Abstract--Based on the internal model principle, repetitive controller (RC) is capable to reduce periodic torque ripple by generating a compensating action that consequently need to be synchronized with the original ripple. However, the synchronization is difficult to achieve using the conventional RC when the sampling frequency is not integer multiple of the speed (known as fractional delay issue), or when the speed varies widely. To solve this problem, this paper presents a fractional delay variable frequency torque ripple reduction method for PMSM drives using the combination of anglebased RC and deadbeat current control (DBCC). Four aspects of innovations are included in the proposed control to improve the synchronization. The experimental results show that the proposed control can effectively reduce torque ripple even during speed and load transient.

The angle-based repetitive controller (RC) has been applied successfully in literature for torque ripple suppression in variable speed permanent magnet synchronous machines (PMSMs). The RC is preferred because of its learning capability which allows the cancellation of all periodic ripple from multiple sources. However, its tuning method has not yet been proposed. Since the structure of the repetitive controller is found similar to the disturbance observer (DOB) which is convenient to tune through pole placement, this paper merges the angle-based RC and DOB into a novel angle-based repetitive observer (ARO) for the first time for the torque ripple reduction in PMSMs. ARO takes advantages of its DOB structure that it can be designed separately and can easily be added into an existing control loop. Taking advantages of its RC nature, ARO can tackle a wide frequency range of torque ripple even in the presence of measurement noise. The experimental test results have confirmed the robust performance of ARO under five conditions, including the ideal integral delay, the fractional delay, the load transient, the speed transient and the detuned mechanical parameters conditions. The execution time of the ARO is less than 10 µs.

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The synchronization between power grid and distributed power sources is a crucial issue in the concept of smart grids. For tracking the real-time frequency and phase of threephase grids, phase-locked loop (PLL) technology is commonly used. Many existing PLLs with enhanced disturbance/harmonic rejection capabilities, either fail to maintain the fast response or are not adaptive to grid frequency variations or have high computational complexity. This paper therefore proposes a low computational burden Repetitive Controller (RC) assisted PLL (RCA-PLL) that is not only effective on harmonic rejection, but also has remarkable steady-state performance while maintaining fast dynamic. Moreover, the proposed PLL is adaptive to variable frequency conditions and can self-learn the harmonics to be cancelled. The disturbance/harmonic rejection capabilities together with dynamic and steady-state performances of the RCA-PLL have been highlighted in the paper. The proposed approach is also experimentally compared to the synchronous rotation frame PLL (SRF-PLL) and the Steady-State Linear Kalman filter PLL (SSLKF-PLL), considering the effect of harmonics from the grid-connected converters, unbalances, sensor scaling errors, d.c. offsets, grid frequency variations and phase jumps. The computational burden of the RCA-PLL is also minimized, achieving an experimental execution time of only 12 μs.

High bandwidth and accuracy of the current control loop are fundamental requisites when a fast torque response is required or for facilitating the reduction of torque ripple in high performance drives, especially at high speed. One of the most suitable control methods to achieve these goals is dead beat current control (DBCC). Many types of DBCC have been proposed and implemented in literature. This paper proposes a DBCC incorporating two new functionalities. One is a two steps current prediction to improve prediction accuracy when current measurements are taken place before each sampling period; and particularly to reduce the overshoot during transients when mean value is used as current feedback. The second is a novel compensation method for the rotor movement to eliminate offset errors which occur at high speed. Moreover, the dynamic and steady state performance of the proposed DBCC is assessed in simulations. On the basis of the simulation tests, the control parameters are tuned for experiments and the performance of the proposed functionalities are verified. Finally, the advantage of DBCC, compared with a classical dq PI current regulator, is verified in experiments.