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  1. #1
    Join Date
    Dec 2012
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    0

    What stepper drivers are people using

    Hi all.

    I was wondering what stepper drivers and I suppose stepper motors people are using?

    I am in the uk and found some stepper motors from cnc4you. They look okay. In Bipolar series mode they have a current rating of 2.1A. I am thinking of using these with a 24V supply.

    Was wondering how well this would work with the machine? If not these then wondering what other people are using and if they are happy.

    Thanks

    Trev

  2. #2
    Join Date
    Oct 2004
    Posts
    590
    In matching steppers, drives and power supplies you want to look at the amp rating of the motor and also the inductance of the motor. If you take the square root of the motor's inductance and multiply it by 32 you'll get the optimum drive voltage for that motor. Then you want to make sure that your drives are rated for that voltage and that you choose your power supply accordingly. For the power supply current rating you can take 2/3 of the total current that the combination of stepper motors in your system will require for the wiring scheme you use. For example 3 motors at 2.1A each wired bipolar series would be 6.3A. Two thirds of that is 4.2A. If your motor inductance was 3mH then your optimum voltage would be 1.73 x 32=55.4V . To summarize then with those motors and my made up inductance value you would want drives that can output 2.1A at 55.4V and a power supply that can deliver 4.2A at 55.4V. Using lower values will still work but at the cost of optimum performance.

  3. #3
    Join Date
    Mar 2003
    Posts
    35538
    If you take the square root of the motor's inductance and multiply it by 32 you'll get the optimum drive voltage for that motor.
    Actually, that is the maximum voltage you should use, not necessarily the optimum voltage. Technically, the optimum voltage is the minimum voltage required to deliver the required performance.
    For example, if a 48V power supply will allow your motors to spin at 1000 rpm reliably, but you only need them to spin at 600 rpm, then you can use a 36V power supply and your motors will run much cooler, with no loss in performance. (Usable rpm is proportional to Voltage)

    The biggest issue here, though, is that it's extremely difficult to calculate the minimum voltage that you need for a particular application.
    So what most people do, is use the above formula and choose the maximum voltage.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

  4. #4
    Join Date
    Dec 2012
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    0
    Thank you both for the replies. This is excellent information. I had read similar from trawling the forums and its nice to have that confirmed here as well.

    As all I have done so far is read I was wondering what real installations people have. I initially want to cut plastic and wood but I can see aluminium being on the cards as well.

    I have found some stepper motors from cnc4you.co.uk. Here is the datasheet

    http://cnc4you.co.uk/resources/60BYGH301B.PDF

    The NEMA23 3.1Nm motors they sell seem to have 12.8mH +/20% inductance when wired in series bipolar and a current of 2.1A.

    Using the maths provided by OCNC( thanks ) we get

    3.58 * 32 = 114V approx.

    Now I want to use a 24V power supply or possibly a 32V supply. This suggests that I probably can use it but I will not be able to run the motors quickly. How quickly will probably be a case of trial and error and would depend heavily on what material I work with.

    Any thoughts?

  5. #5
    Join Date
    Oct 2004
    Posts
    590
    My experience with wood has been that I want feed rates of between 100 and 200 ipm. With less than 100 ipm I just wouldn't be happy with my machine.

    If you wired your motors in bipolar-parallel you would get better speed because the inductance would be less but your drives would need to have a higher current rating than if you went bipolar-series. The power supply voltage would of course then be less.

    Your motors definitely provide enough torque to drive the Momus. For comparison this is a motor that is frequently used for this size machine and actually a bit larger.

    If you haven't already done so you might want to read the support documents at GeckoDrive.

    Chris

  6. #6
    Join Date
    Dec 2012
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    0
    Hi Chris,

    Well I need to get some idea of what 100IPM translates to. For example if we go with the momus machine. The pulley appears to have a circumference of 1inch.

    So 100 rotations per min gives us 100IPM. If we have 1.8degrees per step then we are looking at 200 steps * 100 = 20,000 steps per minute.

    20,000 / 60 = 333 steps per second.

    So now I need some figures on what my motor might be like at this kinda rate? Does this sound excessive?

  7. #7
    Join Date
    Oct 2004
    Posts
    590
    A 10 tooth XL pulley has a pitch dia. of .637 which equals a circumference of ~2 inches. A 200 step motor using a full-step driver therefore needs to see a 100Hz signal to travel 1 inch. (A 200 step motor using a 10 micro-step driver needs to see a 1000Hz signal to travel 1 inch.) 100ipm with this pitch pulley is 50rpm. From what I've seen of various stepper motor performance graphs most steppers of the type being used in DIY cnc machines maintain their maximum torque up to about 300rpm. After that the torque usually starts to fall off rather rapidly such that by the time you get up to 1000rpm you're done. From 300-600rpm you can usually get rapid movements (ie., low load movements) depending on the machine construction. I can rapid my modified Momus at over 300ipm without any difficulty. This equates to 150rpm so I still have a bit of head room. I tested it once to see how fast I could actually go and I think it was just upwards of 500ipm. To be factual though I'll have to check it again as this was some time ago and to no useful point (which explains my vague memory of it). My top cutting speeds usually tend to be between 180-200ipm. You shouldn't have any trouble with those steppers running a Momus. You just may have trouble getting the most efficient use of the combination of motor, driver and power supply.

    Chris

  8. #8
    Join Date
    Dec 2012
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    0
    Thanks for taking the time to explain this Chris.

    As you say I might find there is a problem driving the machine with these motors at 24V or 32V. They can perform at higher voltages but I suppose the only way I would find out is to actually test it. Are there any standard graphs to show motor speed and torgue with respect to voltage?

    What setup do you have? Are you using Gecko drives?

  9. #9
    Join Date
    Oct 2004
    Posts
    590
    I use Gecko 201's. The step motors are Nema 23 425 in-oz. I have them wired bipolar-parallel so you can see that with the inductance at 6.8mH my maximum voltage before heating effects take over is 83V. I've been using a 24V power supply to run these and they work quite well. I hope to swap out this month the 24V supply for a 72V one that I bought this summer. I'm curious to see what the difference will be. There's a certain amount of 'elasticity' in the motion of the machine that I think is caused by the 'low voltage'. I'm hoping that the higher voltage will 'stiffen' the motion. The cutting area on my machine is X (26") by Y (21"). My machine is not enclosed.

    Here's a link to [ame="http://www.youtube.com/watch?v=6iVfFjpNkz0"]a short video[/ame] of it running first at 350ipm cutting a 10" helix in air and then again at 450ipm cutting the same helix. This was part of a trouble shooting question that came up regarding a jitter in a file. We were trying to determine if it was related to the Smoothstepper. It turned out that the lookahead in Mach3 was set too low.

    Here's a link to a pdf of a torque vs. speed curve. You can see that performance starts to fall off a about 5 rps (300rpm) in the high torque motor. Just google 'stepper motor performance graph' if you want to look at some more.

    Chris

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