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

R Rogers wrote:

>Erie Patsellis <erie@...> wrote:
>replies inline...
>
>depends, on my router, I run a 30,000 rpm spindle, feeds are usually
>around 45-90 ipm, cutting .063 al. my machine spends as much time
>traversing as cutting (8'x12' table) so, once again, it depends on your
>machine and particular needs. I can tell you that I've retro'd servos
>onto an identical, only smaller version of my machine,and were it not
>for the customer insisting servos are better, would have steppers.
>AAMOF, I recently did a machine in Kentucky that was r&p drive, with 4:1
>reduction (1" per rev) and had the same situation, steppers would have
>flat out beat the pants off of any servo, on that machine, in that
>situation (routing mdf all day long), but alas,
>
>/// I wish it were possible for that to be proven. Because I'm still confused about this apparently. An Nema42 1100 oz-in stepper is considered large and that is all it will develop and only when it is ice cold (first few minutes of operation). I'm using 40 lb-in continual torque Nema42 servos on a B'port that stall at 5 times that. 200 lb-in. Equates 3200 oz-in at stall and this can be at 80% of the 2500 RPM (no man's land for steppers). Now factor in the reduction which most steppers systems arent using of 2.5:1 and it's roughly 8000 oz-in at stall to the ballscrew. It will snap alot more than a 3/8" endmill and never even hiccup. Now factor in the losses from heat/resistance in the steppers windings. Pushing an 800 lb gantry all day at very low RPM would create quite a bit. I drive the 1200 lb knee of this mill with the same size servo and just finished a job consisting of 1280 7/8" drilled/reamed holes 3" deep in steel, it never missed a step. It will pull a 1" rougher 1"
> deep in steel all day long. Still think a stepper will beat the pants off any servo? Retrofitters stopped using steppers for a reason.
>
>Ron
>
>
>

the question is, do you need that much force? I have 1100w AC
servomotors I could put on my router and it would flat out scream, but
do you need that? (maybe for bragging rights). up until about 3 weeks
ago (when we closed our sign shop), my router ran an average of 4 days a
week, 6 hours a day, never missed a single beat, as a matter of fact, it
hasn't missed a beat since installing mach1, over a year ago. The rack
and pinion arrangement on my router is rater unusual, as the 4" pinion
is reduced 10:1, giving a .4" pitch per rev, with triple stack 34's it
can rapid at over 600 ipm, but I'm not going to run at those rates all
day long, too much stored energy if/when something ever goes awry, call
it what you will, but risk management is what I call it. The router in
Kentucky that I retro'd was an odd duck, and was (I believe) designed
from the ground up for steppers. It has sat for over 2 years and the
owners just wanted to get it running, with servos.

>///
>
>
>
>the customer wanted
>servos, and I had to reduce the motors significantly to get them into
>the sweet spot while actually doing work.
>
>
>
>>>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.
>>
>>
>>
>>
>>
>Oh, were it so, in fact, as you increase frame size, you also increase
>winding size, with a conmensurate increase in inductance, and the corner
>speed drops. Some of the nicest stepper driven tables I've seen use nema
>23 triple stack motors and achieve some exceptional 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.
>>
>>
>>
>>
>>
>well, since power (oversimplified, negating losses) is torque x speed,
>pretty easily, we know that the speed will be approximately 1/4th, so
>roughly your 1 1/2 hp 230v motor will max out about 3/8 hp.
>
>/// Not so, Running servos at reduced voltage ONLY sacrifices speed with no loss in torque providing the power supply can provide the current required. Voltage is speed, current is torque.
>
>
>
>////
>
>
>

exactly, since we're mulitplying speed times torque, reducing one factor
reduces the product proportionately. While we can fill the archives
with silly bantering back and forth, can we agree to disagree? I use
both steppers and servos, depending upon abut 30 variables, only one
being cost.


erie