ac motor tripping on single phase cause?
This is probably a split phase capacitor start motor where all or part of the capacitance used in starting is switched out once it is running, and that would be the first place I would have looked as you have done. The rcd rating is about 12 times the full load current and this would not protect it against an overload, so it sounds like there is a fault in the motor or the feed to the motor and the only way to find it is to take the circuit apart and do insulation checks on each bit. Intermittent faults like this are hard to find as when a test is carried out the fault doesn't always show.
Motor overloaded, or very low voltage applied (such as using an extension cord with wiring that is too small).
