The aim of this work is to study the influence of technological defects in stator windings electrical insulation system on the inception of partial discharges in the stator slot of air-cooled electric rotating machines. By means of Comsol Multiphysics finite-element modeling software the electrophysical processes in high-voltage stator insulation with corona-protection coatings are numerically studied for both Resin-Rich (RR) and vacuum pressure impregnation (VPI) technologies. The most important results are the conclusions made about the possibility of slot partial discharges in the insulation system of stator windings, taking into account the possible types and sizes of technological defects of the anticorona coating in the slot area. The verification of the results of mathematical modeling by experimental study of slot partial discharges on test samples with artificial defects of anti-corona coatings made possible to establish the most dangerous types and geometric sizes of defects, which lead to the inception of slot partial discharges, which determined the practical significance of the results. Quantitative dependencies between the geometric dimensions of coating defects and the characteristics of slot partial discharges (inception voltage, maximum apparent charge, and average current) are experimentally established. The obtained data have practical importance for the development of the optimal technology for manufacturing the system of electrical insulation of the stator winding of high-voltage air-cooled electric machines.