I also think its the capacitor values. Has all been OK up to now ? Can you check the resistance of the motor windings using an Ohmmeter and it insulation resistance using a "Megger " to illuminate the motor .I have wired three ammeters into a converter circuit and like you, was surprised by the differences in line currents and voltages.I then wired a large rotary switch in circuit so that I could adjusted the capacitors, with the motor running, so that the voltages and currents in each line was about the same. I also found that when the motor sounded/hum normal the capacitance was about right. But, all changed when load was applied. I found the values are only correct for one setting, near enough generally. I think modern converters will have some automatic means of adjusting capacitor values.I've never got round to fitting a pilot motor which idles way behind the lathe cabinet equalising the voltages, but apparently they are a good idea. Think of a triangle with each side of the same length, balanced 60 degrees, then another with un equal sides.I'm pleased Inverters came along.John