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Compound-structure permanent-magnet synchronous machine (CS-PMSM), which is composed of a stator machine (SM) and a double-rotor machine (DRM), is a new power-split concept for hybrid electric vehicles (HEVs). To compare the CS-PMSM system with Toyota Prius based on the planetary gear unit, an 80 kW radial-radial flux prototype machine is designed. Fractional slots are employed for SM and DRM and the slot openings and pole-arc embraces of the two machines are optimized to decrease torque fluctuation. As the outer rotor is the structural and magnetic common part of SM and DRM, the magnetic circuits of the two machines are coupled. To obtain the thinner outer rotor and to decrease magnetic saturation of the outer rotor during machine design, the influences of the thickness and pole-arc embraces of permanent magnets on magnetic coupling are investigated by finite-element method. The magnetic field in the outer rotor may reach saturation, due to the impact of armature reaction on the flux distribution in the outer rotor. Under the condition of armature reaction, the flux distribution and electromagnetic torque are analyzed.