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  1. #1
    Join Date
    Feb 2015
    Posts
    113

    Power supply current

    Hello,
    I need help choosing power supply and If you could explain to me why the manufactured recommended for me power supply with output 80vdc and 12A !! Why 12A ? As I understand from searching on the web it should be
    Current*=*Amps*needed*per drive***Number*of*drives *0.7
    I have :
    4x LAM Technologies 2 phase stepper motors ( 7.1 Arms, 1.85 V, 0.26 Ohm, 2.2 mH (each phase))

    4x LAM DS1078 driver rated 4.0-10.0A (RMS) respectively 5.6-14.1A (peak) each phase.
    The manufacture suggest me to use a power supply with 80Vdc output voltage and 12A output current. Is 12A enough for the 4 axis?
    Appreciate your help
    Murad Assadi

  2. #2
    Join Date
    Mar 2009
    Posts
    533

    Re: Power supply current

    Part of the difficulty in knowing accurately is that a CNC doesn't run all 4 motors at a full 7A all the time. Therefore the "running average current" is not the sum of the full current rating of your motors.
    It is a statistical guess on the manufacturer's opinion. Your average current will vary with what job you are doing.
    Secondly, if you were to assume that your four 7A motors run full bore, then you would need a 28A supply - significantly increasing your cost for the "just in case" need.

  3. #3
    Join Date
    Feb 2015
    Posts
    113

    Re: Power supply current

    Thank you for replaying, actually I don't need power supply that would cover any possible mode of usage? But what i need sometimes is a heavy duty motion on more than 2 axes at one time specially that I'm using 2 motor for x axis (one as a slave), so u recommend me to stay with 12A or choose one with more amperes ? And what would happen if I choose for example power supply 80v and 20A ? The motor and driver rated in Arms current not Apk (peak) .
    Thanks for helping
    Murad

  4. #4
    Join Date
    Jun 2009
    Posts
    251

    Re: Power supply current

    This is the same post with the same question from the same OP.
    Assadi, don't do that.
    I already answered on the other post.

  5. #5
    Join Date
    Mar 2009
    Posts
    533

    Re: Power supply current

    The driver is usually adjusted to the motor requirements - in your case 7.1A to get maximum torque, and to ensure that the motor current will not exceed 7.1A.
    Gecko (http://www.geckodrive.com/support/st...ly-basics.html) indicates that the 7.1A rating is not used for power supply current estimation but rather to use 2/3 of this value, if your motors use parallel windings, which I'm assuming you do to get the maximum torque,- which would give you 4.7A per motor X 4 motors = 19A

    Your 80V @ 20 A power supply would be fine and have enough oomph for your "heavy duty motion".

    Regarding your concern that the manufacture gave too low a value may be due to them not knowing you are driving 4 steppers. They may have assumed 3 steppers, which would come to more like 14A. Then, if you factor in a lower "duty cycle" it may well be enough for most of your situations. Measuring the average current of your power supply under various loads will finalize your answer.

    @tommy,
    It appears he wasn't comfortable/satisfied with the answers he got so he asked again. I didn't see his 1st request as I'm not on this forum often.

  6. #6
    Join Date
    Feb 2015
    Posts
    113

    Re: Power supply current

    @KOC62,
    Thank you for helping, the manufacture know that I'm using 4 stepper but even they still recommended me the 80v 12a, and @tommy this is the reason why I'm asking again !! 80v 12a or 80v xxa !!
    Thank you all for help
    Murad Assadi

  7. #7
    Join Date
    Feb 2015
    Posts
    113

    Re: Power supply current

    I think I should share this with you, I contacted the manufactured & asked them if they could explain to me why they recommended 12a, the answer is "Our products have high efficiency and it draws about* 1/4 of the rated motor current when it is in stand still with 50% current reduction, and about 1/2 of the rated motor current when the motor is running. The correct way is always to start from the mechanical power and then apply 0.7 efficiency factor to the result"

  8. #8
    Join Date
    Mar 2009
    Posts
    533

    Re: Power supply current

    How has this information changed your view of the matter?
    What power supply rating will you actually use?
    What about your concern when the motors are asked to work hard with "heavy duty motion"?
    This was the reason that pushed for a beefier power supply of 19A.
    Actual power supply average current measurements during your heavy loads will finalize your answer, which is hard for us to know.
    The general wisdom is to follow the manufacturer's suggestions, until you know why to change it.
    Best wishes on your journey to happy CNCing.

  9. #9
    Join Date
    Feb 2015
    Posts
    113

    Re: Power supply current

    Thank you KOC62, I think it will not change any things, I'm still going with power supply rated between 12a - 20a, BTW I'm a physiotherapist and I don't have any knowledge in electricity and I post this just for people's like me when they search for information, so maybe it will be more easy next time, and again and again and again appreciate you help & support .
    Murad assadi

  10. #10
    Join Date
    Jun 2009
    Posts
    251

    Re: Power supply current

    A quick explanation on power and supply currents in drive and motor electronics:
    In this actual case the 12A given by the drive and motor manufacturer is correct and it will work properly without a doubt, you have to look at stepper drives as switch mode power supplies since they have the same exact function besides being controlled by a computer or PLC or something else, so a motor requires 7A at 3V or thereabouts and the power supply is 80V so the drive will try to push all 80V to the motor but as soon as it gets to 7A the drive will lower the current to the motor and this happens very fast (approx 20000 times per second) so the draw on the power supply subsides and so on and so forth. The AVERAGE power drawn from the power supply is theoretically 0.4A plus losses on the drive and motor and cables. This is for one drive and motor. From experience: you can happily drive 3 axes at 1A with a 24V/1A power supply. Also from experience if i remember correctly: i have several ( over 10 ) of very powerful drives that i do a lot of testing and work with and at 12V they draw 4.1A max. for a 5A motor but the same drive and motor use 2A at 25V.

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