Improved Stator Flux Estimation Method for Direct Torque Linear Control of Parallel Hybrid Excitation Switched-Flux Generator

The hybrid excitation switched-flux (HESF) machine, which exhibits advantages including simple and robust rotor structure, good flux-regulation capability, and sinusoidal flux linkage, is suitable for use as a generator in dc power systems. In this paper, a direct torque linear control (DTLC) scheme is investigated for voltage regulation of a parallel HESF (PHESF) generator. This scheme can achieve direct and smooth control of the instantaneous torque to improve the system steady-state and dynamic performance. However, the stator flux estimation accuracy will affect torque control quality. Therefore, a practical stator flux estimation method is presented for both direct torque control and the DTLC schemes of the PHESF generator. It contains a third-order low-pass (LP) filter, a high-pass (HP) filter, and two simple coordinate transformation modules. The third-order LP filter and the HP filter can effectively filter out high-frequency harmonics and the dc drift in the back-EMF, respectively. Then, the produced leading and the lagging phase angles can be compensated by the two coordinate transformation modules. Hence, this method has same amplitude-frequency and phase-frequency characteristics as those of the pure integrator. Meanwhile, its dc gain is zero. The DTLC scheme for the PHESF generator dc power systems using the improved estimation method exhibits good steady-state and dynamic performance, which are verified by experimental results.