Old thread, I know. But I felt the answers were not that good.
The RATED voltage of a stepper motor, is at it's rated amperage. The amperage running trough a conductor (wire/coil/inductor) is what proportionally generates the magnetic field and heat.. In a conductor, you cannot change it's resistance if you discount heat changing it.
So for all intents and purposes, the resistance (OHM) in your stepper phases, is 24Ω. The manufacturers have rated the phase amperage (the thing that creates heat and could melt a conductor/destroy it), at 0.2A (200mA).
So let's apply Ohm's law. Voltage = Amperage * Resistance.
0.2A, times 24Ω, is 4.8V.
This is why your stepper is rated at 4.8 volts.
You need to understand that the 200mA rating, is for the HOLDING/PERSISTANT current running trough the coil (RMS current), but not the peak ratings. "Technically" the peak current could be anything, but the higher it is, the shorter time it can "be" that value before heating a part of the coil to breaking point.
You can't "change voltage, without changing amperage". That's just not possible. So let's say your supply voltage is 24 volts. The resistance is the same, so this would mean that the amperage would be: 24V, divided by 24Ω, equals 1A.
If you notice, the PEAK amperage isn't specified on the motor, but on the driver. If you supply 24V to the driver, the peak amperage would be 1A for this motor. The driver just can't deliver more than that.
So how the hell does the driver "limit the current"? Well that's the whole task of the driver. If we could supply the stepper with a constant voltage, it wouldn't need an driver module, but just a few transistors. The driver is rated with a max voltage before it breaks, and uses switches (DIP-switches usually) to set what current it should output as a maximum. So when the motor is in a "hold", the driver WILL output 4.8V, and not 24V that is coming in to it. (If you set it to 200mA that is)..
The driver needs to be set to LESS or equal to the rated amperage, otherwise the motor coils gets "too much" current trough them. But the drivers task is to limit the current by reducing the voltage, not to do "both", because that's not possible.
But why do drivers have such higher voltage than the rated voltage on a motor?
Because higher voltage will induce the current WAY faster. If you apply 4.8V to this motor, it will "react", but will be very very slow. It takes more time to reach the rated current (to the factor of tens of milliseconds) for ONE step.. And since the current is what gives torque to the motor, it will not be able to spin fast at all with this voltage. (One rotation is 200 steps. If ONE step takes 12-13ms, that's over 2 seconds to just perform one rotation with full torque). So we absolutely need to decrease the time to achieve rated current. And with higher voltage, we can achieve that.
But there's a tradeoff. Of course there is.... (read up on inductance, cause what I write now isn't completely correct) In that tiny moment where you apply 24V to the coil, "1 amp will run trough the wire", and that's way more than what it's rated for. There's a lot of heat created. But the current builds trough the coil faster, and thus we can reduce the voltage way faster to keep the rated amperage. So in practice we get two problems; there is more heat generated in general, and at a point it will break the motor. Second is that the motor will JUMP to it's next step WAY faster, instead of "slowly moving to it", and this will induce vibration into the whole engine. This vibration is as fast as each driver-pulse, and at for example 200 pulses each second, the vibration is at 200 times each second (200hz). This is what makes the motor sound. And the stronger "jumps" it makes, the stronger the sound (search youtube for "stepper music" and you'll see how nicely we can tune this sound frequency). The motor might even get so fast "jump" that it overjumps into the next "step". Thus missing a step.
The stepper voltage rating is derived from Ohm's law. It directly relates to it's rated amperage. A 68V driver will deliver 4.8V if it is set to 200mA. And a motor has no problem with any peak voltage/amperage just as long as it's not for TOO long to destroy it.