Quote Originally Posted by Juranovich View Post
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.

Quote Originally Posted by Juranovich View Post
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.