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

Mariss, you are still ignoring the fact that you can't get 600 oz-in
useable torque out of a 600 oz-in stepper. In practise you will get
something like 150 oz-in reliable useable torque at cutting speed in
real-world situations. In your example the car with the 5-speed tranny
also has the throttle limited to 1/4 travel.

You have also ignored acceleration. Acceleration can make quite a
difference to the overall machining time, especially with shorter moves.
At higher speeds the stepper is running out of puff so you need to limit
the acceleration. As most controls use trapezoidal acceleration the
extra available torque at low speeds can't be utilised. The servo
however will be capable of putting out over twice it's rated torque over
the full rev range during acceleration. Which car will get to max speed
first, the one with a 5-speed tranny and a throttle limited to 1/4
travel or the one with nitrous injection?

It is possible to argue for ever on this topic and in my opinion it
comes down to the following:
A well designed stepper system will outperform a poorly designed servo
system and vice-versa.
At higher powers it is very difficult to achieve decent performance from
steppers so servos win out pretty much by default.

And now for the biggie: A poorly designed servo system will probably be
slow but reliable. A poorly designed stepper system will be slow and
very unreliable. How many of us have the knowledge and resources to
properly design a drive system? Most of us simply aim high and hope for
the best. If I get the numbers wrong I will take slow and reliable over
slow and unreliable any day. Slow but reliable will get the job done.
Slow and unreliable just makes scrap.

Les


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".
>
>