Power-factor testing of rotating machinery is a valuable Doble power factor test technique that provides information about the integrity of an insulation system. This test technique has been in existence for many years and Doble has been maintaining a database of results since the late 1950s. (The Doble test database is updated on a fairly regular basis.) The Doble power factor test is effective in detecting insulation contamination, the quality of the semiconducting and stress gradient coatings, the amount of void content, partial discharge damage, insulation delamination and the quality of the impregnation process.
Here is an article by Eileen Duarte that you may want to read. This article reviews the test technique and explains how to interpret the results. It also includes several case studies.