## 1 Introduction
In [15], a double-rotor PM machine is proposed in which one rotor is directly connected to the shaft, and another rotor rotates with displacement relative to the shaft by the spring system. Depending on the rotational speed, two rotors will have a corresponding displacement angle. Therefore, the total flux linkage, which is the vector sum of two flux linkages, is adjustable. However, all the self-actuated mechanical flux-weakening methods cannot be controlled manually, which may be problematic when emergencies occur. Moreover, the spring system is difficult to calibrate and requires regular maintenance. The actively controlled mechanical flux-weakening method uses an external force to control the flux-adjusting elements. A switched flux machine with movable iron pieces outside of the stator that is controlled independently is introduced in [16]. When the iron piece is moved to the closed position, the PM flux is short-circuited, reducing 60% of the flux linkage compared to the conventional switched flux machine. An actively controlled version of the axial-flux PM machine in [15] is proposed in [17], and a radial-flux version is presented in [18], which has the minimum flux linkage in the equilibrium position and the maximum flux linkage in the aligned position. In [19], a dual-rotor PM machine based on bidirectional flux modulation is proposed for the standalone DC power supply. This design requires a conventional winding whose pole pair number is equal to the outer rotor, and a modulation winding. Two sets of windings are connected in series as their frequencies are the same. The inner rotor is connected to the servo motor. Therefore, the inner PM flux linkage vector is adjustable and the sum of modulated vector and the outer PM vector, namely the total flux linkage, is controllable. This active-controlled mechanical flux-weakening method suits the machine with a small saliency ratio. Nevertheless, two sets of windings are needed, which have the following shortcomings: First, two pairs of windings have different coil pitches, which are relatively complex to manufacture; second, the turn numbers of two pairs of windings are different for reaching the maximized flux controlling range. This paper proposes a novel mechanical flux-weakening spoke-type PM generator. In only one set of windings, all three working harmonics have the same frequency, which can be added as vectors based on the flux modulation principle. By adjusting the modulator ring within a certain angle, the flux can be effectively controlled without the risk of PM demagnetization, and the variable-speed constant-amplitude voltage control (VSCAVC)can be achieved. A commercial servo system using the position control mode is sufficient for dynamic performance, which is also cost-efficient, robust, and easy to control. The principles of flux controlling and working harmonic selection are introduced. To investigate the influence of the pole pair number and the performance of the novel design, four cases with different rotor poles and winding configurations, i.e., 12/7, 12/8, 12/10, and 12/11 stator/rotor pole pair combinations, have been designed, optimized, and compared. Finally, the 12/7 design is selected, and its VSCAVC characteristics are analyzed.