The voltage going to the motors is 12V (for a 12V supply, and the wires will take some off). But it isn’t on all the time. They might be on for 20%, which would average 2.4V. When the steppers move more quickly, they make effectively a reverse voltage, which means only 6V might be reaching the motors, which would make them be on for 40%, which is still 2.4V average.
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