If you have a clamp on ammeter, like an Amprobe, it would be good to see the no load current draw of the motor. Of course, running on a converter could very well be part or all of the problem if something were wrong with the converter. Have you checked the line voltage on each leg? A serious imbalance could cause some problems as well.
Although a megger will tell you about the condition of the insulation of the motor, you can also check for leakage which will sometimes result from poor insulation. Check for voltage (AC) between the motor frame itself, cleaning the paint off or finding a good bare spot, and the power supply ground wire. Not the neutral, but the ground. In theory, there would be zero volts measured at that point, but if the insulation is "leaking" a small voltage may be present. Start on a high range on the meter, about the supply voltage(which we hope isn't there) and work your way down until you can measure it if it is there. Might only be a few volts. That presents a load that shouldn't be there.
It's also possible, though less likely, that there is a problem with the relay. That's why you need to do these other checks.