Re: [CAD_CAM_EDM_DRO] Re: What makes a motor a servo?

WHAT MAKES A MOTOR A SERVO ??????

We used to have a computer parts surplus store.
They had nice sized servo motors (probably from big
multi disk drives). I never understood just how to classify
these motors. They seemed to be just a big overly precision
series motor. We were cautioned not to take them apart
(same caution for steppers). I was familiar with steppers as they
were used on radar antennas. I used to be able to go to Audels
and get a little idea about the design of motors and the characteristics
of different motors. Some generators were easy to hook as motors,
others not. We took the old Aircraft Generators and made
welders out of them. Surplus Center still sells those and I have
saved one for use on a VW Air Compressor (fire on two, compress
on two. Volks Air and still have enough power when not compressing
to power the Welder). And while on the subject, I still have trouble \
understanding how a DC Brushless motor works.

Bob McKnight





On 8 May 2005 at 19:41, Mariss Freimanis wrote

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

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