A new efficiency-improving permanent magnet rotor structure for a tooth-coil-winding open-slot axial-flux machine is proposed. The composite-body-based rotor construction used in the machine has no electro-magnetic losses in the rotor body itself. The construction also significantly reduces the eddy current losses in the permanent magnets and improves short-circuit strength. A prototype of an axial flux generator for mobile working machine was optimised, analysed and measured. The tooth coil windings produce high-amplitude harmonics in the air gap and large slot openings also reduce the rotor flux due to the large air gap reluctance. Both phenomena encourage finding a rotor structure decrease the amount of harmonics travelling through the magnets. It is found that steel laminates inserted on top of the magnets have positive effects on both cases. However, the adverse effect is that, quadrature-axis inductance increases. 2D FEA and 3D FEA analysis was used to verify and optimize the laminations geometry. Computations are verified by real measurements of a 100 kVA test machine.