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

On 11 May 2005 at 4:11, Mariss Freimanis wrote:

I guess my point was missed entirely. My fault, let my try again.

Servo motor:

1) Torque is independent of speed. If you have 60 in-oz of torque,
you have 60 in-oz at 6 RPM or at 6,000 RPM.

2) If you need 600 in-oz, you have to have a 10:1 reduction.

3) If you have a 10:1 reduction, your maximum speed is 600 RPM. You
have run out of RPM.


Step motor:

1) Torque is the inverse of speed. If you need 600 in-oz, you use a
600 in-oz motor.

2) Your reduction ratio is 1:1

3) You still have usable torque for rapids at 1,500 RPM.

Or try it this way: You have a car with a manual transmission.
Unfortunately it has only one gear, 1st. You red-line at 20MPH; you
have run out of RPM.

Same car, same horspower but now a 5-speed transmission. You red-
line
at 120 MPH. You still have usable torque for your 120 MPH "rapids".

---------------------------

Doing work takes power so power is everything. It is the mother of
the twins named Torque and Speed. That's why you don't see NEMA-
17
motors driving Bridgeports no matter how suitably geared.

Physics is neither deceptive or cares if it's agreed with or not. It
simply is.

The example I set up was a practical one. Though at its limits, the
stepper is delivering 600 in-oz of torque, else how would I be able
to measure it? The servo also is at its limits at 60 in-oz, go beyond
that and it will burn up. That is a fair comparison.

The servo needs a 10:1 reduction to get 600 in-oz. Its 7,500 RPM no-
load gets turned into a maximum possible 750 RPM. Looks on the level
to me.

The stepper torque measures 100 in-oz at 1,600 RPM. Dyno says so;
the
same dyno that also ran on the servo. My calculator agreed by
predicting what the dyno would say except it said 1,683 RPM. Seems
honest there as well.

So. The critera of 600 in-oz at 150 RPM (30 IPM on a 5 TPI screw) was
met and exceeded by both. The stepper held that torque out to 300
RPM.
The servo could deliver that torque out to 685 RPM but so what? It
was only necessary to 150 RPM.

The other criteria, a 100 in-oz rapid, was won by the stepper at
1,600 RPM to the servo's 725 RPM. Why? The servo "red-lines" (runs
out of RPM) at 750 RPM.

My point is a servo has an unfavorable speed-torque curve compared
to
a stepper's for a hi-torque/low-speed, hi-speed/low-torque dual-mode
operation. The above is intentionally set up to point it out.
Comments?

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:
>
> >
> > OK, so my question is about power in the calculation. at what
point
> > do you select power for either type of motor ?
>
> Don't. I don't agree with Mariss that power is the bottom line.
> You need to select a torque and RPM curve that fits your
> application. Power is just the result of these 2 factors, but
> not something to be directly chosen. The Power number in Mariss'
> calculations is what made the servo appear to be performing worse
> than the stepper. The example he posted is completely deceptive
> since he is assuming that the servo is putting out maximum power
> when it is rapiding. That is not the case. At slow cutting speeds,
> the servo is actually using less power than the stepper, yet this
was
> not pointed out. Where do you spend your time, cutting material or
> doing rapids?
>
>
> >
> > Is my NEMA42 800oz in motor just as good as a NEM34 800oz-in
motor? I
> > thought there were reasons the NEMA42 were not as good.
>
> NEMA is just a frame size and has no bearing on the motors
performance.
>
>
> > As for servo's. how do you calculate for a 230 volt motor that is
> > rated for 1-1/2 hp and 5,600 rpm when you are going to use a
Gecko on
> > it with a 66 volt power supply ?
>
> Sounds like trial and error to me.
>
>
> Scott




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