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  1. #1

    Join Date
    Dec 2021
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    10

    Choosing the right stepper motor ?

    Hello all,

    Firstly, sorry if I am not in the right section because there is a bunch of it

    I am building/planning my first CNC that I aim to mill Aluminium with. It's a small desktop CNC but I try to make it sturdy.

    But I have a hard time choosing stepper motors... I have established the size standard I will use : NEMA23 form factor.

    I think, I have established the torque I need : about 2.8nM (430oz I think) -> Is it "correct" coupled with SFU1605 LeadScrew ?

    I plan to use either a "OpenBuild Black Box" or a "CNC xPro V5" controller, so as I wish to use the internal stepper driver, I will limit myself to 4.2A of peak current.

    What I really don't get about the spec sheet of motors is the relation between current peak and torque, it does not seem to be related like a traditional motor. I see 4.2A with 3nM of torque, sometime I see 6A with only 2nM of torque. Is the length of the motor influencing this ?

    Maybe you can give me a general rule about stepper to help me choose ?

    Thank a lot.

  2. #2
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    Jan 2018
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    1368

    Re: Choosing the right stepper motor ?

    With nema23:
    Beings you know roughly what torque you want, my further opinion would be:

    You want as low an inductance / resistance as you can get.
    You want to go against the grain and use higher voltage drivers like DM860T.
    Using a minimum 60vdc on n23/24 sized motors gives you high max velocity.

    Something like:
    https://www.omc-stepperonline.com/nema-24-stepper-motor/p-series-nema-24-bipolar-1-8deg-3-5-nm-495-74oz-in-5-0a-60x60x97-8mm-4-wires.html

    https://www.omc-stepperonline.com/stepper-motor-driver/digital-stepper-driver-24-72a-18-80vac-or-36-110vdc-for-nema-34-motor-dm860t.html

    I'm basically using these or something very close to it and haven't been disappointed.

  3. #3

    Join Date
    Dec 2021
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    10

    Re: Choosing the right stepper motor ?

    Quote Originally Posted by dazp1976 View Post
    With nema23:
    Beings you know roughly what torque you want, my further opinion would be:

    You want as low an inductance / resistance as you can get.
    You want to go against the grain and use higher voltage drivers like DM860T.
    Using a minimum 60vdc on n23/24 sized motors gives you high max velocity.

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

    https://www.omc-stepperonline.com/st...or-dm860t.html

    I'm basically using these or something very close to it and haven't been disappointed.
    Thank for your reply.

    What is the influence of voltage on the stepper ? The torque ? The torque/speed ratio ?

    Because they specify torque like a constant on spec sheet. I would like to run them at 24VDC with the included stepper driver (on the controller) due to space issue on my CNC.

  4. #4
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    Apr 2004
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    141

    Re: Choosing the right stepper motor ?

    Stepper motors generate a lot of back EMF, which increases with the motors RPM. With only 24v fed to the motor controller, you are limiting yourself with regards to max usable rpm of your motors.

    If you are concerned with space in your controller box, consider using integrated stepper motors like this one, and run it at 48v. If you look at the torque curve chart, please note the difference between 24 and 48 volts. At 24v, the torque drops to 1.2nm by 250 rpm, but at 48v, the torque stays up above 1.2nm until close to 850 rpm, or more than 3x the usable speed on that motor just by bumping the input voltage up 2x.
    https://www.omc-stepperonline.com/es...c-1000cpr.html

    However, I would HIGHLY recommend finding the space for the motor / controller mentioned previously. You will get much better performance and be far less likely to be replacing drivers / controllers in the future.

    I hope that this helps.

    John Z

    Sent from my SM-G991U using Tapatalk

  5. #5
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    Quote Originally Posted by karnalta View Post

    I would like to run them at 24VDC with the included stepper driver (on the controller) due to space issue on my CNC.
    Current (amps) = torque
    Voltage = velocity

    You will get shockingly bad rapid speed from that.
    24vdc is designed for nema17 on 3D printer teritory

  6. #6

    Join Date
    Dec 2021
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    10

    Re: Choosing the right stepper motor ?

    Thank a lot for these infos, I think I will adapt my design to fit drivers in it and run it a 48VDC.

    Happy new year

  7. #7
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    Nov 2013
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    1834

    Re: Choosing the right stepper motor ?

    Hi,
    all steppers lose torque the faster they go, it's plain physics. The measure of how bad the degradation will be is inductance, the lower the better.

    A 400oz.in stepper of 8mH inductance will have less than 10% of it torque (40oz.in) at 1000 rpm. The same size stepper and still 400oz.in but 1mH inductance will have
    about 45% of its torque (180oz.in) at 1000rpm.

    Most manufacturers know that first time buyers have no idea what inductance is so they make really high torque (sounds good right!) but high inductance motors and
    sell them cheap to sucker these people in. A 500oz.in 8mH motor has got to be better than a 400oz 1mH motor......well no, at speed the 400 oz.in low inductance motor will beat
    the 500oz.in every time.

    In 23/24 size motors look for 1mH-2mH, 1mH preferred and reject anything over 2mH.

    The next thing is to run the stepper at as high a voltage that you can find drivers for. At the current time there is a whole crop of 80VDC divers, get some and use an 80VDC supply.
    Why you would pay for 80VDC drivers only to use them at 60VDC is a mystery to me.

    My Vexta 5phase steppers that I used for seven years, were second hand when I got them, and they're still going strong and they have drivers that apply 150VDC plus. I used to run them,
    without missing steps at 2400rpm, voltage is key to getting ANY stepper to run fast.

    Craig

  8. #8
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    Re: Choosing the right stepper motor ?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    Why you would pay for 80VDC drivers only to use them at 60VDC is a mystery to me.
    Craig
    I just do that to avoid any damage from voltage spikes (just in case). Some supplies show a +/- 10% margin.
    Like with toroidal, their idle voltage is higher than when their loaded so I allow for that.
    Max supply I'd buy myself for the 80v drivers is a 72.

    Anyway, Nema23/24 on 60vdc absolutely fly, don't really need any more than that. (And that's on 3mh inductance motors, even better when they're lower than 2).
    With nema34 size using 90vdc on the DM860T makes a big difference too.

  9. #9
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    Nov 2013
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    1834

    Re: Choosing the right stepper motor ?

    Hi,

    Max supply I'd buy myself for the 80v drivers is a 72.
    Then your wasting margin.

    Craig

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