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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?
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.Quote:
Originally Posted by m_c
This article seems like a good starting point:
http://www.geckodrive.com/support/power-supply-basics/
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.
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.Quote:
Originally Posted by Neale
These are the drivers I have:
https://uploads.tapatalk-cdn.com/202...7871b3a171.jpg
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.
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...
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
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……Quote:
Originally Posted by Neale
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 [emoji482]
1. With a significant overload, the transformer will overheat leading to breakdown of insulation and/or burnt out winding. However, with short-term overload (say, no more than a few seconds with 2x overload) you will not see any significant effect and the transformer isn't likely to suffer. Output voltage will drop slightly below the nominal value but not enough to worry the stepper drivers. The motors will not notice. And even that level of overload is unlikely. Go 625VA and be safe to add a fourth axis if you like.Quote:
Originally Posted by JonnyFive
2. After my last posting, I went back and looked at your picture of your drivers. I was surprised to see that there was no current control setting. Usually, some of the switches are used for setting maximum current. So maybe these drivers have some kind of software setting, or they are pre-configured for a particular current. Did they come with specific motors? If so, ignore what I said because the manufacturer has already dealt with this one!
3. As per 1 above, if you go for an over-rated transformer you could safely add a fourth axis - just wire it in parallel with the existing drivers. Fourth axis is usually rotary and unlikely to even take as much current as the XYZ motors. Or add another dedicated transformer? Do not wire a second transformer in parallel with the first! Their output voltages unlikely to match closely enough. You do not need any connections between the two transformers. However, I doubt that you would need that anyway.
4. Yes, just connect transformer secondaries directly to the AC in terminals on the driver. If at all possible, you are recommended to wire each driver directly to the transformer, probably via a terminal block of some kind which can take all the wires. Do not daisy-chain more than one driver together (i.e. wires go to first one then continue to the next). Whatever rectification and smoothing is needed by the driver is handled by its internal circuitry.
Just to clarify something on the current requirements, a 5A stepper motor won't draw 5A from the power supply, except in very specific circumstances which are not actually likely to occur.
Say a stepper is rated 5A at 10V while stationary (nice round figures for simplicity, but voltage is likely to be 3-5V for most common steppers), that means at idle, it will consumer 50W (5V x 10A).
If you power that via a 50V transformer, that means at idle the drive is only going to pull 1A (50W/50V). (assuming 100% efficiency - typical efficiencies are going to be in the 90s)
However, once the motor starts spinning, the motor requires more voltage to maintain the rated current due to the generated back emf, so at some point the drive will reach a point where there is not enough voltage for it to maintain the full 5A current, and this is the point where maximum power supply draw will be pulled. As you move above that point, which I'll call the peak power point, current draw will actually decrease.
However (yes another however!), stepper motors are not that efficient, so in practise the current actually drops of before reaching the peak power point, due to that pesky inductance which limits how fast the current can be changed in the coils.
@Neale those drives are software configurable.
I'm not entirely sure why they bother with microswitches for some settings, given other critical settings have to be set in software.
@JonnyFive if you need the software, I've got the install file for V2.1.8, which lists your drives.
So there’s an app I need to configure the drivers? I did not know that. Presumably they are set to 6A from the factory, am I likely to need to change it?Quote:
Originally Posted by m_c
Just read the manual for the driver, looks like you need to connect via the comm port to tweak the current. I ain’t gonna do that [emoji1787] The manual says the current will be adjusted automatically.
Let’s talk fuses, how does one work out what amperage to use? Just add up all the current draws within the circuit? Add a bit on top for luck?
I've been researching wire sizing for my build and here's what I've come up with:
1) Cable from wall socket to cabinet - 3 core, 2.5mm2 flex
2) 240Vac within cabinet - 2.5mm2
3) Earthing - 1.5mm2
4) 24Vdc signals - 0.5mm2
5) Proximity switches - 3 core 0.5mm2 CY
6) VFD to spindle - 4 core 1.5mm2 CY
7) 55Vac within cabinet - 2.5mm2
8) A+ / A- / B+ / B- - 4 core 1.5mm2 CY
9) EGND / VCC+ / EA+ / EA- / EB+ / EA- - 1mm2 CY
10) PUl+ / DIR+ - 0.5mm2 twisted
How does that all sound?
:beer: