As industrial automation equipment becomes more compact and precise, the structural design of a harmonic reducer has become an important factor in motion system selection. Cup type, hat type, and pancake harmonic reducers use the same basic strain wave gearing principle, but their different flexspline structures result in different characteristics in axial size, torque capacity, rigidity, hollow space, and mechanical integration. For applications such as 3C automation, semiconductor equipment, CNC machinery, and precision inspection systems, choosing the right configuration requires more than simply comparing reduction ratios or dimensions. This article compares the three major harmonic reducer configurations and explains how to select the most suitable solution based on the actual requirements of an industrial motion system.