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Thread: CNC Power Supply

  1. #1
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    Question CNC Power Supply

    Greetings!
    As soon as I discovered this forum I figured out that it might be just the right place to place some questions.
    So I'm designing a small scale CNC Router with movable gantry. The dimensions are X=700mm Y=1000mm Z=150mm. At the begining, the idea was to cut bodies for solid-body electric guitars, although it has general purpose.
    I want your advice, about the power supply for the motors I use.
    They have the following parameters:

    Holding Torque (Nm ±10%) 2.8 (about 420 oz-in)

    Current / Phase(A) 3

    Resistance / Phase (ohms±10%) 1.6

    Inductance / Phase (mH±20%) 6.8

    Detent Torque (N cm) 12

    Rotor Inertia (g-cm 2 ) 800

    Length (mm) 112

    Weight ( kg ) 1.4

    I am confused with the choice of power supply. At my local electronic store there are three options.
    1. 12V/12A impulse power supply.
    2. 24V/14.6A PSU.
    3. 24V/10A PSU.

    Which of them will fit best?

    Thank you, in advance.

    P.S.: Excuse my poor English.

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  2. #2
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    Photo

    Here is a render image made with AutoCAD.



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    Hello Tomov,

    Non of those PSU's will be any good for those motors, they will work with the motors but you will be massively under powerd on the voltage resulting in slow speeds.

    For best performance from those motors you need around 80V (it actually calculates to 83V).
    The way to calculate voltage is to use the motors Mh inductance rating. Take the square root of the inductance and times by 32 this is then the ideal voltage for best performance for that motor. . . . IE Sq/rt 6.8=2.6 x32 =83.2V

    The amps of the PSU dont need to be the total amps of all the motors, to work out PSU amps required you total all the motor amps and divide by 68% this is the amps required for the PSU.
    IE: 3A motors x 3 = 9/68%=6.1A PSU required.
    Allowing a little bit more wont hurt but more than say 70% and your just paying for a larger PSU than you need. . . . More Amps wont hurt anything just your wallet.

    So for a 3 axis machine using 3 of the same motors you will need a 83V @6.5Amp PSU.

    Also if your using differant sized motors then you calculate the PSU volts to the smallest motor used.

    Your drive's will probably be the limiting factor because most drive's max at around 70V and for these motors you will need some large drive's for best performance ideally about 90V. You always want the drives to be able to handle slightly more volts than the motor/psu combo.

    You could use these motors with 70V drives and say 65V PSU but you will be running below the motors capabilty in regards to speed.

    The matching of motors,drives and PSU is very important and the KEY to getting best performance from Stepper motors.

    Most that use these size motors tend to build there own PSU's using toroidal transformers often it works out the cheapest way of getting a PSU to give best performance.
    It's not very difficult to do if your prepared to get your hands dirty and are comfortable with electrics.

    Hope this helps.



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    hemsworthlad, thank you for the fast response.

    Having your opinion in mind, I browsed the website of one of the local dealers. They are having drivers for both bipolar and unipolar stepper motors, but with the following limitations:

    60V/5.5A - this is the top rated driver circuit,
    all the other options are up to 2.5A/32V. Regardless the motors I have chosen.
    Could you suggest an option for any of the components - motors, drivers, breakout board, power supply suitable for my needs ?

    And one more thing, is there a simple way to calculate the torque of the motors needed to move the gantry and the spindle?

    It seems that most of the kits on eBay are not properly rated - motors and power supply... a shame... :S



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    Just had a thought.? Where those figures you quoted for wiring Bi-polar parallel or serial.
    The MH seems a little high so I have a feeling it's serial your quoteing(which motor suppliers often quote) in which case you would be better off wiring them parallel. This would lower the volts needed but up the amp's.
    Parallel is the better option because it gives more torque as the speed rise's, torque drops off very quick when wired serial, parallel gives a better all round performance.



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    Here is the web page of the supplier SIRIUS-PCB

    The page is in Bulgarian, but the motor parameters are in English.

    Ooh and one more thing - What is RATE VOLTAGE ? I have seen some motors with "RATE VOLTAGE about 4 or 5 volts...

    Last edited by Tomov; 12-09-2010 at 03:12 PM.


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    Quote Originally Posted by Tomov View Post
    hemsworthlad, thank you for the fast response.

    Having your opinion in mind, I browsed the website of one of the local dealers. They are having drivers for both bipolar and unipolar stepper motors, but with the following limitations:

    60V/5.5A - this is the top rated driver circuit,
    all the other options are up to 2.5A/32V. Regardless the motors I have chosen.
    Could you suggest an option for any of the components - motors, drivers, breakout board, power supply suitable for my needs ?

    And one more thing, is there a simple way to calculate the torque of the motors needed to move the gantry and the spindle?
    Bi-polar is the way to go and wire them parallel not serial( see post below)

    Post the link to the supplier and I'll have a look for you.

    Regards calculating torque yes their is a scientific why to work it out but it's quite in depth and my life's too short to waste time crunching numbers.
    A simpler why in my view is take look at several other builds of a similiar size and material to what you intend building and use what they used has a guide making any adjustments you think maybe needed to suit your build. Maybe ask how there motors etc perform, most are only too happy to advise.

    If you give some details on machine and what components you intend using IE: Ballscrews, acme or R&P belt etc and linear motion IE: profiled rails, round rail, flat bar & bearings style etc plus frame construction materials and i'd be happy to give some help with motors drives etc.



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    Quote Originally Posted by Tomov View Post
    Here is the web page of the supplier SIRIUS-PCB

    The page is in Bulgarian, but the motor parameters are in English.

    Ooh and one more thing - What is RATE VOLTAGE ? I have seen some motors with "RATE VOLTAGE about 4 or 5 volts...
    IF the motor labelling says some thing like . . . Rate: 4v then this will be the motors rated voltage.! Depending on motor wiring IE: serial/parallel sometimes it will say some times not.?. . . often when high volts 5+ they tend to be quoting serial connection.

    I looked at the link and to be honest I wouldn't go with any of those drives they look a bit weak and not very good quality.
    Do you need to buy local or are you prepared to buy outside of Bulgaria, IE: from states or else where in europe.?



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    Okay, I'll start from here, then.

    1. So the frame, as seen on the picture is made from steel rectangular pipes. The dimensions are 50x60x3 mm.

    2.The gantry is made out of 18mm thick MDF board.

    3.The linear slides are profiled - round hardened steel bar (20mm in diameter) both for X and Y axis:


    4. The lead screw I bought form the local hardware store. (don't shoot!) It's metric thread M10 with 1.5mm pitch. I intend to buy ballscrew and nuts in future. But for now it's just that - I made the nuts by myself out of polyacetal (kind of hard, wear-resistant plastic) The screw will be fixed at the both ends on two radial bearings with nuts (on each side):


    5. Here are the Z rail (d=16mm) and one of the linear bearings:


    6. One of the motors I have (Nema23, 2.4 Ohm - coil resistance from what I've measured):



    Here is bigger photo of the project:


    And, about buying from here (Bulgaria)... I'm starting to think for a way to order the drivers and the breakout board from the web. Just looked at Gecko Drive website.



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    Ok thank you now we have a better idea what were dealing with.

    First because you have a mix of home made and off the shelf compnents then the calculated way wouldn't be much use anyway due to unknown force's so I'll take the rule of thumb approach with a mix of real use experience. . . . Never failed me so far.!

    I think you already know the T-rod is the weak link so every thing I'm suggesting will be working on using real screws.
    The threaded rod in theory will be more demanding on the motors than BS due to higher friction but in practice this wont be the case.? You wont be able to run the t-rod very fast because it will wear the nut away in a short amount of time if you do.? . . . which you wont want to do every week.!!

    3Nm nema 23 motors will work great on this size machine, most 3Nm motors in europe when wired parallel will have motor rating in the region of 4.2A & 2.7V with inductance around 3Mh which would need a PSU approx 57V @8.5a

    Drives capable of handling these motor specs are easily available in europe and not overly expensive most come from china and copies of leadshine drives which are a proven design.
    Personally I prefer Gecko drives and have used the cheap leadshine drives and even some not so cheap real leadshine and the Gecko's just out perform them in every department IMO and all though on first inspection they look expensive they dont work out that much more and the extra capabiltys and performance make it worth the extra IMO.
    Thru experience I've come to find it doesnt pay to buy cheap components and trying to do it on the cheap only costs you more in the long run but the drives and PSU are particularly important items not to cut corners on.

    So my 1st recommendation would be a G540 with motors to suit it's 3.5A limit if you dont mind the extra cost and buying from states. . . Or something like these from europe Zapp Automation

    The Gecko 201x is also a top drive for the money and allows motors larger than 3.5A to be used and for the little extra compared to the above would be my second option if I wanted to use larger motors or intended to upgrade
    the machine at a later date.
    This upgrade potential is very important and the single biggest reason why not to cut corners on drives and if done wisely your drives and PSU which are some of the most expensive items of the build will go with you from machine to machine. Motors are cheap and can be got pritty much anywhere.

    Another potential weak spot I recommend you think about is not using those unsupported rails on the Z axis.! No point having supported rails on the other axis if the main axis that supports the cutting end flexs.??



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    I was pleasantly surprised after I looked at the Gecko Drive G540, but included in a kit, along with 3x 380 oz.in motors, cables and the included switching power supply. Also I think that the price is a bargain
    After some modifications on the construction - ballscrews and nuts instead of the ISO threaded ones and supported rails for the Z rail I am looking to buy that kit:


    - 48V 7.3A switching power supply
    - Gecko G540 4 axis stepper motor driver
    - 3 High-torque Nema 23 380 oz-in stepper motors. These motors already have the current set resistors installed in a pro-grade metal shell. No soldering or crimping required. These are optimized for the G540 -- don't get caught up in the stall torque game, since your motors don't run at stall! These motors use all of the 3.5A the G540 has to offer.
    - 3 High-performance flexible motor cables: (1) 6'and (2) 12' cables

    I think it's the best at the comparison price/quality(functionality).

    Any weak spots in this kit ??

    P.S. Here our mains voltage is 220V/50Hz.



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    No weak spots and will be perfect for your needs. You wont regret buying a kit like this.

    We have same voltage here in UK, I think the PSU in this kit will probably be dual voltage but would check before ordering.

    The G540 is a brilliant piece of kit and comes with every thing you need in one neat little box and IMO for the money it cant be beat. Matched in a kit like this you will have a powerfull piece of equipment that will serve you very well and give you great upgrade potential.

    Will be the best money you will ever spend and wise choice if you go for it.



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