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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > 1st Build, will this be a decent Design?
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  1. #61
    Join Date
    Jun 2010
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    4252

    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

    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!

  3. #63
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    Jun 2010
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    4252

    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

  4. #64
    Join Date
    Jul 2018
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    6248

    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

  5. #65
    Join Date
    Oct 2009
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    733

    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.

  6. #66
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    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

  7. #67
    Join Date
    Mar 2003
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    35538

    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

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

  8. #68
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    Jun 2010
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    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

  9. #69
    Join Date
    May 2005
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    3920

    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.

  10. #70
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    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

  11. #71
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    Aug 2009
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    230

    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

    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)

  13. #73
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    May 2005
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    3920

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

    The electrical ampacity ratings of wire are based on a number of things including the temperature rating of the insulation. If you are in an area where the NEC applies acceptable current ratings can be defined in the code (for specific usage). Transformers are an interesting discussion but I think I’d side with others and have to question the wisdom of 18ga wire in a secondary operating at greater than 15

    The reality is comments from a transformer designer / engineer would shed more light on the issue. The other problem here is the fact that far too many online sellers have no idea what they are selling. The 18 ga could easily be the primary.
    Quote Originally Posted by __Britt View Post
    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... ???

  14. #74
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    Aug 2009
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    230

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

    The other problem here is the fact that far too many online sellers have no idea what they are selling. The 18 ga could easily be the primary.
    The Great Internet Giveth, and the Great Internet Taketh Away... (low prices, and competency, respectively, in this case).

    18 on the primary would tend to make more sense...

    I wonder if dropping a line to Antek might be enlightening... they do manufacture torroidial transformers from scratch, so... ???

  15. #75
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    Re: 1st Build, will this be a decent Design?

    I do not understand where the problem is. A toroidal transfomer of 1kW capacity and 10 or 20 A output at 50+ V is an off-the-shelf item in America. Also in Europe and elsewhere. Retailers have them. What's more, several mfrs will wind a transformer (eg 1 kW) to your specs. Just Search.

    There are lots of them on eBay too, although you will have to crawl through the listings in most cases to get the actual specs.

    Cheers
    Roger

  16. #76

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

    So I've found this and from what people have said I think it will be suitable, does anyone have any thoughts before a go ahead and buy it... - https://uk.farnell.com/multicomp/vtx...P_LastViewed_1

  17. #77
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    Re: 1st Build, will this be a decent Design?

    It's a fine toroidal transformer, but at 40V it might leave your motors a bit slow.
    (You did say you were looking for >50 V?)

    Cheers
    Roger

  18. #78
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    Jul 2018
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    6248

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

    Hi BFP - The Farnell transformer is 122 pound cost (not weight my keyboard doesn't have Pounds) yet switching supplies are cheaper and easier? 2x48V 500W SPS are 39pound each? plug and play? That gives you 2x10A which is overkill! & it can be cranked up to 52V
    Peter

    https://www.omc-stepperonline.com/power-supply/page/2

    Your inductance is 1.8mH so your Vmax is 32*(1.8)^0.5=43V the SPS is adjustable from 42V to 52V perfect....

    2x 48v x8A which is a better match is 23pound each a bargain...

    https://www.omc-stepperonline.com/po...nc-router-kits

  19. #79
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    Re: 1st Build, will this be a decent Design?

    The bit which puzzles me is that 2 of the trannies were <40 quid each, but the slightly higher voltage version with the same powwer rating was 122 quid. Something a shade strange there.

    The advantage of a tranny/diode/cap PS over an SMPS one is that you cannot blow up the tranny by dumping reverse charge from deceleration into it, but you can blow up the SMPS that way. A second advantage is that the RFI from the SMPS can send your driver mad: such drivers are not built to handle that sort of incoming RFI.

    YMMV.

    Cheers
    Roger

  20. #80
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    Jul 2018
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    6248

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

    Hi Roger - I have similar ones on three routers (all from Stepperonline) and they work fine. I appreciate the issue of back EMF with SMPS but the inactive motors act as a power sink and if your really concerned you can put caps on the output, plus they are designed and sold for steppers and this issue is incorporated, designed and dealt with in the electronics. Cost of units is proportional to demand. So I expect the higher cost units have less demand. I notice with Stepperonline various things come and go. I expect cost and stock holding are very sensitive to demand. Cheers Peter

    I had to look up YMMV

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