4325
« on: June 12, 2018, 07:35:18 AM »
Hi,
there are some predictive formulas but they require an accurate assessment of motor variables beyond those the are made available to us by the manufacturer.
The best figure of merit to determine whether a stepper will perform well at speed is its winding inductance. The higher the inductance the longer it takes for the current to
increase and as current and torque are closely related, the greater the drop off of torque with speed.
You might check out the Leadshine website, they don't as a rule publish an inductance figure but they do publish Speed-Torque diagrams which give you an excellent means of
comparison between products and for expected performance in practice. It is interesting to note that some of what appear to be the best and strongest performing steppers behave
very poorly at high speed, maybe as little as 5-10% of rated torque at 1000 rpm whereas a smaller and lesser unit will retain 25-30% at 1000 rpm and be better than its supposedly
bigger and better sibling.
In the attached pic you can see that the applied voltage is a major determinant in torque at speed, the higher the applied voltage the better the stepper will go. Consider then Geckos's drivers
which are 72V capable or Leadshines AM882's which are 80V capable.
I use Vexta 5 phase steppers for my mill and chose the genuine Vexta drivers as well. The drivers are hooked directly to the 230VAC line. Their peak output to the stepper is about 150V and
man do those steppers sing! I paid quite a premium for them though.
The bottom line is that stepper loses torque bigtime at speed, you can expect even good and well designed units with high voltage drivers to get no better than 30% rated torque at 1000 rpm.
If your machine is such that you need greater rotational speeds then you want proper servos.
Don't be taken in by the advertising hype about closed loop steppers, that is to say steppers with encoders and smart drives, they would have you believe that they are as good as servos, not even
close. They will also tell you that they don't lose steps, also fallacious. They are still steppers and a closed loop stepper loses steps (and faults 'following error') in exactly the same circumstances that
open loop stepper loses steps. Provided a stepper is used WITHIN its performance envelope it DOES NOT lose steps. If its used outside its envelope it will, closed loop or not.
Genuine servos have rated torque right from zero speed to rated speed. Some manufacturers publish the peak torque only, it looks really impressive but what you should base your judgment on is
'continuous stall torque'. When you do so you might be surprised that a servo has less torque than a stepper of the same physical size, and of course may well be two or three times more expensive.
IT IS NOT A TYPO! Steppers perform very well at low speed, better than servos even, but servos come into their own above 500-1000 rpm and will carry on giving rated torque where a stepper has long since
stalled.
AC Servos and servo drives are decreasing in price but are still a considerable premium over steppers. I think it may be worthwhile that you experiment some more with steppers until you are familiar
with their strengths and their limitations. Do yourself a favour and get some decent high voltage drivers and matching power supply, you'll never get the best from any stepper until you do. I have been
told that steppers magnets can weaken with age and so you might need to purchase new to ensure that you get rated torque.
The link I gave you previously tells you that microstepping is about making steppers run smoothly not increased resolution, as seductive and logical as it sounds. A microstepping regime of 8 to 16
microstepps per full step is about right, more is giving you only marginally better smoothing but upping your signaling rate maybe beyond your controller/BoBs capability.
Craig
Guest
Show Posts