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hamonic
it comes my interested to understand the logic and calculation.
let say, my single motor require 2.8V 1.68A.
If I have 12v PSU, the current I need for this motor is proportional to PSU, which is (2.8/12)×1.68A=392mA
and for same logic if using 24V, the calculation is (2.8/24)×1.68A=196ma
Above is correct situation for the driver work?
Depends on the mental frame in which you want to represent the things. For a dc motor, as the voltage is increased, the current also increases (like the ohms law with supply voltage and coil resistance/impedance). So for a dc motor the voltage can be increased with purpose of increasing the current and this way to produce more power. For the stepper motor, when the voltage is increased, the current does not increase because the driver limits the current instead. And the 2.8V number its as much a "requirement" as much as a representation of the coil resistance in relation to the maximum current. From the 3 it suffices that manufacturer gives 2 so user can deduct the missing one. So from that 2.8V and current number you can deduct that your coil resistance should be ~1.67ohms (measure it with an multimeter).
In reality the parameters of the motor come down to what is maximum current, which depends on the wire used to make the coil. Specifically the wire sectional area, a thicker wire would allow for more current and vice-versa. So a certain wire section (gauge) is used in coil, and with the number of turns comes to a certain coil length, which further begin to show as other two parameters, inductance and coil resistance (or impedance). These and only these are the "hard-coded" parameters built into the motor. The voltage at which users are going to run the motors is not a such thing, but its a user choice, you can choose to run at different voltages.
If you run the motors at 12v you can set the driver at 1A (the peak current). If you feed a psu 24v, then again you can set the driver at 1A also. Just the frequency would be different because in 2nd case the coil flux builds up faster, or in other words the coil energizes faster, potentially providing more speed, better said more power at higher speed. But voltage as parameter mostly depends on driver board max voltage input. The coil is insulated with a laqeur which should hold like mains voltage rating, so the coil wont fail because of voltage, but only because of current. The 4 motors in series would make a 6.68ohms coil and the inductance would be 4 times of a single motor, so you probably "really" need 24v to make the Z axis able to move practically, even with a low speed, depending on load too. You could try maybe find a construction solution with a very lightweight Z and only 3 motors, imo that should be better than four.