low or medium inertia for router?

[Boltz]

Hi, I'm looking for replacement x-axis motors for a CNC router with a fairly heavy steel gantry in the 500 lb. range. I'd like rapids of at least 1200 IPM. I'm thinking I'll need something in the 1.3 KW range.

Should I be looking only at low inertia motors? The literature describes the medium inertia motors as being suited to "heavier loads". Do the quick direction reversals required in normal router operation dictate a low inertia motor?

Does anybody have recommendations for what the best bang for the buck is in the used market?

-Jim Hart

[Al_The_Man]

What size motor is on there at the moment and what type and ratio of reduction?
Al.

[Boltz]

Hi, Al. I tried this last year, and didn't get much useful information, so I'm trying it again. You might remember me as the guy with the weird 8-pole Minarik servo motors.

The attached photo shows the Minarik nameplate, with what I consider to be a somewhat exaggerated peak torque number. Experience has shown that these motors are barely adequate to control the rather heavy steel gantry. Several days of tuning slightly improved performance, but I still have to run very gentle accelerations to avoid overshoot and settling problems.

Would you mind answering the general question of low vs. medium inertia for a heavier gantry first? Let's say you were starting with a clean sheet to design a high performance servo system for a router. Would you be looking at low or medium inertia motors, given that the machine is in a production environment, and expected to run all day everyday?

I've surveyed the used market on ebay and here's the short list of manufacturers I'm considering:

Allen Bradley family, including Reliance, etc.
Parker
Indramat
Yaskawa

There's a lot of Fanuc stuff out there, but I haven't been able to find spec sheets.

thanks,
-Jim Hart

[Al_The_Man]

Your motor/load inertia ratio is very important when it comes to matching the motor to large gantry's like this, generally a fair amount of reduction is required in order to implement efficient sizing of motors.
Have you played around with any of the sizing programs such as the one on the Kollmorgen site?
You can change the numbers and see the effect of changes on the motor/load inertia ratio.
In particular the difference in accel/decel rates.
This type of program should get you in the ball park.
Al.

[Boltz]

Your motor/load inertia ratio is very important when it comes to matching the motor to large gantry's like this, generally a fair amount of reduction is required in order to implement efficient sizing of motors.

Are there drawbacks to oversizing the motors by 50%? Aside from the fact that you are paying for performance you may rarely use? My understanding of inertia ratios is "the lower the better".

-Jim Hart

[Al_The_Man]

The industry guide lines used to be to keep the motor/load inertia ratio to < 10:1.
The inertia ratio is reduced by the square of the reduction.
Al.

[mike_Kilroy]

boltz, if u want more than 30% chance of getting the performance u want, u really should play what if with a servo motion sizing program. the kollmorgen one AL mentioned is fairly quick to learn.

http://kollmorgen.com/en-us/support/.../motioneering/

how can u keep Jmotor:Jload > 1:5 if u dont know Jload?

No one can answer is it better to use a low inertia motor or a medium inertia motor - without doing motor sizing with the specific machine data; there is not one correct answer.

Saying lower inertia ratio is better is also not true - it depends on the machine and performance wanted once again. For instance, imagine going lower and lower so you end up with load being .1x the motor. Now go to .05x the motor: it is 2x as low so 'better' right? Nope. since torque to accel is directly linear to inertia TOTAL, you just reduce ratio by 1/2 but the total inertia didnt change significantly so it takes the same torque to accel in both cases. BUT now the motor has to go faster to make the same move so it actually takes longer to accel with the same torque! so no, lower is not always better. there actually is a sweet spot for most machines - and you only find it by doing motor sizing.

oversize motor? maybe big disadvantage if Jload is already only .1x Jmotor; oversize 50% and u just reduced your acc/dec by half!

do motioneering sizing and u will know why you have overshoot and settling problems today - it can be the inertia ratio is way off so you cannot tune it, OR it could be the motor doesnt have enough torque - without motor sizing, its a crap shoot which is the cause so a crap shoot whether to go low inertia, high inertia, or bigger or smaller motor.

[Boltz]

Um, I'm not exactly starting from scratch here... the system has *adequate* performance with the existing motors. If the acceleration is set toward the low end of the range it's possible to cut aluminum with no visible overshoot.

The goal here is a small incremental improvement in performance, and to move to a motor readily available on the used market.

The Allen Bradley MPL A330P is readily available, has twice the stall torque, slightly more peak torque, and is rated for 5000 RPM. Different drives required, of course.

-Jim Hart

[mike_Kilroy]

Um, I'm not exactly starting from scratch here... the system has *adequate* performance with the existing motors. If the acceleration is set toward the low end of the range it's possible to cut aluminum with no visible overshoot.

The goal here is a small incremental improvement in performance, and to move to a motor readily available on the used market.

The Allen Bradley MPL A330P is readily available, has twice the stall torque, slightly more peak torque, and is rated for 5000 RPM. Different drives required, of course.

-Jim Hart

Jim, I must stand by my reply. without motor sizing for your mechanics, going the route you are going will not guarantee any improvement and may make it worse. You never said you need more cutting torque, & with little additional accel torque, this larger inertia motor may indeed may it worse. It will of course get you a motor you feel is more readily available from ebay but that may be all you get.