Heat is generated directly in the windings because the low-frequency current flows through the copper conductors and produces I²R losses inside the metal itself, not in the surrounding air or oil. This means the thick paper insulation is heated from the inside outward, exactly where most of the moisture is trapped. As a result, a strong temperature and vapor-pressure gradient is created inside the insulation, which drives moisture toward the surface much faster and makes vacuum removal far more effective than external heating methods.