Thanks for the explanation.
If you were going to use a SMPS, how much extra over the nominal current would be recommended?
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Thanks for the explanation.
If you were going to use a SMPS, how much extra over the nominal current would be recommended?
In terms of current draw, there is no real reason to overspec a SMPS. Once it's overcome the initial start-up surge and charged up it's various internal capacitors, and the stepper driver capacitors, the average current draw is minimal at idle/slow speeds. At idle, with say a 5V 2A stepper motor, it should be seeing a combined total of 2A at 5V through either or both of it's windings, so 10W. Now say you've got a 2A 50V SMPS, that's a 100W source. The stepper driver (which is essentially a fancy SMPS), although switching currents might peak at 2A, the various capacitors will smooth that somewhat before it reaches the SMPS circuitry.
Even at higher speed/load where you happen to be in the optimum point to be applying 50V to get the full 2A through the motors, due to the switching nature of the stepper drivers you're not likely to draw the full 100W capacity. Plus once you get beyond the optimum voltage/current point, back emf will limit the amount of current the available voltage can push through the stepper motor windings, and power consumption will actually reduce. This is why it's recommend for a typical 3 axis machine, you only need a supply capable of delivering 2/3 the maximum combined current.
The real big problem with SMPS's is there inability to handle the regenerated energy from stepper motors/drivers as axes are decelerated. During deceleration all that kinetic energy gets dumped back into the power supply, which can cause it to go overvoltage. SMPS rarely have the ability to handle that, with the better ones simply shutting down (often until they get power cycled), and the cheaper ones releasing their magic smoke. Bigger SMPS will normally have bigger capacitors so they'll absorb a bit more energy before going over voltage.
You can get around this by installing a reverse energy dump circuit which will dump any regenerated energy to a big resistor rather than into the supply, but for the cost, you should be able to build a suitable linear supply that will avoid all these problems, be far simpler, and have less bits to potentially go wrong.
So after almost a year, the mechanical part of this build is still progressing .... very slowly!
For the electronics, I have just got 3 DM860T drivers from stepper online to power my 3.1Nm Nema 23's.
I would quite like to wire up the motors just to check everything is working correctly, so starting to think more about BOB and PSU.
The drivers can take either AC of DC input.
I like the idea of AC as it make the power supply build a little simpler, but is there any downside to using an AC input?
Any advice for BOB? I plan on using linuxCNC.
Thanks,
If you have a spare arduino lying around and a PSU that can run 1 motor temporarily, you can test them reasonably easily just to check everything is okay.Quote:
Originally Posted by diycnc
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Dont have a PSU to hand, so looking to put together a simple 70vac transformer, but im struggling to find a suitable toroidal transformer.
Only 230v to 70v transformers I have found so far are max 500va, which im not sure is sufficient for 3-4 motors. The current design is 3, but want to leave room for a 4th if i choose to upgrade later
https://www.rapidonline.com/vigortro...-x-35v-88-0007
This company will make any voltage you want for you. https://airlinktransformers.com/cate...standard-rangeQuote:
Originally Posted by diycnc
I use a 500V a toroidal with 4 nema 23 on 2 different machines with out problems. The next size up generally is 625Va
Mine went from TME. Quick sending and good prices.
https://www.tme.eu/fr/katalog/transf...100028/?page=1
Best regards
I think i need to check my PSU calcs. Can someone point me towards some example PSU design calcs?
Also, any idea if there is any benefit of AC vs DC?
This is quite a good introduction...
http://www.skillbank.co.uk/psu/
The stepper driver must be rectifying the AC into DC anyway, though with little or no smoothing. My guess is that making drivers that can be fed with AC is done merely to reduce the cost of the power supply. Whether a smoothed DC supply noticeably reduces motor vibration compared to AC will need some research.
kit
There is absolutely no difference. I've built loads machines using both and see no performance or reliability differences between AC or DC.Quote:
Originally Posted by diycnc
The AC is simpler for obvious reasons so my suggestion is go with AC.
Also, 525Va transformer will do what you need. 625Va will give you a bit more overhead, which you won't need, but does increase inrush and that could give you issues with MCB trips. However easily solved with D rated MCB.
Resistor ?Quote:
Originally Posted by m_c
Thanks JAZZCNC. Will go with AC.
Only 70v toroidal transformer I can find is 500va, which I think should be plenty.
https://www.rapidonline.com/vigortro...-x-35v-88-0007
What size MCB would you recommend?
I pointed you to one in post #47Quote:
Originally Posted by diycnc
I went with a 10A Type D MCB in the end after trying a 6A Type D MCB that kept tripping on start up.
Note that my transformer was 750VA, 2x24v in series, (I'm running 5 steppers, with 2 steppers on X and Z) so you could try the 6A one first as they are not too expensive.
Thanks for the MCB advice. Will try 6A type D to start with.
After a little more reading, going to go for a 500va 230v primary, 2x 30v secondary. This should give me nominal 60vac when wired in series, and max voltage of 69vac when considering the mains can vary +10%, and transformer output by +6%.
A 2 x35 would give max of 81vac which is above the driver max of 80vac.
What's the recommended connectors for the high voltage wires from the PSU to the drivers?
Just been watching some of Joe Harris videos, and have had discovered the world of DIN rail connectors!
I finally understand what the rows and rows of plastic blocks are in everyone's control boxes now.
Nearly always needs a 10AQuote:
Originally Posted by diycnc
Directly wire from the transformer output to Fuses then to drives. Fewer connections the better. Every connection or connector is a potential problem area so fewer the better.Quote:
Originally Posted by diycnc
I guess you are referring to the fact that the psu will have a single positive cable and single negative cable output, and you need to connect these to 3 or more drivers.
Daisy chaining the drivers power input is not advised, so separate runs should be used.
Maybe Jazz has a neater solution but most people use a distribution block to connect each power output to multiple outputs, often mounted on the psu board.
Every connection is a potential problem down the road so if you can limit connections you eliminate potential problems. So why have connection blocks on the PSU when you can remove those connections by wiring directly to fuses.? Put the fuses on or close to PSU.Quote:
Originally Posted by routercnc
Also, note that when using AC drives your only using a Transformer so often there's no PSU board involved because you don't need Caps or Bridge.
Just wondering is there a reason why we use a MCB before the PSU, but then a fuse before each driver?
Do they not both perform the same job? Why not use the same everywhere?
One drive should only ever pull x amps. Multiple drives connected to the same PSU can draw more than x amps quite normally. That's the theory at least - although at this point I think we're trying to stop things going up in smoke rather than protecting the drives themselves.
Totally understand the need for protection before and after the PSU, just wondering why we don't use MCB on both sides, or fuses on both sides.
I still have a lot to learn about the electrical side of things, so this might be a silly question.
Ah yes now your question makes more sense. I'm no expert so I'm sure someone will put us right, but the way I see it is the MCB is placed before the PSU because it's 3-4x (however many drivers you have) more likely to trip, so easy and inexpensive resetting is helpful. It's also very obvious when it's tripped rather than digging out fuses. There's also the chance of inrush to the torroidal tripping it, so again more likely. It's also a convenient way to isolate the PSU. As for the fuses on the drivers, they're cheaper than adding an MCB.
The main reason is protection.
The MCB protects the transformer in case of a short circuit. You don't actually need an MCB type fuse, any fuse type of correct size will work but due to the nature of Transformer and inrush, etc MCB is just easier to reset when trips.
The fuses for the drives protect each drive in case of short circuit or excess current draw. The transformer can supply many more amps than each drive can handle so if, for instance, motor shorts out the drive will keep dumping current up to it's rated output or until it destroys its self. some drives have built-in current protection but it's not advised to rely just on this, hence why a fuse.
Fitting a fuse same size or just slightly higher than the drives rated load will offer it some protection. In fact, I often fit fuses at a lower rating than the drives max rated current if the motors are not rated at or close to drives Max current.
Drivers are rated to 7A, and motors just to 4.2A. Probably will be running them at over 3.14A ref I think.
Would a 4A fust be okay?
Currently looking at the industrial DIN rail fuses holders like this one, unless anyone can suggest anything better/cheaper.
https://www.rapidonline.com/mersen-c...holder-26-5786
Also looks like most people are using double pole MCB's on the live and neutral before the PSU. Is this essential? I thought only the live would need one?
5A would work better. Gives some safety to drives but won't trip when motrs are being worked hard.Quote:
Originally Posted by diycnc
These are what I use, cheap and easy to wire and change.Quote:
Originally Posted by diycnc
https://www.ebay.co.uk/itm/5X-FS101-...72.m2749.l2649
No don't need double pole.Quote:
Originally Posted by diycnc
I prefer these to Din rail connectors when creating Busbars. Lot cheaper as well.
https://www.ebay.co.uk/itm/Set-6Posi...72.m2749.l2649
Thank for your help JAZZCNC.
When it comes to settings the driver current, the drivers have peak and REF values labeled on them. Which one should I be concerned with?
I found the same question in routercnc MK3 build thread, but couldn't find an answer.
Use Peak values.Quote:
Originally Posted by diycnc
Transformer, fuses, mcb, and terminal block on order. Wish me luck!
Whilst I wait for the psu parts to arrive, I was thinking about the driver fuses. With the current setup, if one fuse blows on say the z axis, the other axis will just continue regardless, potentially damaging the part or even the machine. Is there a software or hardware tool to stop this happening?
Would a 3 pole fuse holder work? Or is each fuse still independent of the others?
What drives you using.?
Each fuse is still independent.
You might be able to detect an unpowered driver with a properly configured fault output and a controller that is checking for this signal?
DM860T drivers from stepper online.
https://www.omc-stepperonline.com/download/DM860T.pdf
Nice, looks like you making good progress:)
Just stumbled across this on YouTube and thought it was worth sharing. This guy cuts aluminium on a 3D printed MPCNC better than most videos iv seen from people with a "propper" diy cnc router! Mighty impressive!
Hope my machine can perform even half as well as this.
https://youtu.be/zefnrPRvc9Q
Very impressive! That just goes to show how important it is to spend time learning how to use the tools you have to their maximum capability.
I'm certainly tempted to try out my router on aluminium. I hadn't really though it was up to any serious work it until I saw that.
It's the adaptive tool paths that allow him to get away with it as they take very lite cuts but at full depth. If you notice when profiling using typical profile toolpath he was using a very small depth of cut(DOC) and feed rate because the machine wouldn't be able to handle taking correct DOC to suit the tool diameter. However Still impressive to say 3D printed.!Quote:
Originally Posted by Kitwn
Transformer arived today. Very please to see it came with a mount bracket too. Couldn't wait to test it so wired it up, only to spend half hour trying to figure out why there was no output voltage... turns out my multimeter had died! Try again tomorrow with a new one.
Thinking of building a small wooded cabinet for all the electronics, maybe with a plexiglass window if I can keep my wiring looking nice and tidy. Are wooded cabinets considered safe?
I don't think theres anything wrong with a wooden cabinet - plenty of people do it. I think metal is probably better at keeping moisture out, (electrical) noise in, and means anything that manages to go bang is contained by a strong earth-connected box, but its probably overkill for many of these machines. From a financial perspective, my metal cab certainly wasn't cheap, and I'm pretty sure its not going to add any functionality to my build. I have a sneaking suspicion mine is mainly for aesthetics....Quote:
Originally Posted by diycnc
Another option is to grab yourself a server rack with a glass front (reasonably inexpensive on ebay - especially the kits or 2nd hand), or use a large PC case.