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A compound-structure permanent-magnet synchronous machine (CS-PMSM) system used for hybrid electric vehicles (HEVs) enables an internal combustion engine (ICE) to operate within optimum efficiency region independent of road conditions, thus decreasing the emissions and fuel consumption. In this paper a radial-radial-flux CS-PMSM is investigated. It integrates two synchronous machines radially, one is called double-rotor machine (DRM) and the other is called Stator Machine (SM). A downsized prototype machine of the CS-PMSM was designed and manufactured, and the tests were performed. The back electromotive force (BEMF) and efficiency of the DRM and SM are measured and analyzed, respectively. The BEMFs of both machines are approximately sinusoidal, which shows a good design of the magnetic structure. The two machines have efficiencies above 85% in a wide range in both motor and generator modes, but the peak efficiency is not well-content, which is analyzed by investigating the loss distribution. The temperatures of the DRM and SM are further measured, showing a good thermal behavior of the stator but serious overheat of the inner-rotor windings, which must be improved.