Stepper Motor Max Practical Speed


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    Question Stepper Motor Max Practical Speed

    I have some inexpensive Chinese stepper motors so the inductance is a little high and will limit the speed. From a practical sense this won't be an issue because my machine is a plywood build hobby machine. My cutting speed won't need to be that great.

    So I calculated the theoretical max rpm based on inductance.

    For rapid non-cutting motion can I exceed the max theoretical rpm? Based on your experience is there a practical limit to over speed for a stepper?

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    Default Re: Stepper Motor Max Practical Speed

    a lot of what you can get out of a stepper motor depends on a lot of factors. the machine mechanics are a big part of this. you can gear them to to operate in a better place based on the torque speed curves to get more performance but until you get your machine built it hard to say.

    the one thing that is important on what you can get out of a stepper motor is running them at the highest rated voltage you can get out of the motors and drives. so the cheap motors and drives are probably rated at 50 V so i would use a 48 V power supply if this is the case.

    so when you decide on the drive mechanics all this will come together when you start tuning things.in general the faster you run a stepper motor the the more power they will loose. they do best at lower speeds and drop off like a stone the faster you run them.



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    Default Re: Stepper Motor Max Practical Speed

    For rapid non-cutting motion can I exceed the max theoretical rpm?
    You may find that you can't even achieve the theoretical maximum. There's a lot of friction and inefficiency involved in the real world.
    Once the machine is running, you fins out what speed it stalls at, and back off 25% or so.

    I don't know anyone with a stepper powered router that didn't wish it was faster. Most people run them right at the edge of reliability.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Stepper Motor Max Practical Speed

    Hi Hope - Most steppers peak out around 1000rpm. They have max torque at near zero speed and drop to nearly zero torque at 1000-1200rpm. Its pretty safe to call it linear from 0-1000rpm where 1000rpm is zero torque for design purposes. I've attached a typical torque curve for a NEMA 24 motor. This motor has a 3.8mH inductance so its not real low. The variables for speed are the mechanical restraints (inertia and acceleration, and friction) the electronic restraints are quality of the drive and its voltage. As everyone says use the highest voltage possible if you need speed. If your designing a router say keep the motor at say 100-400rpm for cutting speeds and 700-9000 for rapids. So you need to gear the motor to stay in this band and tune the motors against your inertia (structures) once running.... Cheers Peter

    Attached Files Attached Files


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    Default Re: Stepper Motor Max Practical Speed

    +1 to everything Gerry and Peter just said. Pretty much all you can do with a stepper is find the point where it stalls and back off to make a safety margin. And 9 times out of 10 you would wish it could spin faster.



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    Default Re: Stepper Motor Max Practical Speed

    Quote Originally Posted by peteeng View Post
    Hi Hope - Most steppers peak out around 1000rpm. They have max torque at near zero speed and drop to nearly zero torque at 1000-1200rpm. Its pretty safe to call it linear from 0-1000rpm where 1000rpm is zero torque for design purposes. I've attached a typical torque curve for a NEMA 24 motor. This motor has a 3.8mH inductance so its not real low. The variables for speed are the mechanical restraints (inertia and acceleration, and friction) the electronic restraints are quality of the drive and its voltage. As everyone says use the highest voltage possible if you need speed. If your designing a router say keep the motor at say 100-400rpm for cutting speeds and 700-9000 for rapids. So you need to gear the motor to stay in this band and tune the motors against your inertia (structures) once running.... Cheers Peter
    9000 meant to be 900rpm cheers Peter



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    Default Re: Stepper Motor Max Practical Speed

    You hAve good advice here but repeating the key point you likely will never achieve the theoretical max. As noted determine the stall point then back off.

    If this is a larger router you might want to consider servos if speed bothers you. Well the lack of speed. If you are not familiar with machine controls though going with servos might be more difficult than you want to deal with on a first build.

    A quick edit here: fast machine can be dangerous. If you want a machine with fast rapids or even fast feed rates please design in suitable safety measures.



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    Default Re: Stepper Motor Max Practical Speed

    Quote Originally Posted by Project_Hopeless View Post
    I have some inexpensive Chinese stepper motors so the inductance is a little high and will limit the speed. From a practical sense this won't be an issue because my machine is a plywood build hobby machine. My cutting speed won't need to be that great.

    So I calculated the theoretical max rpm based on inductance.

    For rapid non-cutting motion can I exceed the max theoretical rpm? Based on your experience is there a practical limit to over speed for a stepper?
    Which stepper?
    I have nema 23's on a small mill that are 3.5mh ind. On 36v they bombed out even with drivers set at higher currents. Like at <1800mm/min (<350rpm)
    Ditched the 36v supplies and 50vmax drivers and upped it all to 60v.
    They now fly better, current is now also set lower. I've taken them to 3000mm/min (600rpm) no motor issues but screws sounded rough so backed it off a touch.



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    Default Re: Stepper Motor Max Practical Speed

    Quote Originally Posted by peteeng View Post
    Hi Hope - Most steppers peak out around 1000rpm. They have max torque at near zero speed and drop to nearly zero torque at 1000-1200rpm. Its pretty safe to call it linear from 0-1000rpm where 1000rpm is zero torque for design purposes. I've attached a typical torque curve for a NEMA 24 motor. This motor has a 3.8mH inductance so its not real low. The variables for speed are the mechanical restraints (inertia and acceleration, and friction) the electronic restraints are quality of the drive and its voltage. As everyone says use the highest voltage possible if you need speed. If your designing a router say keep the motor at say 100-400rpm for cutting speeds and 700-9000 for rapids. So you need to gear the motor to stay in this band and tune the motors against your inertia (structures) once running.... Cheers Peter
    There are a number of thing in the "if I had to do it all over again" one of them is motor selection.

    I don't have nearly as linear a curve nor the rpm range of your drive.

    https://www.omc-stepperonline.com/do...rque_Curve.pdf

    For the time being I'll have to make do with what I have. Anyone have a link to tuning/setup for stepper systems?



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    Default Re: Stepper Motor Max Practical Speed

    Hi Hope - if you use digital drivers it's all done automatically. Peter



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    Default Re: Stepper Motor Max Practical Speed

    Quote Originally Posted by CitizenOfDreams View Post
    +1 to everything Gerry and Peter just said. Pretty much all you can do with a stepper is find the point where it stalls and back off to make a safety margin. And 9 times out of 10 you would wish it could spin faster.
    Actually after working with an engineer the last couple of weeks developing a model for stepper motor motion design, I have to say I have a renewed respect for steppers. Everything we thought we knew when we started has been turned upside down. Using a carefully inertia matched design and keeping them in their element, their low down torque can be harnessed to achieve amazing results. I'm just annoyed it would be such a major job to retrofit my plasma table to the design. We are talking about a 460% improvement in acceleration which means we can get to > 400 inches per minute in < 0.5 seconds. In fact I designed one axis that could get to 800 inchs per minute in 0.5 seconds. I know we don't have cutting forces to contend with but with plasma acceleration is king!

    Rod Webster
    www.vmn.com.au


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