient way to express each. Mechanical Watts is
velocity times force or torque times RPM. Mentioning torque without
RPM is pointless.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

> 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