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

Kim Lux 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.
>
>

Oh man! You are in BIG trouble, unless these motors really ARE 600 Oz-In
continuous, which I seriously doubt. A 3/4" endmill will deliver LOTS of
linear thrust against the table. I designed my machine to be able to
deliver
1000 Lbs to the table for one minute. These are 1/8 Hp (continuous)
motors, with an estimated 1 minute capacity of 1/2 Hp. I determined that
1000 Lbs number based on a 1" end mill and a 1 Hp spindle as a worst-case
condition.

>Could someone please explain to me the difference between the torque
>ratings of servo motors versus stepper motors ?
>
>
>
>

Well, stepper motors deliver maximum torque at zero speed, and the torque
falls off pretty much linearly, starting at some low speed like 100 RPM.

Servo motors deliver full rated torque up to the speed where motor back EMF
plus resistance drop in the windings equals the applied voltage. Apply
more voltage, and the torque extends to higher speed. But, you have to
watch the thermal conditions. On a stepper, as long as you don't exceed
the current per phase limit, you don't have to think about it. With a
servo,
the manufacturer supplies a continuous torque rating, which the motor
can deliver all day. Then, there is a peak torque rating, usually based on
some current above which the magnets will be demagnetized. but, you
can only apply the peak torque for some time interval, such as one minute
every hour, or 1 second every minute, or some similar duty cycle.
Or, they give you an RMS maximum, averaged over some time interval,
and you can manage the loading any way you want, as long as the RMS
average over the interval does not exceeded the spec. These specs are
essentially impossible to determine without testing a few motors to
destruction, that's why we let the manufacturer make these measurements
for us.

Jon