4' x 4' x 8" Router Build

[JonnyFive]

Is anyone able to shed some light on how I size my transformer for the motors? I have 3 x 5.5Nm Nema 24s and 1 x 4.5Nm. The motor data sheet says current = 5.8A and the driver max peak current = 6A. Is it just a case of adding them all together? Do you downgrade a bit based on the assumption not all motors will be drawing max current at the same time?

[m_c]

Generally for 4 motors about 50% of total combined capacity will give plenty head room.

I generally go for 50%, then round up to the next transformers size if going for an unregulated supply.

[JonnyFive]

Generally for 4 motors about 50% of total combined capacity will give plenty head room.

I generally go for 50%, then round up to the next transformers size if going for an unregulated supply.

Thanks for the feedback. I’ve asked around in a few places and been told multiple things from 1/3 to 125% of the total current rating which hasn’t made anything any clearer. Let alone the type of power supply - do I go for a switching PSU or just a transformer? AC or DC? I’m still not sure where to go.

This article seems like a good starting point:

http://www.geckodrive.com/support/power-supply-basics/

[m_c]

Gecko is a good resource, as it's written by people who actually understand how drives work, and the principles.

1/3 is probably OK if you're not pushing the motors to their maximum speed with high acceleration.
125% is most definitely a figure thought up by somebody who doesn't understand the power requirements of a stepper motor.

My preference is for a basic unregulated supply, with a transformer, rectifier and diode, however I normally used drives that use a basic DC input and dump energy back into the supply under decelleration. If you're using drives that accept a DC or AC input, then they don't do that (the drive input is rectified, so can't physically dump energy back out it), and I'd pick whatever power supply will physically fit better.

[Neale]

The main reason for using one of the cheap Chinese switch-mode power supplies is that they are cheap, small, light, and cheap. And did I say cheap? However, as has been mentioned in this thread and also alluded to in the Gecko document, they do not suffer overload gracefully. That is, they will trip out if they detect any combination of overload, overvoltage, or whatever (and assuming that the protection circuitry is up to it) and the stepper drivers do not like this - any loss of power is going to mean at best restarting the job and at worst a ruined job as the software has now lost control of machine position. That is why a SMPS needs to be over-rated for the load for those rare just-in-case situations when all the drivers are hitting peak load together. The "linear" power supply - transformer, rectifier, capacitor - will, within reason, accept peak overloads without flinching and just carry on, as long as the average load is within its capabilities. This is why they seem to typically be under-specified as the stepper motors only demand peak current in relatively short bursts and seldom all together.

As m_c says, it used to be the case that stepper drivers would only accept DC and you therefore needed some kind of power supply for them. These days, there seem to be more and more drivers around that will accept an AC input of the appropriate voltage directly and contain all the circuitry needed to generate DC internally. Then you just need the corresponding transformer and it's all a lot easier.

I have a CNC router and CNC mill in my workshop that use the classic transformer/rectifier/capacitor combination, and a CNC lathe that I am currently updating which uses newer drives which accept AC. All of these work just fine. I do use the cheap Chinese SMPS boxes, but for some combination of 5V/12V/24V for the control electronics depending on the machine. The SMPS boxes are fine for that application.

[JonnyFive]

The main reason for using one of the cheap Chinese switch-mode power supplies is that they are cheap, small, light, and cheap. And did I say cheap? However, as has been mentioned in this thread and also alluded to in the Gecko document, they do not suffer overload gracefully. That is, they will trip out if they detect any combination of overload, overvoltage, or whatever (and assuming that the protection circuitry is up to it) and the stepper drivers do not like this - any loss of power is going to mean at best restarting the job and at worst a ruined job as the software has now lost control of machine position. That is why a SMPS needs to be over-rated for the load for those rare just-in-case situations when all the drivers are hitting peak load together. The "linear" power supply - transformer, rectifier, capacitor - will, within reason, accept peak overloads without flinching and just carry on, as long as the average load is within its capabilities. This is why they seem to typically be under-specified as the stepper motors only demand peak current in relatively short bursts and seldom all together.

As m_c says, it used to be the case that stepper drivers would only accept DC and you therefore needed some kind of power supply for them. These days, there seem to be more and more drivers around that will accept an AC input of the appropriate voltage directly and contain all the circuitry needed to generate DC internally. Then you just need the corresponding transformer and it's all a lot easier.

I have a CNC router and CNC mill in my workshop that use the classic transformer/rectifier/capacitor combination, and a CNC lathe that I am currently updating which uses newer drives which accept AC. All of these work just fine. I do use the cheap Chinese SMPS boxes, but for some combination of 5V/12V/24V for the control electronics depending on the machine. The SMPS boxes are fine for that application.

Thanks Neale, really appreciate you taking the time for such a detailed response.

These are the drivers I have:



JazzCNC suggested running them at 55VAC and just using a transformer which is what I have in my control schematic.

I’m not looking to build this on the cheap, I want it to be right and I want it to be reliable. Although I can move my gantry easily turning the ballscrews by hand the gantry will be heavy when it’s finished, I’m guessing 50kg, so there will be momentum creating voltage spikes and the system needs to tolerate them.

[Neale]

Decent drives, and I would be happy using 55V AC with those.

I have a home-built CNC router, roughly 1500x750mm cutting area, and the gantry is about 1m long and fairly heavy - especially with a decent spindle motor on it! I drive it with 2 20mm ballscrews. 5mm pitch as 10mm wasn't so common back when I built it, and given that most of my work is smaller but needs a lot of XY movement (i.e. not just long straight cuts) actually that's not too bad a compromise, and I can still get 5m/min out of it which I find acceptable. Personal choice there, and reflects the kind of work I do. Classic "your mileage may vary" situation! However, when you do the sums, you might be surprised to see just how big a component the rotational inertia of the ballscrews contributes to the torque needed to accelerate and decelerate it. Very roughly, on my machine, the ballscrews need about as much torque to get them turning at speed as the gantry itself needs. Surprising result, but this is exactly why some aspects of machine building are not intuitive. 25mm ballscrews would significantly reduce performance purely due to inertia, and why NEMA34 motors often reduce and not improve performance over NEMA23/24 for this size machine.

What you might want to consider when you get to that point is stepper motor current. There are endless arguments about whether the data sheets talk about average, RMS, peak, etc, current. All are different, and usually you don't know which the motor manufacturer is quoting. I would go out on a bit of a limb here and upset the purists by saying that those data sheet values are only a starting point. Basically, the more current you can put through the motor the better it will perform, but the hotter it will get. So my technique is to start with data sheet values, then check the motor casing temperature after the machine has been used for a bit. If you can comfortably hold the motor, then crank the current up a notch. If it's just a bit too hot to hold, turn the current down a bit. Stepper motors typically run hotter than you might expect. Once you have the current about right, then you can finish tuning the machine max acceleration and speed parameters, knowing that you are getting about the best performance available. Other opinions are available...

[JonnyFive]

Any thoughts on these transformers? Are they up to the job?

https://uk.rs-online.com/web/p/toroi...ormers/2238314

This one has the same output for a lot less spend:

https://amzn.eu/d/j9rrGWB

A bit more grunt for a little bit more cash:

https://www.rapidonline.com/vigortro...-0-55v-88-3845

[Neale]

I'm using a couple of Vigortronics transformers myself. Seem to do what they say on the tin! I bought from Rapid but only because I didn't know they were available via Amazon...

500VA is probably enough although because the price difference was small at the time, I went up to 650VA for my router.

[JonnyFive]

I'm using a couple of Vigortronics transformers myself. Seem to do what they say on the tin! I bought from Rapid but only because I didn't know they were available via Amazon...

500VA is probably enough although because the price difference was small at the time, I went up to 650VA for my router.

That’s good to hear some real world feedback, think I might be reaching a conclusion on this……

Based on the Gecko recommendations of 1/3 total current I’d need a 425VA transformer. I’m inclined to go a bit bigger so the transformer isn’t working so hard - if I go for 625VA that takes me to roughly 50% total current as you suggested Neale.

Just to clarify for my own sanity:

1) What are the consequences if I’ve under rated the transformer? The motors will struggle in the situation that they’re all maxed out? How does this actually manifest itself? Motors stall? Feed slows down and start to get rubbing on the cutter?

2) You mentioned checking the motor temperature and adjusting the current - how do you adjust it? Is it a software setting?

3) If I want to add an extra motor sometime in the future (4th axis) I can just add another transformer and wire it in parallel to the first one to give more grunt?

4) The drive accepts AC, I don’t need any kind of smoothing or additional circuitry - just wire direct to the drives from the transformer?

Thanks for your help