Electrical – How to measure Stepper motor current when using EasyDriver A3967

I would suggest driving the motor with 72% of rated current, or about 310mA. That should yield the same power dissipation as would be expected from unipolar drive at 440mA. When the motor isn't moving, the voltage would have to be about 31 volts. I wouldn't expect insulation problems at 31 volts, or even 50, but I don't know how far one could safely go. Perhaps measure the AC voltage when hand-spinning the motor quickly to see if there seems to be a limit.

Looking at the two links you provided the voltage rating of the stepper motor is shown as 12V which you've well exceeded by applying 20V. That may caused it to exceed the 750mA rating of the driver or burnt out the motor, but when operated at 12V the rated current is shown as 330mA so that driver should be OK.

In general 'supplying' 3A shouldn't be a problem, because if operated within spec it won't draw that much current. But I guess you really mean you've attached to a 20V / 3A supply which would have been delivering less current in practice. So yes 3A would be too much, but I doubt it was really drawing that much current unless you also have some other problem, or the motor windings shorted because of the higher than rated voltage.