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uncle_bob
The very fundamental problem is that people seem to be looking just at current and not at voltage or resistance (or max torque). Current by its self means absolutely nothing at all. You need the rest of the data. I can *easily* design a NEMA 17 motor that will blow away you 2.5A gizmo when mine is running 0.1A..
All circumstances aside. I'll try say what these parameters mean to me. These could mean differently to everyone else, with a margin that is normal, for everyone sees things differently. Voltage/resistance is the factor that gives the final value in the rising edge formula. What is important for the stepper to function as it needs to, is to have that factor several times lower than what ohms law would give, that is for the purpose to have the actual rising edge engaged in its first part, where it rises fast e.g. di/dt is high, and not in the latter part where it rises painfully slow. Resistance with inductance gives the time constant tau at the coeficient of e^(-t/tau) and thats their impact. I think i should care for rise time specifically, simply put, because there is lots of it. Beyond that, after the Ipk has been reached, input voltage isnt what it normally would be. You said those words believing the power relation still holds like in an normal dc load, but i think it doesnt. Actually after the Ipk is reached, the *input* voltage doesnt matter in the way you expect it to, because the driver job is to limit the current. It does that in a what that you can say that either the driver limits the voltage down to fit ohms law for the given coil, or probably bettter you can say the driver chops it (e.g. "chopper" driver). In power switching to get the average is applied some principle named volt*second balance (e.g. webbers). So it doesnt matter what the voltage is at input, the one going through the coil will be dynamically chopped off, and will average to a different level, hence we cant simply multiply the input voltage with the current to get the coil power state. If we want to do that in comparable terms we would need to apply something like volt*second balance. And in case of higher voltage input then duty will be smaller and then you can see it doesnt go the way of increasing input voltage to get proportinally more power. We do say things like that sometimes, but as an abstraction or oversimplification for the purpose of highlighting something else. However increasing the voltage does bring an important benefit, and that is it betters voltage/resistance ratio that is the final value (the natural point would be reached if no intervention) in the curent rise formula so the current increases much faster than it would increase with low voltage, and this way also contributes to improving the average. But ultimately this is very beneficial exactly for the purpose of helping with maintaining torque at speeds.
Maybe some motors may have a "max torque", that is they have a bad behavior at start and low rpm, then they get to a nice torque plateau, then at higher rpm they again behave worse. E.g. like a automobile gas pedal or something. In stepper motors i see nothing similar as a principle. Some things that do that are missing, and the last thing that could do that is rotor inertia but characteristically too low to for the large scale impact, though may be seen its effect on some graphs. One of the things given, holding torque is somthing i dont care for in my printer, because the x and y are horizontal, and may care for if z would had belts, but its on self blocking screws and z motors are off between slices - this may be different with rostocks or milling heads that get a force exerted on head. Instead, what we would certainly care for is in any 3d printer is dynamic torque, that is a dynamic parameter that starts from a detent torque value that is the torque from one full step position to the other i guess like in a solenoid. Also a note about it, normally this should of mentioned methodology or its measuring params, otherwise its not applicable or comparable directly. Instead, this is exactly why they give it as a max, that isnt like peak of a parabola, but instead is just the max start point under best conditions - and after that things only go downhill. And then at speeds, this torque keeps decreasing untill at some speed, the torque tends to zero. And that happens because current cant rise fast enough. So if there is a parameter that goes down, then it goes down more or less depending on its critical conditions, which determines where we end up. We could have a good start and be in trouble mid way, or we could have a lesser start but still be in good shape mid way.