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  1. #1
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    Apr 2008
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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
    ger21
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    Re: Which stepper motor and power supply option?

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

  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

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