In this paper, the distribution pattern of harmonic waves in ac electric motors is investigated. On the basis of this, a novel exact and universal approach for calculating differential leakage related to the harmonic waves was developed. This approach applies to any ac electric motors, regardless of winding type, single-phase or three-phase, regular or nonregular, integral-slot or fractional-slot, symmetrical or asymmetrical. Since this novel approach substitutes the approximation method adopted by conventional approaches with an exact one, it distinguishes itself by offering precise results, general applicability, and fast calculation.