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  1. #31
    Quote Originally Posted by John S View Post
    Lee the accepted figure is 2/3 for a parallel wired motor and 1/3 for a series wired motor.
    These are figures that Marriss Fremanis at gecko drive came up with and are repeated as law.

    So a 3a motor in parallel take 2a and 1a in series.

    However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

    In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.

    One machine I used to convert I used 650 VA transformers at 50 volt AC, in a cost cutting exercise, due to rising copper costs, I dropped to 500 VA and saw no difference, a drop to 400VA still didn't make a difference but going to 330 VA did,

    So the drop from 650 VA to 400 VA halved the transformer cost.
    Shouldn't be taken into account the motor amps/torque vs. the machine weight/working material?

    If I put 3A motors on a belt driven PCB milling machine it is impossible to use that power assuming to have idle-current reduction.

    I think the 60% rule for the power supply works only with toroidal transformers which have an instantaneous power at least 25% higher than the nominal, and with one PSU for all motors because it is unlikely to have all axis under high loads at the same time.

    John, what rated current had the motors and drives for your conversion where you used reliably a 400VA transformer?

  2. #32
    Joe,
    Truthfully can't answer this question at this time.
    Using laptop with parallel port is a no no but the USB card 'should' be OK.
    So far although we have two cards neither has been tested because I'm in the process of converting a manual mill to CNC and it will be used on this, probably about 2 weeks away from testing given this has to fit in with my regular work.

    It will be tested on a Dell to start as the 'test bench ' is so equipped but I'm also interested in seeing if the laptop will run it OK
    John S -

  3. #33
    Quote Originally Posted by John S View Post
    However I have had clamp ammeters fitted to power supply leads and running a system under rapids and loads i have never seen these figures, they have always been far less.

    In fact in many cases the figures have been very small, far smaller than I would have imagined so the 50% is a far more acceptable figure.
    Part of the reason for your low readings is the sample frequency of the clamp meters is low relative to the frequency of the current the motors are drawing, so it tends to miss the peaks and generally not be reliable, especially if the meter is not true-rms. An oscilloscope reveals a lot more.

    However, it is still true to say the rms current, and therefore power drawn, is substantially lower than the 2/3rds 'rule' implies. It's easy to show why - just look at the motor ratings to work out the power. For example the common 3Nm motors require 4.2A and 2.73V. Power is current multiplied by voltage, so P=4.2*2.73=11.466W. That's per phase though, and it's a two phase motor, so 2 times that which gives 23W. You're driving it from about 70V, so the current is pulsed such that the mean power does not exceed this rated power. Same formula again, so the mean current is I=P/V=23/70=0.33A.

    That's why you see a low current (about 0.33A) per motor, not 4.2A or some significant fraction of that. The reason it helps to have the power supply rated for quite a lot more (i.e. 2/3*4.2) than this anyway is that the peak currents can be substantially greater, so you need a transformer capable of delivering that. That's not the only way to solve it though.
    Last edited by Jonathan; 26-03-2013 at 02:13 AM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  4. The Following User Says Thank You to Jonathan For This Useful Post:


  5. Quote Originally Posted by John S View Post

    It will be tested on a Dell to start as the 'test bench ' is so equipped but I'm also interested in seeing if the laptop will run it OK
    Me too. Be good to hear how it gets on... Thanks John

  6. Quote Originally Posted by John S View Post
    Joe,
    Truthfully can't answer this question at this time.
    Using laptop with parallel port is a no no but the USB card 'should' be OK.
    So far although we have two cards neither has been tested because I'm in the process of converting a manual mill to CNC and it will be used on this, probably about 2 weeks away from testing given this has to fit in with my regular work.

    It will be tested on a Dell to start as the 'test bench ' is so equipped but I'm also interested in seeing if the laptop will run it OK
    Any news on this John?

  7. #36
    Still 2 weeks away <g>
    Problem is my day job is emergency repairs and I never know what comes thru the door to upset planned jobs.
    Starting to wire the box up Friday, fingers crossed and then sort the machine out.

    machine is all converted mechanically, just need covers and bellows re fitting.
    John S -

  8. OK be glad to hear when you get it up and running. Cheers

  9. #38
    Quote Originally Posted by dudz View Post
    What's the delivery time like? I bought some from the same seller last December and they took 7 weeks to arrive. Might of been due to the Christmas post, etc. I am just wondering if they "normally" arrive sooner?

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