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

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 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.

////

While it would be nice to think that all of this is black magic, in
reality designing any machine (cnc or otherwise) is an exercise in
compromise. Compromise between cost/performance being the pertinant one
here, I for one prefer steppers, you can say what you want, and are
welcome to come over and try to stop my 800 lb gantry moving at 200 ipm,
using any or all of your body mass. I've sheared 3/8" straight router
bits with it, and I'm assuming that there's enough headroom in the
available forces to do any work I need done.


erie


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