On Tue, 12 Feb 2019 12:10:16 -0800 (PST), wrote:

snip

The suggestion re carbon in the motor could well be worth looking at. I reaplced the brushes in the motor with new ones about 3 months ago, so it could be 'dusty' in there.

Mine was as I saw when I blew it out (outdoors).

That said, it trips even when the control knob is set to somethign that doesn't immediately run the motor.

I think one side of he motor can be energised, even if it isn't
actually doing anything and that can then cause a leak to ground.

A (potentially) simple test would be to unplug the motor (often a plug
on the side or end of a short cable) and see what happens then. No RCD
trip the chances are it is the carbon issue.

Cheers, T i m