1st Build, will this be a decent Design? - Page 6


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Thread: 1st Build, will this be a decent Design?

  1. #61
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    Default Re: 1st Build, will this be a decent Design?

    if there was a time when we universally had good capacitors?
    Not really. Older caps were often 'smaller', or lower capacity, while same volume. Modern caps can have a higher capacity in the same volume, but I doubt they are any worse. (I am 74 yrs old).

    What does happen a bit more today is a squeeze on safety margins in order to cut costs. For instance, 240 VAC is ~340 VDC peak. People used to use caps rated at 630 VDC for this; today many cheaper designs use caps rated at only 400 VDC.

    Yes, 4 off 100 uF caps in parallel can be better than 1 off 400 uF. Lower ESR for a start - all things being equal!

    Cheers
    Roger



  2. #62
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    Default Re: 1st Build, will this be a decent Design?

    Hey guys, I've got all the parts, but the actual Toroidal Transformer ready to buy in my basket, I'm having a big problem trying to find one with similar ratings as the one posted in Gers link on page 3 of this Thread... the one he posted has 56v 1000w 18a but I'm struggling to find one like that, I've found plenty around the 48-56v range with 500-1000w but the ampage is usually around 7-12 mark I just don't know if any of these would be suitable or should I go for higher amps lower watts...?? Sorry in advance for my poor electrical knowledge!



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    Default Re: 1st Build, will this be a decent Design?

    The motors require power to run the machine. The power comes from the transformer. If the transformer is not big enough and cannot handle enough power, your motors will be starved and the machine will run slow.

    That said, I do have to ask just how much power you will really need. I monitor the power going into each axis, and I find that they rarely draw more than 50 W each! The gearing provided by ball screws driving linear bearings is a huge benefit. However, if you are driving acme threads on a cast iron dovetail, your power needs will be far greater. (I have not gone back to look.)

    In principle, the higher the voltage, the faster a motor can accelerate and run, so voltage is your first concern. It may be that you would rarely drive more than one axis very fast, so your peak power demands will be somewhat less than you think.

    One viable course of action for you is to get the best transformer you can with a 56 VAC rating, and to try it. But leave room in the physical layout for a bigger transformer in case what you have is just not big enough. Ah yes: build all the rest to handle the maximum power you might need, even if that is more than the first transformer can provide.

    Cheers
    Roger



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    Default Re: 1st Build, will this be a decent Design?

    Hi BFP - It goes like this.
    1) You have 4x4A motors. They do not operate at full current or all at the same time so: some people say you need 60% of max some say 70% conservative says 100%= 16A. 16x0.6~10A min. The voltage is not super important. The motors will run on 12V poorly 24V quite well but the faster they spin the more back EMF they make and the PS has to overcome that (plus other things but let's keep it simple) . So 45-60V is great. 12A/16A= 75% I'd expect this to be fine especially with a simple PS vs a switching source. You mention a 12A PS?

    You can also split the PS so say the Y axis (which has two motors is supplied by one 8A PS) and the others are supplied by another 5-6 or 8A source but one PS is simpler. Cheers Peter

    Last edited by peteeng; 09-20-2019 at 08:19 PM.


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    Default Re: 1st Build, will this be a decent Design?

    Phil,

    Don't forget that the rectifier will multiply the voltage of the transformer by a factor of about 1.6.

    BryggaCNC.com


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    Default Re: 1st Build, will this be a decent Design?

    a factor of about 1.6
    Yes, and NO.
    The peak voltage will be that high, under no load conditions. Once you have a decent load the average DC voltage will be close to the nominal AC voltage. But rate the capacitors above the peak!

    Cheers
    Roger



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    Default Re: 1st Build, will this be a decent Design?

    See Section 6 here:
    https://www.geckodrive.com/support/s...or-basics.html

    The DC voltage is 1.4x the transformer AC voltage. There will be a small voltage drop under load, but it's minimal.
    You want to end up with a DC voltage between 48V-60V. Voltage is roughly proportional top speed.
    Do you know the inductance rating of your motors?

    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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    Default Re: 1st Build, will this be a decent Design?

    Better note that the drop from nominal (be it 1.4x or 1.6x) will depend a bit on the transformer itself (actually, its internal impedance). An old E core tranny will drop more than a modern toroidal tranny.

    Regardless, do NOT over-volt your drive unit. The semiconductors inside it won't like it.

    Cheers
    Roger



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    Default Re: 1st Build, will this be a decent Design?

    Given a fixed voltage if your current goes up power goes up.

    Rating power supplies for servo/stepper duty is a bit of a black art. Few implementations will have all drivers running at full capacity at the same time. Beyond that stepper drives themselves might never apply full current to the windings. The simple answer would be to add up the stepper current ratings but that can over size the power supply for many installations.

    One thing to say here, an 18 ga secondary would have me concerned for a 1000 watt 56 volt secondary.



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    Default Re: 1st Build, will this be a decent Design?

    18 ga? Possibly adequate if a solid wire, but not good as flex. I err on the 'robust' side.

    Cheers
    Roger



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    Default Re: 1st Build, will this be a decent Design?

    Disclaimer: I am not an electrical engineer; nor do I play one on TV.

    Well, FWIW there is this:

    https://www.powerstream.com/Wire_Size.htm

    At 56v, 1000 watts is about 17.9... lets call it 18 amps.

    18ga wire, according to that chart, should be good up to about 16 amps in free air; but only 2.3 amps in a bundle... this leads me to thinking that the wire in question is a bit on the light side for the task at hand. On the other hand; I often see transformers wound with lighter gauge wire than what is feeding them, so... ???



  12. #72
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    Default Re: 1st Build, will this be a decent Design?

    All great info again guys thanks for this, I'm slowly learning something every day!... Ger, the Inductance for The Motors is 1.8 (+-20%, when wired up in Bipolar Parallel)



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