Hi @SIGURD,
I’m not sure why the voltage between a phase wire and ground is relevant here - the motor coils are energised by current going through them, which a controller achieves by varying the voltages of the phase wires relative to each other (not necessarily directly relative to ground).
The motors are driven by energising the coils with a particular order and timing, so it’s likely more of interest to measure and compare the voltage across different phase pairs over time, to see whether the voltages across each phase are similar or different between the two controllers, or ideally measure the current through the coils rather than voltage across them, to more directly correlate with the magnetic field strength.
@Mark-Belbin’s phase current plots here are perhaps worth a look 