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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Help with final motor specification decision
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  1. #1
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    Help with final motor specification decision

    Hello

    It would be really nice if I could get your opinion for a stepper motor buying decision. It is for a 4 motor 3 axis CNC machine for wood machining, featuring a 2.2kW air cooled spindle.

    The setup is:
    – 2005 ball screws with 12mm machined end for joint coupling
    – 8mm/12mm round or 14mm/12mm keyed plum flower type shaft coupler (depending on choice below)
    – 20mm linear rails

    The drives are 4 x DM556T (made by Leadshine), featuring the following current table choices:
    Peak RMS
    – 3.8A 2.7A
    – 4.3A 3.1A
    – 4.9A 3.5A
    – 5.6A 4.0A

    The choice is between:

    1) NEMA23
    Current / phase 4.2A
    Resistance 0.9 ohm
    Rated Voltage 3.78V
    Inductance 3.8mH
    Holding Torque 3Nm
    Phase / Wires 2 / 4
    Shaft 8mm

    2) NEMA34
    Current / phase 4.0A
    Resistance 0.31 ohm
    Rated Voltage 1.36V
    Inductance 2.86mH
    Holding Torque 4.5Nm
    Phase / Wires 2 / 4
    Shaft 14mm

    Price is not an issue here, as both solutions are virtually identical in price.

    I saw a number of posts online that recommend NEMA 23 over NEMA 34 based on smaller motors having a larger torque at higher RPMs – as required by the 5mm ball screw. However what attracted my attention to this NEMA 34 motor was the very low inductance (even compared to the NEMA 23) while also providing a higher holding torque. Unfortunately I don't have any torque diagrams available.

    Further, the NEMA 23 has a 8mm shaft, whereas the NEMA 34 has a 14mm one. The 20mm ball screw is 12mm diameter with respect to the coupling connection. Would you be concerned going from 8mm to 14mm with the coupler?

    Could you please let me know your opinions?

    Thanks,
    Christian

  2. #2
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    Re: Help with final motor specification decision

    None of them.
    For one, with your drives limited to 50v max nema34 are out of it.
    For second, 3.8mh inductance is too high.

    https://www.omc-stepperonline.com/ne...m-4-wires.html

  3. #3
    Community Moderator ger21's Avatar
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    Re: Help with final motor specification decision

    A motor like this would be much better, with the drive set to 5.6amps.
    https://www.automationtechnologiesin...-single-shaft/
    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
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    Re: Help with final motor specification decision

    Hello

    Thank you both for your replies and recommendations. Both are really great suggestion of motors with very low inductance. I am particularly interested now in the StepperOnline motor.

    Dsap1976 – Could I ask you please about your setup. I saw you posting in another thread where you mentioned going from 7Nm NEMA 34 to 4Nm NEMA 24 while running at 60V. The motor you pointed me to has 1.85mH. The formula I was taught to use to calculate V(max) is 32 x SQRT( Inductance in mH ), which would result in 44V max supply voltage. Are you running a similar low inductance motor at 60V despite that? I am trying to understand if I am making a mistake with the voltage calculation.

    Thanks,
    Christian

  5. #5
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    Re: Help with final motor specification decision

    Hi,
    the simple rule is use the highest voltage drivers and power supply you can get your hands on. The higher the voltage
    the faster you can go before the inevitable degradation of torque defeats yor stepper.

    Craig

  6. #6
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    Re: Help with final motor specification decision

    Hello

    Yes, sure. But there is a recommended maximum for each motor, right? The above formula seems to be the general recommendation on the internet, but for very low inductance motors this also would results in a lower recommended maximum supply voltage value.

    I was curious how far anybody here is actually pushing it. So if you are running a very low induction motor at a high supply voltage it would be interesting to know. Are people using different formulas to calculate the recommended maximum?

    Thanks,
    Christian

  7. #7
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    Re: Help with final motor specification decision

    Hi,

    But there is a recommended maximum for each motor, right?
    No. Have you ever seen on any of the stepper spec sheets a maximum recommended driver voltage? At most they list the insulation voltage withstand, usually 500V.

    but for very low inductance motors this also would results in a lower recommended maximum supply voltage value.
    No, the whole point about getting the lowest inductance motor you can is to go as fast as you can without undue degradation of torque. Don't throw it all away
    by using a lower driver voltage, use the maximum you can find.

    I drive my Vexta steppers with genuine Vexta drivers. The drivers are powered direct off the 230VAC line but internal to each driver is a DC power supply of about 150VDC.
    Thus the drivers are supplying to the stepper 150VDC pulses......they go like hell. I routinely run them at 2400rpm but have run them at 3000rpm without losing steps.

    Craig

  8. #8
    Community Moderator ger21's Avatar
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    Re: Help with final motor specification decision

    But there is a recommended maximum for each motor, right?
    Gecko came out with that formula, and they claim that higher voltages don't offer any benefit.
    Lots of users have found that higher voltages do indeed run faster, and modern high end drives keep the motors cool at those higher voltages.
    The only real limit is to keep the motors from getting too hot. (<80°C)
    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)

  9. #9
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    Re: Help with final motor specification decision

    Thanks Gerry - now that you mention it, I am sure that the Gecko documentation is exactly where I had my information from.

    Christian

  10. #10
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    Re: Help with final motor specification decision

    The only real limit is to keep the motors from getting too hot. (<80°C)
    I used nema 34 on x and y axis. nema 23 on z, all with the same MA860H driver and all voltage 70 volt, I found nema 23 motor hotter than nema 34. I think < 80 deg C, good info ... I was thinking to put air cooler on Z, cancel it ...

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