PSU size calculation

[ian]

Hi Jazz CNC

I've have build a first version as a proof of concept (see picture attached), but am now starting a new "better" build and am looking at what the ideal PSU should be so that I can buy the right one (or multiples if needed).

The aim is to build a bench-top CNC router, able to cut a lot of engineering plastics of up to 600mm x 600mm x 40mm with reliable precision and accuracy of 0.01mm at the most optimale speed (which seems to be 3500mm/min+) and at 3200 resolution. The machine will be used to make prototype enclosures for GPS trackers for cattle and sheep and accuracy is important due to the products requirements and to achieve good surface finishes and potentially also cut PCB's. Ideally the machine will be housed in a sound/dust enclosure as it will be in my office and I will need easy access to it to build prototypes and keep an eye on small production batches, making 100's off enclosures weekly, so the machine will operate ±4 hours per day and must be reliable. I will also cut some aluminium plates for making prototype moulds.

I'm not yet sure how achievable and realistic the above is as the budget is also somewhat limited, but I intend to build incrementally from the basics (solid frame and linear motion) and add more automation and features later on (assuming I can define a good end-design in the beginning and build towards it).

So far I have the following items for my next iteration:

X & Y
3 x 800mm 1605 ballscrews, C7
3 x pairs 800mm Hiwin 15mm rails and 2 HGR15CA carriages per rail

Z
1 x 250mm 1605 ballscrews, C7
1 x pairs 300mm Hiwin 15mm rails and 2 HGR15CA carriages per rail

4 x Nema 23 motors, 4Nm (want to change to closed loop steppers and drivers as step errors can ruin expensive components)

40mmx80mm extrusions for the frame (need to change these as they are not rigid enough)

Blackbox controller (from Openbuilds), 24v power supply, limit switches (ALL of this needs replacement with better components - these were only for my proof of concept machine build)

I'm currently working on the designs for the plates which is likely to be 15mm aluminium.

What would the ideal PSU specs be for such a machine and the closed loop motors?

[JAZZCNC]

Well like I say with those 50Vdc drives then you really don't want to run much above 45Vdc, I certainly wouldn't run them at 48vdc because it's like playing Russian roulette.

I would build a PSU using a toroidal transformer rated at 500Va with 30V secondary outputs wired in parallel. This will give you approx 43Vdc and 16A but usually, the incoming mains fluctuates anywhere between 230Vac and 250Vac so in use you'll probably end up around 45Vdc.
If you know your incoming voltage is stable and 230vac then you could go with a 32Vac transformer which will give you 45Vdc. Personally I wouldn't because a few less volts won't make a big difference to the speed but certainly will blow a drive up.

Now regards the speed and running at 3200 resolution, by which I presume you mean micro-stepping.? Then first off I need to say that Micro stepping shouldn't be classed as resolution as that is a function of the ball screw and micro-stepping is more intended for smoother running of the motors. However, because of how steppers work there is a limit to what is useful and usable. Most steppers can't resolve down to high micro stepping levels and 3200 is about at the Max that is useful.
Now the side effects of running at this level of MS are lower torque and much higher demand on the controller. For instance, you'll double the amount of pulse frequency required for 3200 than 1600 but have less torque with very little gain in motor smoothness. If your motion controller is limited to say 25Khz then this will half the velocity, so it's only worth running at high MS-like 3200 if you have an excess frequency in the controller.
1600 Ms is more than enough for any stepper system and doesn't tax the controller.

Now with those 2 phase closed-loop steppers and 45Vdc you'll get around 900Rpm that is usable and will max out around 1200-1300Rpm before stalling.
So with 5mm pitch and 900rpm then provided your axis are not binding or sticky and the gantry isn't silly heavy then your Max velocity will be around 4500mm/min and you may push it just above 5000mm/min.
A realistic MAX cutting velocity of 4000mm/min is about what I would estimate with those motors/pitch and voltage.