With a modern micro stepping driver, the current in the coils is a constant frequency sine wave when the motor speed is constant. As you change the speed the drive is still a sine wave, it's just the frequency that changes. That's not really surprising since you are using the stepper as a multi pole synchronous AC motor. If you run the motor slowly, then ohm's law does indeed apply. It's only at high speed that the inductance of the rotor has any real impact.
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