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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Question regarding stepper motors, inductance and voltage supply
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  1. #1

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    Question regarding stepper motors, inductance and voltage supply

    I have 900oz-in (3.3mH inductance) nema 34's on my mill being driven by KL8060 drivers and a 48V regulated power supply. I can reliably rapid at 200ipm without any lost steps or issues but I've always wondered if I can get a little more speed out of the machine. I'd like to build a new toroidal power supply for the motors, but am having difficulty finding concrete answers for acceptable voltage.

    Many "established" sources state the maximum voltage you should supply steppers with is Vmax = 32 * sqrt(L) - in my case this is roughly 58V.

    Many cnc builders state you should always supply steppers with the highest voltage available that your stepper drivers can handle.

    So which one is it? I've read that supplying steppers with increased voltage can cause overheating issues but can't wrap my head around why this happens if my drives are limiting the current flow. I'm curious what/if any consequences will occur when I supply my motors with 80V?

  2. #2
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    Re: Question regarding stepper motors, inductance and voltage supply

    My amateur understanding is that the 32 sqrt rule is not a maximum but more of a target for decent performance.

    The real maximum is usually the driver allowable input voltage.

    How good are your ways?
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  3. #3

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    Re: Question regarding stepper motors, inductance and voltage supply

    That's what I figured as well. The gecko website words it in a fashion that makes it seem like the 32 sqrt rule should never be exceeded - although I know of many who have exceeded it.

    Surprisingly, my ways are very good (not linear rails good but better than a lot of G0704's I've seen online). A frequent supply of way oil from my oiler does seem to make a big difference. I can rapid at 225-250ipm but not reliably enough to commit to running at that speed - I'm thinking a voltage increase would get me there.

  4. #4
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    Re: Question regarding stepper motors, inductance and voltage supply

    Hi,
    I use 5 phase Vexta 23 size steppers and genuine Vexta 230VAC input drivers. The drive rectifies the incoming AC and buck regulates that down to about 150VDC.
    It is the 150VDC that is applied to the motors. They run at 2400 rpm no trouble at all, in fact I have had them running at 3000 rpm.....so the high voltage sure makes them sit up and take notice!

    I say use as high a voltage as you can find drivers for.

    Craig

  5. #5
    Community Moderator ger21's Avatar
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    Re: Question regarding stepper motors, inductance and voltage supply

    The gecko website words it in a fashion that makes it seem like the 32 sqrt rule should never be exceeded
    Gecko is the one that came up with that "rule". Mainly to prevent excessive motor heating.

    Before that, the rule of thumb was 20-25x the rated voltage. With quality modern drives, motors run much cooler, even at higher voltages.
    Gerry

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  6. #6

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    Re: Question regarding stepper motors, inductance and voltage supply

    Quote Originally Posted by joeavaerage View Post
    It is the 150VDC that is applied to the motors. They run at 2400 rpm no trouble at all, in fact I have had them running at 3000 rpm.....so the high voltage sure makes them sit up and take notice!

    I say use as high a voltage as you can find drivers for.

    Craig
    Wow 150VDC @ 3000rpm is pretty crazy, that's servo territory!

    Quote Originally Posted by ger21 View Post
    Gecko is the one that came up with that "rule". Mainly to prevent excessive motor heating.
    That's interesting information to know. Seems like a liability "rule" on their part. I've never heard of modern day steppers burning up due to overvoltage, usually they fail because of incorrectly set current and the drivers fail due to overvoltage

  7. #7
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    Re: Question regarding stepper motors, inductance and voltage supply

    Hi,

    Wow 150VDC @ 3000rpm is pretty crazy, that's servo territory!
    The steppers are quite small and low torque, about 70-80 oz.in The saving grace is that they are direct connected to a low lash (< 2 arc min) 10:1 planetary
    gearbox which boosts the torque to 705 oz.in but reduces the rpm at the output shaft to 300 rpm, which is most certainly not servo territory. They have proven to be
    great little performers and the premium I paid for them in the first place has been amply rewarded, but servos they are not.

    I am building a new mill and have bought three 750W B2 series (160,000 cpr encoder) Delta servos, one braked. They perform on a new and whole different level
    than the little Vexta steppers. In fact modern AC servos eat any stepper ever made, including closed loop steppers. Don't be fooled.....steppers will never rival an
    AC servo.

    Craig

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