In this study, a mechanical flux adjusting technique for the switched flux permanent magnet (SFPM) machines is presented to improve the flux-weakening capability by extending the speed range. The ferromagnetic material pieces, that is, flux adjusters (FAs), are located on the outside surface of the stator poles equipped with permanent magnet (PM) and used to weaken the PM flux via short-circuit. To explore the most suitable machine topologies for such flux adjusting technique, the performance of conventional SFPM machines with different stator/rotor pole combinations, multi-tooth, E-core and C-core SFPM machines is investigated and compared by employing the FAs. It shows that the maximum speed and flux-weakening capability of the conventional SFPM machines can be significantly increased if the FAs are used. On the other hand, the multi-tooth, E-core and C-core SFPM machines have inherently large inductance. Thus, the flux adjusting technique has less influence on their flux-weakening performance. Besides two-dimensional (2D) finite-element analysis (FEA) and analytical methods, 3D FEA accounting for end-effect is also used to predict the results. Finally, the analyses are validated by experiments.