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  1. #1
    Next in the list (now that the motors are here) will be the PSU. I think I understand what I'm doing but I wanted to ask if someone could stop me before I make any (or at least too many) terrible mistakes :) I know this is a rehash of threads to probably occur once a fortnight but its for a reason. I'm setting out the details because it's not just about buying the right thing - I want to make sure my understanding of what I'm buying and why is correct.

    I have these 3.1Nm steppers and I plan to wire them as parallel: 4.2A, 3.2mH and 2.73V. Doing some magic calculations (based on magic spells found elsewhere on this forum) leads me to an operating voltage of 57.2V. The PSU would therefore need to provide 57.2V@12.6A. But wait...

    I plan to buy PM752s which can provide, inter alia, 4.09A or 4.64A. In fact they support up to 5.2A but the motor has a maximum of 4.2A. This would mean the appropriate setting would be 4.09A therefore the PSU would now only need to provide 57.2V@12.3A. But wait, again...

    Now as far as I understand it, I don't need to make the full 56.2V available, as nice as the idea might seem to me. There would be a performance drop but as the motors are probably outperforming the rest of the machine anyway, would this necessarily present a problem? Common figures seem to be 42V and 48V. Is dropping from 56V to 48V a travesty of epic proportions?

    So what about the current? If needed, I could run two PSUs wired in parallel to provide more current (ie 2 x 42V@7A would effectively be able to provide 42V@14A). However, much as the full 56.2V may not be needed, so too the full 12.3A may not be needed. For an unregulated supply, the PM752 docs suggest the PSU might only need to supply 50%-70% of the motor rating. Picking an arbitrary 75%, the PSU would now only need to provide 42V@9.3A. While 50%-70% would actually be 6.2A-8.7A, I assume (no! not assumptions! nooooo!) that more safety margin is always better than less.

    Zapp's SPS407 - with 42V@7A(continuous) and 9A(peak) - would work on its own using the 50%-70% figures but would need to be doubled up to make my fairly arbitrary 75% minimum - though if doubled then for the same price I could consider the SPS705, two of which would provide 68V@10A. I gather this could be magically fettled to provide the ideal 56V-57V at the slightly-less-than-ideal 10A, but would move the cost bracket from 60-70 to 120-140.

    CNC4You's 48V@12A, by contrast, would provide 97.5% and a few extra volts all on it's lonesome, but would still be under the ideal voltage.

    I notice that Zapp also bundle their PS806 68V@6A with 3xPM752s and 3Nm motors, but that 6A falls below the 50% so I am presuming it would be too far a stretch in this very-similar-but-a-tiny-bit-different scenario.

    Then we move onto sourcing the bits and building one but for some reason that makes me a little jittery compared to getting something made by someone who is competent to build something I plan to plug into the mains!

    All of this leads to a point, eventually. If I choose "money" then it's 48V@12A@57. If I choose "voltage" then it's 56V@10A@138.

    Am I on the right tracks and am I considering the right things to balance? I've picked up this idea that I can run on less volts, but am I being daft (well, the anwer to that is always yes) to think I should be considering anything less than 56V in this scenario?

  2. #2
    Looks like you're after an explanation of why, not what...

    I would consider the voltage you get from the common formulas (20*V, 32*L^0.5 etc) to be a 'recommended minimum'. You will get better torque at high speed if you use 70-75V, plus seems a waste to get PM752 drivers if you're only going to run them on 42V.

    A linear power supply handles current surges better than a switch mode power supply (smps), which means you can safely use a lower rated current than the smps. A 500VA transformer is fine for 3 motors, probably 4 and that's only 500/70=7.1A.

    The current in a 2-phase stepper motor on a microstepping driver is, at intermediate speed, approximately sinosoidal and out of phase by 90. This means that the maximum current is the maximum of I*(sin(thing)+cos(thing)), which is I*2^0.5 where I is the current in one phase - so 2.1*1.414=2.97A. This is 70.7% of 4.2A. Similarly if you work out the rms current for both phases (just integrate the previous formula squared etc), it is 2.1A - hence 50%. I'm guessing this is where your 50%-70% approximation comes from.
    Old router build log here. New router build log here. Lathe build log here.
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  4. #3
    Ok the calcs you've done are close and correct in some ways but some of what you read I think you miss understood.? The 752s Manuel and 50-70% was referring to current not voltage. So you still need 57V but only 8-9A current.

    That said has you know the voltage is what gives you the speed in a stepper so dropping to 48V means your only getting 85% of the speed you would at 57V. PLUS these motors will happly run at 70V thou I always run them at 65-68V to give a better margin for Back emf. So really at 48V your only getting 69% of the motors potential at 70V.?

    Now in speed terms to the machine it can be dramatic in terms of the motors corner speed or point torque starts dropping away.
    IE @48v then expect the corner speed less than 700rpm and @70v approx 1000rpm.
    On a 5mm pitch screw then it makes the usable feed rate difference between @48=3500mm/min and @70v=5000mm/min.

    Without knowing more about pitch and what you want to cut then hard to say best route to take.? .. BUT . If the voltage can provide the speed in combination with your screws to give the feed rates you need to cut what you want then it's not an issue really simple has that.!!

    Remember high rapids mean pretty much nothing in real world cutting unless you intend to do lots of positional moves like drilling or have a massive long machine.? . . Mostly just bragging rights.!!

    That said I prefer building my own supplies so obviously can build it to my exact needs so nearly always 65-68V using 75V drives simply because I can.! The amps just depends on how motors and number of them.
    It's easy building a toroidal PSU and often cheaper plus safer for the drives has they handle back Emf much better and gives a nice consistent supply of power.

    For me it would be "Build own to my exact needs" other wise if speeds/feeds it allows are OK use the 48V@12A and save the money.! Certainly wouldn't buy 2x68V@5a has could easily build for less than price one.!!!

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  6. #4
    Quote Originally Posted by Jonathan View Post
    Looks like you're after an explanation of why, not what...

    I would consider the voltage you get from the common formulas (20*V, 32*L^0.5 etc) to be a 'recommended minimum'. You will get better torque at high speed if you use 70-75V, plus seems a waste to get PM752 drivers if you're only going to run them on 42V.
    I think that's it. It's easy to ask for the answer (and I often do) but it's sometimes better to ask for correction as a learning exercise.

    I agree about the drivers. It was my understanding that the system could be run with lower voltage, I was trying to establish if doing this would lead to an earthshattering change or just knock a few percentage off the numbers. I would prefer to go for the higher-spec drivers and utilise them as best I can.

    Quote Originally Posted by JAZZCNC View Post
    Ok the calcs you've done are close and correct in some ways but some of what you read I think you miss understood.? The 752s Manuel and 50-70% was referring to current not voltage. So you still need 57V but only 8-9A current.
    Yep I got that bit, though I guess I didn't really make it clear. The reduction to 42V or 48V wasn't because I was applying a percentage reduction, I was just observing that several of the CNC places were offering that size of PSU and if I wanted to buy off the shelf then I was limiting myself to what was on the shelf in the first place!

    Quote Originally Posted by JAZZCNC View Post
    Without knowing more about pitch and what you want to cut then hard to say best route to take.? .. BUT . If the voltage can provide the speed in combination with your screws to give the feed rates you need to cut what you want then it's not an issue really simple has that.!!
    Ahah. Of course, the benchmark against which the performance needs to be measured. I was asking the wrong questions again! Not "will it work" (because the answer can only be "it depends") but "what do I need it to achieve?"

    Quote Originally Posted by JAZZCNC View Post
    For me it would be "Build own to my exact needs" other wise if speeds/feeds it allows are OK use the 48V@12A and save the money.! Certainly wouldn't buy 2x68V@5a has could easily build for less than price one.!!!
    I keep looking at it and thinking "that looks quite straightforward", but then I think about plugging it into the mains and I have a vision of mushroom clouds rising over Derby. Some might say it would be an improvement, of course.

    I'm going to end up trying to build this, aren't I? Bugger. Well, most of the components are cheap enough,

  7. #5
    Quote Originally Posted by Rogue View Post
    I'm going to end up trying to build this, aren't I? Bugger. Well, most of the components are cheap enough,
    Yep probably.?? BUT it is Honestly so easy you'll wonder why you fretted afterwards and if you need a hand holding then just shout.!

  8. #6
    So...

    ...this 500VA 35V-35V transformer would let me connect the two outputs in series to get 70V@7.1A. By the time it has lost a few volts through the additional components (rectifier etc) I would have 68.6V (I think it's 1.6V drop for suitable rectifiers?). This 30V-30V would give me 58.4V - with 57.2V being the minimum rather than the optimal that I had thought it represented - so it would be the bottom line option.

    Daft question, is it feasible to knock the output down a bit from a transformer, short of fiddling with the number of turns?

  9. #7
    No the 35V is AC so needs to be rectified which makes it times 1.4 so that would be 35 +35=70 x 1.4= 98Vdc. . .Boom.!!

    You need 25V to give 70Vdc.

    Yes can fiddle with turns but why would you.? Just buy the appropriate transformer.??

  10. #8
    Quote Originally Posted by JAZZCNC View Post
    Yep probably.?? BUT it is Honestly so easy you'll wonder why you fretted afterwards and if you need a hand holding then just shout.!
    I fret because I'm used to being an expert in my chosen field and here I'm dealing with things that I have absolutely no experience (or confidence) in. I am very good at imagining all the horrible ways things could go tragically wrong, though. Each action taken or order placed commits scarce resources, and every mistake is a painful waste. Otherwise I'd quite happily glue everything to everything else and pump lightning through it to see what would happen

    Hand holding? I would rather throw my ideas out and be corrected so that my understanding improves, but I'm sure some handholding creeps in unwittingly. My apologies if it does because that's not what I want to do.

  11. #9
    Quote Originally Posted by JAZZCNC View Post
    No the 35V is AC so needs to be rectified which makes it times 1.4 so that would be 35 +35=70 x 1.4= 98Vdc. . .Boom.!!

    You need 25V to give 70Vdc.

    Yes can fiddle with turns but why would you.? Just buy the appropriate transformer.??
    Ahh, yeah that might not be what I'd want then. I was asking about the fiddling because I was wondering, I certainly don't want to fiddle...

  12. #10
    Quote Originally Posted by Rogue View Post
    Ahh, yeah that might not be what I'd want then. I was asking about the fiddling because I was wondering, I certainly don't want to fiddle...
    You can wind a few extra turns on, or vice versa, then add the appropriate insulation. It's a crude but successful way of tweaking the voltage of a toroidal transformer that's not quite what you want...it shouldn't be needed here though as you can just buy the right one.
    Old router build log here. New router build log here. Lathe build log here.
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