Was about to get a toy- then I did some research...

[Juranovich]

Do not rely on the OV protection of these drives, they will not save you from more than just a few volts. So if you get a spike then they will fry them.
They are good drives provided you leave a good safety margin.

You need to leave at least a 10% safety margin on the voltage and I wouldn't run much above 60Vac with 70Vac Max.

Regards the Transformer I use 750Va with these drives without any issues. I wouldn't go above 750Va because you will get troubles from inrush.

Do NOT use a regulated PSU like as been suggested by one member as it could cause you troubles with voltage clamping.

Now this gets fun! So, given that I want steppers around 4Nm. They usually (based on my google searches) come rated at 4-5A and 2-3mH inductance meaning total drawn amps are in the range of (Atot_= 2/3*pcs*Arated) 10.5-13.5Atot and Vmax (=32*√mH) 45-56V.. Now based on what I've picked up in this discussion I want to give my drives as much voltage as possible (minding 10% safety margin) and restrict the PSU to 750VA due to inrush. Say I supply 60V to 70Vmax drives, that means the 750VA PSU supplies 12.5A of current i.e. steppers should be rated at 4.5A (~12Atot) or lower in order to get max performance out of them (naturally I could have higher rated steppers, but that would be wasted monies, correct?). Am I correct in this logic?

Moreover, is the performance of the steppers linearly related to the current supplied or is there some leverage at play, i.e. say I supply a stepper with 4.5A, now will a 5A (arbitrarily chosen) rated stepper perform equally well as a 4.5A rated stepper would?

[JAZZCNC]

Say I supply 60V to 70Vmax drives, that means the 750VA PSU supplies 12.5A of current i.e. steppers should be rated at 4.5A (~12Atot) or lower in order to get max performance out of them (naturally I could have higher rated steppers, but that would be wasted monies, correct?). Am I correct in this logic?

Well, it depends on how many drives you intend to use now and in the future. Maybe you might add a 4th axis.? In this case, the 750Va allows a little overhead. However, what I didn't mention so as not to confuse the issue is that I've also used 625Va without any issues. But I knew this machine wouldn't need 4th axis because it was fitted with a 4th Axis that was powered from a separate smaller PSU.

Moreover, is the performance of the steppers linearly related to the current supplied or is there some leverage at play, i.e. say I supply a stepper with 4.5A, now will a 5A (arbitrarily chosen) rated stepper perform equally well as a 4.5A rated stepper would?

If using the Same Voltage then the 5A motor will have lower overall performance than the 4.5A motor. It will have a little more torque lower down the range but the RPM will be lower. This is mostly due to inductance because the higher current motor will have more inductance. It will also create more heat which robs performance.
In this case, half an Amp is neither here or there so wouldn't be a big difference and you'd hardly notice it. Thou any gains would be offset unless something else changed ie: Voltage. Everything comes at a cost.!

In a nutshell, Higher Amp's which often = higher inductance means higher voltage to reach the same speed. This is why often a 4Nm Nema23 will outperform a 4Nm Nema34 motor if using the same volts.
It's also why Large Nm Nema 34, 42 size motors require Very high or better still mains level voltages to allow any reasonable RPM's. All the machines I build that use above 8Nm motors use Mains voltage drives.

People often mistakenly think increasing the current will give more torque, which it does up to a point, but it also increases heat which affects the motor's saturation point which then creates resonance etc so stalls at lower RPM.

It's a complicated formula with several twists depending on motor spec, wiring, etc also with the advent of Digital drives allowing much better performance then Old Vmax (=32*√mH) Formula is even less relevant because the motors can be pushed harder and heat is controlled better along with resonance.

All I can tell you is that if you run the motors at the Rated current with a Voltage 10% lower than the drives Max V you'll be getting the best performance. If you need a little more Torque then increasing the current will provide a little extra but will cost in terms of heat and RPM.

[Juranovich]

All I can tell you is that if you run the motors at the Rated current with a Voltage 10% lower than the drives Max V you'll be getting the best performance. If you need a little more Torque then increasing the current will provide a little extra but will cost in terms of heat and RPM.

After much googling and talking to my uncle who's an electrical engineer I think I've got my head around the basics of this. One thing i still find confusing however is that logically you'd want to run the motors at they're rated current but I keep reading that the drives only draw 2/3 of that assuming parallel wiring (hence psu should be sized 2/3*Atot). Now, are the drives able to supply the motors the full rated current even if only 2/3 of that is supplied by the psu (through some magic I don't understand)? Or is this 2/3-rule applied simply due to the fact that the motors are seldom simultaneously drawing all of their rated current?

[AndyUK]

Or is this 2/3-rule applied simply due to the fact that the motors are seldom simultaneously drawing all of their rated current?

I think its that you'd never power all phases of the same motor to 100% of their current at the same time? But I'm sure one of the electronics experts around here will come up with a much more detailed answer! The 2/3rds rule is for parallel wiring, and for series wiring you can get away with 1/3rd.

I'd wager the answer is in here: http://homepage.divms.uiowa.edu/~jones/step/index.html (I just don't have the time to read it all!)

[JAZZCNC]

After much googling and talking to my uncle who's an electrical engineer I think I've got my head around the basics of this. One thing i still find confusing however is that logically you'd want to run the motors at they're rated current but I keep reading that the drives only draw 2/3 of that assuming parallel wiring (hence psu should be sized 2/3*Atot).

Be careful here when talking with electrical engineers ie: Domestic electricians or maintenance electricians because while they know how electricity and how circuits work etc I find they don't always understand or realize how different a Stepper driven machine differs to say typical AC motor system.
A stepper drive uses a chopping system to control the current/voltage that steppers require so it's not straight forward in terms of power draw etc like it is with say an AC motor connected straight to mains voltage.
The drives use a chopping system which uses PWM which only draws current 50% of the cycle on time. This power is taken from the capacitors in the DC system (AC drives just rectify inside the drives to DC) so during the Off cycle time the capacitors are recharging so only drawing power 50% of the time.
This is one of the reasons why the PSU can be sized lower than total Motor ratings. The other reasons being Not all Motors will draw full current all of the time and if they do then it's for very short periods and the Capacitors and drives will deal with any shortfall.

Now, are the drives able to supply the motors the full rated current even if only 2/3 of that is supplied by the psu (through some magic I don't understand)? Or is this 2/3-rule applied simply due to the fact that the motors are seldom simultaneously drawing all of their rated current?

Above should explain this hope fully.! . . . . Don't try to overthink this, I understand the need to understand how it works but if you want to build a good machine then what's been suggested will work great. You could spend weeks or months learning how it all works and you'll still end up back at what's been suggested.
Go with what's proven to work and you won't go wrong.

[Juranovich]

Above should explain this hope fully.! . . . . Don't try to overthink this, I understand the need to understand how it works but if you want to build a good machine then what's been suggested will work great. You could spend weeks or months learning how it all works and you'll still end up back at what's been suggested.
Go with what's proven to work and you won't go wrong.

This! Even though I'll probably end up building my machine according to the various suggestions I've got, I still cannot bring myself to do something without having built an understanding of my own of the matter. Hence, the occasional silly questions :)

[Juranovich]

Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?

My thinking is that any force applied to the spindle in the X direction affects the balance of the entire gantry assembly. Now, the reason I'm double checking this is that when drawing the gantry assembly (as in the attached, albeit, unfinished drawing) I find that the COG is quite far back (the left side mark) if the beam is also included in the calculations. I know some parts are still missing from the drawing, but even when included, the end result will not change much (in fact, what's missing is mostly parts on the left hand side of the mark). Bottom line is, in this scenario my gantry arms would raise straight up from the X-axis carriages (hence, neither would the inclusion of the arms affect the cog) and, essentially, leaving the spindle quite far in front of the front carriage.

Problem is that nearly all gantry arm designs I've looked at will have the spindle closer to the halfway point between the carriages or at least somewhere between the carriages. Now, when I calculate for the Z-axis assembly only (the mark to the right) the COG is naturally much closer to the spindle, and should I use this as my reference point my gantry arm design and more importantly the spindle position (relative to the X carriages) would end up looking much more like what I see others using. My intuition still says I should go with the COG of the entire gantry assembly as my reference point, but seeing that this would end up looking much different to the norm, it makes me wonder if I've overlooked something or simply using the wrong logic?

[JAZZCNC]

Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?

Yes Ideally but again without wanting to sound like a broken record don't let these kinds of details bog you down from the building phase. The COG being off a little like what your showing isn't going to make one jot of difference to how the machine performs or how it affects component life in a DIY environment.

Anyone who's built a machine and worried about these kinds of things will tell you that it was a pointless exercise and it's the smaller details that make a bigger difference to how the machine performs. Details like sturdy ball-nut brackets and adjustability, motor mounts, wire routing, Limit SW placement, access to grease nipples and lubing, etc, etc.
If you build a Sturdy structure and pay attention to key areas like Z-axis then you won't go wrong or notice if COG is off a little, but you will notice if the little details are missed.

My advice is to look around at other builds and pay more attention to the little details and pay less attention to if COG looks a little offset.!

[JAZZCNC]

I still cannot bring myself to do something without having built an understanding of my own of the matter. Hence, the occasional silly questions :)

The only silly question is the one you don't ask...Keep firing away we understand.!
What's I'm trying to say more than anything is don't let it slow you down from building because you'll still end up back at or around what's been suggested.