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  1. #1
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    Which stepper motor and power supply option?

    Would like some advice from the experts please.
    I have 2 options and i have read lots about stepper motors and matching power suppy etc and think i have got this right.
    This is for a small/med size router im building.
    I have 2 leadshine drivers and 1 gecko both rated at 80vdc max.

    So at the moment i have an unregulated power supply with a 800va 50vac tap torriadal transformer which gives me about 71-75vdc output.
    Using this i can buy nema 24 430oz in motors 2.8a with 6.8mh inductance.
    Buy my calc. Max volts for these motors is 83.5v so my transformer should be good for 3 motors for volts and amps.

    Option 2 is nema 24 425oz in 5amp 2.3v motors with 2mh inductance.
    But this will mean i have to buy a new transformer which i would rather not.
    If i do i can get a 500va 30vac tap transformer which will give me 42vdc and enough amps for 3 motors i think?

    So after a long thread start and if my calc are correct because i have enough volts in my existing transformer are the high inductance motors going to be ok or would i be better getting the low induc motors and a new transformer?
    I only want to buy once.

    Thanks.

  2. #2
    Community Moderator ger21's Avatar
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    Re: Which stepper motor and power supply option?

    I'd run the 5 amp motors at 72 volts.
    Gerry

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  3. #3
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    Re: Which stepper motor and power supply option?

    Quote Originally Posted by ger21 View Post
    I'd run the 5 amp motors at 72 volts.
    Me too. Especially if you are using modern drives that cut back the current when idle.

  4. #4
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    Quote Originally Posted by ger21 View Post
    I'd run the 5 amp motors at 72 volts.
    Ok so a bit confused, your saying i should keep my transformer and use the low inductance motors?
    My calculations for max volts for 2mh motors is only 45vdc?
    Could you explain?

  5. #5
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    Re: Which stepper motor and power supply option?

    Hi,

    Using this i can buy nema 24 430oz in motors 2.8a with 6.8mh inductance.
    6.8mH inductance is too high, don't get these motors....no matter what voltage you run them at they will lose steps at speed.

    Look for the lowest inductance motor you can and run them at the highest voltage your supply can generate and/or your drivers can handle.
    The formula you are using to calculate the voltage is just a recommendation.....a ballpark figure. The bottom line is the higher the voltage the
    faster the motor will run without losing steps.

    Craig

  6. #6
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    Quote Originally Posted by joeavaerage View Post
    Hi,



    6.8mH inductance is too high, don't get these motors....no matter what voltage you run them at they will lose steps at speed.

    Ok thanks.
    That clears things up a bit.
    I will get the low inductance motors and use the transformer i have.
    I will keep an eye on motor heating and can always swap transformers later.


    Look for the lowest inductance motor you can and run them at the highest voltage your supply can generate and/or your drivers can handle.
    The formula you are using to calculate the voltage is just a recommendation.....a ballpark figure. The bottom line is the higher the voltage the
    faster the motor will run without losing steps.

    Craig
    Thanks for that. I will get the low inductance motors then.
    I am only going to get 2 motors for now and see how 1 goes on my z axis. If i feel i need a bigger motor and will get a nema 34 for that axis.
    Cheers.

  7. #7
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    Re: Which stepper motor and power supply option?

    Hi,

    I will keep an eye on motor heating and can always swap transformers later.
    Heat generation has nothing to do with the voltage of the power supply but the current delivered o the motor by your driver.

    I thought I might commit to 'paper' some ideas about stepper inductance and how important
    it is to having a stepper run fast and reliably without loosing steps.

    The first picture is a representative diagram of a stepper driver, the voltage source of 24V, driving a winding
    of a stepper with a resistance of 1 Ohm and an inductance of 1mH. Our assumed stepper has a maximum current of 6A.

    The second picture shows what happens if the voltage source is turned on (at t=1ms).....Note that eventually
    the current will get up to 24A an destroy our stepper. The stepper driver, rather than the rather simplified
    voltage source I have pictured, restricts (current limits) its output to 6A, our nominal rated current of the
    stepper motor.

    Fortunately the inductance means that the current does not immediately go up to 24A but it builds up. I have
    sketched the 'time constant', it is a calculation that allows you to know how quickly the current will build up.
    In this case with R=1 Ohm and L=1mH the time constant, that is the time required for the current to build up to
    66% of its maximum eventual current is one time constant, 1ms.

    The third picture is the time delay from when the voltage is turned on until the current reaches the normal
    maximum of 6A. It takes about 0.3ms with 24V input. The stepper would achieve its maximum rated torque at 0.3ms
    AFTER the power was turned on. If we assume that once the current has built up to 2/3 of max, 4A then the current
    would be enough to generate about 2/3 of the steppers maximum torque the delay is reduced to 0.2ms.

    Thus if we stepped our stepper every 0.2ms the current would build up to about 4A at each step and the stepper would get
    to about 2/3 of it maximum torque and enough to drive our axis ball screw say.

    Its rotational speed would be 1 / (200 steps/rev X 0.2ms)= 25 revs/second =1500 rpm.

    If the inductance of the stepper is high, lets say 4mH then the delay for the current to build up to 4A would be
    about 0.8ms and the rotational speed would be reduce to 375rpm.

    The last picture is the delay if we increased the input voltage to 48V, ie double. Note the delay has halved.
    Therefore we can step the stepper faster because the current builds up faster, the stepper runs faster
    without loosing steps. Thus we want the highest possible voltage driver to drive our steppers to make them run fast.

    When you make a stepper you can increase its holding torque by increasing the number of turns of wire in the coils. This
    also increases the inductance dramatically. Most inexperienced CNCers think 'well I'll get that 500 oz.in stepper,
    it must be better than that 400 oz.in one' without realizing that the inductance of the 400 oz.in one is much
    less than the other, and it will go much better when going fast. Manufacturers tend to trumpet the torque their
    motor can achieve but don't specify the inductance.

    Don't be caught.....you want low inductance motors, if a manufacturer or supplier cannot give you that specification or
    some other speed/torque diagram, walk away.

    Craig

  8. #8

    Re: Which stepper motor and power supply option?

    Interesting discussion and very informative for a newbie.

    Question: Is a low inductance motor to avoid lost steps still necessary when using a stepper with closed loop position feedback? I would assume that the closed loop stepper will no longer lose steps. Are there other compelling reasons to stay with low inductance motors?

    Thanks!

  9. #9
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    Re: Which stepper motor and power supply option?

    Hi,

    I would assume that the closed loop stepper will no longer lose steps. Are there other compelling reasons to stay with low inductance motors?
    Rubbish. A stepper motor loses steps when it lacks the torque necessary to step in response to the commanded input. This is particularly pronounced at high speed as the
    stepper torque diminishes.

    A closed loop stepper of the same torque and inductance of an open loop stepper will lose step just the same. Its true that the closed loop stepper will try to catch up,
    ie the controller will insert extra pulses to try to catch up but if the motor is just plain overloaded the extra steps will suffer the same fate as the regular steps.

    A closed loop controller DOES NOT suddenly make a stepper any more powerful, or torquey, or faster.

    The manufacturers of closed loop steppers are playing on your fear of losing steps and are selling you something expensive when you don't need it.

    If you choose a open loop stepper with sufficient torque and low inductance such that you operate that stepper within its torque/speed curve IT WILL NEVER
    LOSE STEPS, ie closed loop is a waste. I have high quality open loop steppers and high voltage dives on my machine and I have not lost a step in the five
    years I have had them except when I try doing something stupid. Even closed loop steppers won't solve the problem if I do something stupid.

    IF you try to operate a closed loop stepper outside of its torque/speed curve it will lose steps. The controller will try to catch up but fail. When the stepper
    lags behind a certain number of steps the driver will fault 'following error'. That is one advantage of closed loop....if the stepper gets out of position
    the driver senses it and faults and stops whereas an open loop stepper will carry on with potential inaccuracy wrecking your part.

    It doesn't matter open loop or closed loop LOW INDUCTANCE is essential if you wish your steppers to spin fast. If you try to use a stepper, again. open
    or closed loop, beyond its torque/speed curve it will lose steps and NO controller can change that.

    Craig

  10. #10
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    Re: Which stepper motor and power supply option?

    If you choose a open loop stepper with sufficient torque and low inductance such that you operate that stepper within its torque/speed curve IT WILL NEVER
    LOSE STEPS, ie closed loop is a waste.
    If you live in an ideal world this is true, but I live in the real world and it isn't even close. There are so many things that affect the torque required that on a homemade machine you simply can't guarantee that you will never exceed the torque capabilities of the motor. A gib strip too tight, or simply trying to drill a hole a little to aggressively, or trying a little too aggressive a cut with an end mill can overload an axis motor whether open or closed loop.

    Also, depending on the closed loop stepper driver, a loss of steps results in the driver increasing current momentarily, thereby increasing torque. So, the closed loop stepper has the ability to act much like a servo in this regard.

    In case you are wondering, I have a G0704 with 570 oz-in 2.2 mH open loop steppers and is a fantastic machine. After using it for a while I learned its limits, but I could easily make it lose steps by simply setting a feedrate a bit too aggressive like I mentioned before.

    I also have a RF-45 sized machine and it has closed loop steppers that are 1600 oz-in 12 mH inductance. That's twelve, not one point two. That machine can rapid faster than the G0704 (250 IPM vs 180 IPM), Both machines have 5mm pitch screws and so the higher inductance motor has to spin faster.

    Bottom line is that closed loop steppers definitely have advantages that can't be talked away by simply saying don't exceed the torque rating of an open loop motor. I will still use open loop depending on the intended use, but for the main 3 axes of my primary mill I would no longer use an open loop stepper.

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