AC motor single phase trips RCD?

I think you mean a 30mA trip RCD (earth trip). If there is an imbalance of current, usually due to an earth fault, in the live and neutral conductors of a circuit the RCD will detect this and switch off (trip).

There may not be anything wrong with your motor. Capacitor start motors can occasionally trip ordinary RCDs, usually when its centrifugal switch switches the starting winding and capacitor, which can give an imbalance for a fraction of a second due to the charging / discharging of the capacitor, or possibly a dirty / faulty centrifugal switch. I come across this occasionaly with single phase capacitor start motors in industrial type equipment, pressure washers and floor polishers etc, but rarely is it a motor fault. If in doubt, contact a local electrician or motor repairer to carry out an insulation test on it and advise, as a multimeter may not pick up an insulation fault. If the motor is found to be O/K and the RCD tripping is a real pain, a possible "cure" may be to install a separate circuit for the motor and protected by a time delayed RCD.

Hope this helps.

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.

RCD's will only detect earth faults, not over-current faults and they are rated in milliamps (mA). I feel that your protective device may be an MCB however this would be 32A and not 30A, whatever I think you have an inrush over-current causing nuisance tripping.

Type C MCB's are designed to deal with small inrush currents such as this and I suspect yours is a Type B. I feel the answer is to protect your lathe with a local Type C MCB of an appropriate rating in line with discrimination requirements - which means it must be of a smaller rating than the next breaker upstream. If you have a Type C MCB then the rating is incorrect and needs a higher one, ie 40A.

If I'm wrong and your lathe is plug in and it is a 30mA RCD that is 'tripping' then you have an earth fault in your motor, which is nothing to do with overload or inrush currents.

EDIT: As an afterthought, just to be clear, your house should never be plunged into darkness because of a dodgy motor. If you have a fault on your kettle you expect the plug top fuse to blow, not the household RCD! Thats what I mean by discrimination, every appliance and conductor (cable) needs a correctly rated protective device.

Remove the motor and have it serviced. I suspect that it needs to have all of the wood shavings removed and new bearings installed. 1/3 HP should not trip a 30 amp breaker unless there is more on the circuit. This small motor should never be started under load. It should be allowed to come to full speed before the load is applied. Use an idler pulley to do this.

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An RCD is for current overloads - not unequal currents so it is overloading. This is not particularly dangerous. There is nothing wrong other than you are overloading a little motor. Put a trip near the lathe so as you don't have to hurry to the basement each time it goes.

You have more than your lathe running on that electrical circuit...go turn the breaker off and see what else goes off.