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  1. #1
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    PS and steppers

    I'm obviously new to CNC and am trying to figure out the relationships between the PS, drivers, stepper motors Amp, Voltage, and inductance. I am using the gecko 251's which max driver limits are 3.5 amps and 50 volts. So if I get a 48 volt power supply, my drivers are at their limit. I know the PS amps should be the motors combined amps x .67 and any motor over the drivers 3.5 amp limit will have there oz-in rating adjusted by oz-in x 3.5/(motors amps). So the steppers amp rating should be 3.5 or less to maintain their oz-in rating (or adjust the size to for the rating loss). The question I have is about the relationship between mH and voltage. I understand that using mH ( sqrt(mH) x 32) you can calculate the peak voltage for each motor (for a 48v PS that would be 2.25mH). Now my quandary, should I look at the motors I'm thinking of using and use the lowest mH rating for the PS voltage rating, use the highest (keeping in mind the 50v limit) and resisters on the lower mH rated motors, or something in the middle.

    Does voltage just control acceleration? Because I intend to use it for wax carving and therefore don't need a lot of acceleration or IPMs.

  2. #2
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    scottluther,
    If I were in your position, I would look at the motors and run the numbers.
    1. What amps are they rated for.
    2. What is the inductance of these motors.

    analysis 1.
    IF the motors are rated for much more than 3.5 amps, then the 251 is probably not satisfactory for this project unless you want to seriously derate the motor.
    analysis 2.
    If you run the 32*sqrt of inductance and the number comes up seriously greater than ~50V then again, this driver is not a good choice.

    There are all kinds of tradeoffs that you have to look at. You are correct that the holding power is ~to the current and the speed is ~to voltage as I understand it.
    Never having milled wax, I can't say for sure, but I would think that you want a lot a acceleration and speed, but agree that it will not take a lot of power.

    Hopefully someone with more knowledge than I will make more sense.
    Art
    AKA Country Bubba (Older Than Dirt)

  3. #3
    Community Moderator ger21's Avatar
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    Does voltage just control acceleration?
    Not really acceleration. Rpm is proportional to voltage. The higher the voltage, the higher the maximum usable rpm will be. The downside, is that higher voltage create heat. Ideally you want to use the lowest possible voltage to give you the performance that you need. Any voltage above that will just create heat. The problem is that it's hard to determine how much is enough.

    Are you using motors with different inductance ratings? If so, then you probably want to go with the lowest, if they are similar. If they are very different, you might be better off with separate supplies for each motor.
    Gerry

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  4. #4
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    The question came because I was trying to optimize my different stepper motors to the gecko's limits 3.5 and 50v. When I was looking at my guesstimating for the oz-in sizes I would use I obviously saw some wide ranges of amp and mH in the same oz-in range and was wondering what the impact would be. My goal was to get the oz-in range with motors close to or below the 3.5 and 2.25 mH range. I came up with these motors:

    Nema 17 62 oz-in. 2.8v. 1.7a. 2.5mH
    Nema 23 177 oz-in 2.5v. 2.83a. 2.5mH
    Nema 23 270 oz-in 3.2v. 2.8a. 3.6mH

    These were as close to the oz-in that I thought I would need and the specs. I am trying to set up a Taig with a small trunnion table for 5 axis. A, X, and Y would have the 177 oz-in steppers, the Z would have the 270 oz-in stepper, and the B axis (3in rotary) would have the 62 oz-in stepper.

    Do I need to worry about heat if I'm under their max voltage ratings? These are all above the 2.25mH number for a 48v power supply.

  5. #5
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    Those motors are going to be screaming at 48 volts. I would recommend starting with a (MUCH less expensive) 12 volt supply. e.g. from a PC power supply:

    Converting a PC Power Supply Converting a standard ATX PC power supply into a bench supply.

    Getting a Compaq HP DL580 Proliant Server Power Supply to Work - RC Groups Converting hot swap PC power supplies into bench supplies.

    For some information on how fast a motor will probably spin given a certain power supply, etc.. see:
    techref.massmind.org/techref/io/steppers.htm#Estimating under "Estimating Maximum Speed"
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  6. #6
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    a 12v powersupply will NOT work with a gecko 540, It requires a supply between 18 and 50V.
    Art
    AKA Country Bubba (Older Than Dirt)

  7. #7
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    Quote Originally Posted by Bubba View Post
    a 12v powersupply will NOT work with a gecko 540, It requires a supply between 18 and 50V.
    Doh. My bad. Still... start with something cheap and smaller and go up if you need to. 24 volts maybe.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  8. #8
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    Does anyone else agree with the 24v comment? I only ask because someone else commented that I would be disappointed with that low of voltage. I was hoping for consensus . That hope has obviously bit the proverbial dust. Anyone else want to throw their weight on one side or the other of the scale?

  9. #9
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    Just to be clear, I recommended starting smaller because those motors are tiny, and not terribly powerful, and your little Taig shouldn't need that much especially cutting wax. My GUESS is that 24 or less will move them around ok, but I could be wrong.

    HOWEVER: I've just now realized that you may be confusing the HOLDING torque of the motors with the actual DYNAMIC torque required to operating the axis. Just to make sure: The torque rating most often supplied with motors is the torque required to turn the shaft against the holding ability of the motor. That means nothing when your goal is for the motor to turn something. You want dynamic or pull out torque when measuring how much is going to be required to turn the axis on the Mill. Actually, torque readings are only useful AT SPEED, and so unless your motor mfgr supplies torque curves, it really doesn't mean much and isn't necessary to design a drive system.

    Here are the steps:

    1. If you have the actual hardware, measure the actual torque required to turn the shaft where the motor will be connected. Measure with a torque wrench, or a weight at a distance like you use to measure motors. Here is an example: techref.massmind.org/techref/io/stepper/measure-torque-rb.htm

    If you don't have it, estimate the weight of the items that the axis screw will be moving.

    2. Figure out what speed you want your machine to be able to move at, as a top speed. More speed = more money.

    3. Use the formula or calculator at:
    techref.massmind.org/techref/io/steppers.htm#Estimating to figure out how many WATTS of drive you need to get that speed. Watts = IPM * Lbs / 531 or Watts = torque (in in-oz) * RPM / 1351. These are "rules of thumb" from Geckodrive. Add a little extra for error and to be certain.

    4. Watts = amps * volts (or more usefully: amps = watts / volts) so you need a current and voltage rating that, when multiplied, reach more than your minimal required watts. For example, if you need 200 watts of drive power, and you are using a 24 volt supply, then you would need more than an 8 amp motor... 200/24=8.333 NOT reasonable! So then you bump up to 36 volts and that would require a 5.5 amp motor. At 48 volts, you would need a 4 amp motor. That seems doable. Again, this is an /example/! You may not need 200 watts of drive. Use step #3 to find out what you really need! A small Tiag with those little motors shouldn't need more than 50 watts for very reasonable speed.

    5. Given an amperage rating, and a voltage, find a driver that will manage that combination. Your Geckodrive will do what was it? 3.5 and 50? So hopefully you don't need more than 175 watts. If you only need 50 watts, your 2.8 amp motors will do just fine at 24 volts. If you need more drive, you will need a higher power supply... it all comes down to knowing what you need. Again, I'm guessing your setup won't need that much.

    6. Once you have a driver, and know the amps and volts, find motors that are rated for that. Those little guys are probably fine. MAX voltage is 32 * the mHs or 80 in that case, but you certainly don't need to run them at 80 volts! LOL...

    7. Finally, pick the lowest supply voltage you need for the drive system. The lower the voltage, the cooler the motors will run and the less ringing and mid band resonance issues you will have.

    That is the COMPLETE process for designing a drive system. Not a guess... I KNOW that works: I learned it from Mariss (of geckodrive) and Roman (of Linistepper and SLAm), and they certainly have it right.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  10. #10
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    Ok James, I bow to your expertise

    Not just the knowledge, but you put it out there in a way that was easy to understand and that I could use to do the calculations. It was extremely helpful and gave me confidence in the choice. Using the 24v number and my motors amp rating to calculate watts and then working backward with the lbs*IPM/531= watts formula because I don't have the exact lbs number or the IPM. I could then substitute different numbers in there.

    So it went something like this 24v * 2.8 amp motor = 67.2 watts for the proposed PS and my motors combination

    Then 67.2 * 531 = 35,683 = lbs * IPM

    Even if I substituted the entire weight of the Taig for lbs, I would still need to exceed 419 IPM for 24v to be under powered.

    So in conclusion... "YOU DA MAN!"

    Since I have you on the hook, one last question (for now), does it matter whether I use a regulated or unregulated power supply? In my situation, I'm not going to get any back power generated from the motors.

  11. #11
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    You NEED to use an UNregulated supply. Motors and motor drivers draw massive current when a coil turns on and release large spikes when the coil is released. A poor little semiconductor trying to keep that regulated is just going to have a nervous breakdown!

    I'm a little weird on this, but I really like car batteries with chargers as power supplies. You put them in a battery box, and NEVER overcharge them (cheap intelligent chargers are everywhere) so they are pretty safe, especially in a shop or garage. And then you can't possible not have enough current handy... Cold cranking amps right? The only trick is you have to stack them for 24, 36, 48 volts and that means the chargers have to play nice when stacked (not a given) or you have to find a charger for that voltage of battery which can be done, but isn't as cheap. It's a bit of a pain, but there can be no finer power supply!

    Other than that, some people have been able to stack PC power supplies... that's scary... but cheap.

    Let us know what you find for a supply? I'd like to collect some good recommendations for the page at:
    techref.massmind.org/techref/io/stepper/power.htm
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  12. #12
    Community Moderator ger21's Avatar
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    Even if I substituted the entire weight of the Taig for lbs, I would still need to exceed 419 IPM for 24v to be under powered.
    On 24V, you're maximum rpm from your steppers will probably be about 1000 best case. With a 20tpi leadscrew, that means you probably won't see more than 50 ipm.

    While the formulas may tell you how many watts you need, it can be far more complicated than that. There are numerous threads here from people who have much larger motors and drives than they need, yet they get poorer performance than others using smaller, less powerful motors and drives.

    To get the best performance from steppers, everything must work together, and compliment each other. That includes the motor, drive, power supply and leadscrew.


    You NEED to use an UNregulated supply.
    And yet 95% of people are using cheap, regulated switching supplies with no problems.
    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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