First Proper CNC Router

[diycnc]

Thanks for the explanation.

If you were going to use a SMPS, how much extra over the nominal current would be recommended?

[m_c]

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.

[diycnc]

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,

[AndyUK]

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.

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.

[andy1989]

-

[diycnc]

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

[Clive S]

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-range

I use a 500V a toroidal with 4 nema 23 on 2 different machines with out problems. The next size up generally is 625Va

[aprex]

Mine went from TME. Quick sending and good prices.

https://www.tme.eu/fr/katalog/transf...100028/?page=1

Best regards

[diycnc]

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?

[Kitwn]

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