Re: CNC Servo motor gear ratio: overheating. EMC step rate. Tablespeed

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Kim Lux <lux@d...> wrote:

> On Sun, 2003-06-08 at 12:52, Jon Elson wrote:
> > Tim Goldstein wrote:
> >
> > Yes your motors will suffer major heating problems set up that

way. If

> >
> > >you want to run at that low an RPM you need to use steppers.
> > >
> > I think not, as his expected load is 2.8% of the peak. These

are very large

> > motors, and he doesn't expect cutting loads, apparently.
> >
> > Jon
>
> I do expect cutting loads. 25 lbs was the empty (ie non cutting)

force

> on the table. We occasionally want to push a 3/4" endmill at 5

ipm.

>
> Could someone please explain to me the difference between the

torque

> ratings of servo motors versus stepper motors ?

Steppers are rated in holding torque. A 200 oz stepper has 200oz of
torque at zero RPM. As RPM increases the torque deacreases
dramaticly, even though current remains the same. During rapids, the
motors are producing nothing close to 200 oz in.

Servos have constant torque (directly proportional to current)
regardless of rpm. They still produce the same torque as rpm
increases.

Brush servos have two current ratings. The max rating is the max
current before damage to the permenant magnets inside. The
continious rating is the most continious current that the coils can
take without overheating. Obviously the continious rating can be
safely exceeded for brief moments (such as acceleration, etc.)
without causing overheating. The armiture inside a servo is very
poor at dissapating heat and therefore the continious current (and
torque) is relatively low.

Steppers have great torque but it deacreases rapidly as rpm
increases. This makes them ideal for a direct drive applications.
Servos on the other hand have less torque but rev much, much higher
making them more suited for belt reduction.

Hope this helps.

Mark

>
>
> --
> Kim Lux <lux@d...>