The earth leakage of an induction motor under normal dry circumstances with clean mains applied should be extremely low, however there are a number of circumstances other than moisture and insulation problems that cna increase the apparent earth leakage of the motor and not indicate a problem.
There is considerable capacitance between the stator windings and the frame, and because of this, there can be a significant capacitive current if there is a significant amount of high frequency component present. This can be due to variable speed drives or other high speed switching devices distorting the waveform.

Tell us more about your application.

;c;