Fault-tolerant capability is one of the dominant advantage of multiphase drives. In case of open-phase fault occurrence, the control strategy needs to be modified in order to maintain smooth operation, resulting in larger amplitude of the phase current. Consequently, the winding losses are increased in comparison with healthy operation. Also, the torque operation range (TOR) is reduced inevitably. This article proposed a unified fault-tolerant strategy that can ensure the widest TOR while minimizing losses. Compared with state-of-the-art literature, the execution of the fault-tolerant algorithm does not require any online or offline global optimization process. Instead, simple closed-form expressions are adopted, which could be considered a feasible simplification. Furthermore, the proposed strategy can be adopted for symmetrical or asymmetrical multiphase machines with any number of phases. Experimental results validate the effectiveness of the proposed fault-tolerant method.

Conventional space vector pulse width modulation (SVPWM) would degrade the operating performance of the multiphase motor drives under open-phase fault. To resolve this issue, existing solutions require the reconfiguration of transformation matrices. However, for the variable positions of faulty phases, the structure of matrices needs to be redefined, which is a restraint to the industrial implementation of such technologies. This study examined the offset of the voltage vectors after fault occurrence. Accordingly, a universal fault-tolerant SVPWM strategy is proposed for the purpose of natural fault-tolerance, as well as a simplified fault detection scheme. Compared with conventional fault-tolerant SVPWM, it is highlighted with the characteristics of minimum reconfiguration, torque-ripple free operation, and applicability to handle various types of open-phase faults using a single transformation matrix. Besides, proposed strategy eliminates the redundant controllers and also reduces the degree of difficulty for fault diagnosis. Finally, the effectiveness of proposed approach is evaluated experimentally with a seven-phase induction machine, which shows that the torque ripple could be reduced by about 40% in the presence of multiple faults compared to the conventional SVPWM strategy.

The proposed technique focuses on the non -sinusoidal power supply control strategy aiming to obtain the optimal air-gap flux density distribution. The electrical constraints for the alignment between rotated fundamental and harmonic air-gap flux are illustrated in detail. The supporting simulation is performed to analyze the misaligned angles between fundamental and harmonic air-gap flux for the traditional methods. Different from traditional flux-oriented methods, the improved algorithm for harmonic currents injection is designed based on rotor flux orientated control (RFOC) in fundamental plane and air-gap flux orientated control (AFOC) in harmonic planes. The synchronous and aligned control of the fundamental and harmonic air-gap flux is achieved regardless of load condition. The experimental verification is implemented based on a multiphase driving platform using a 7-phase induction machine (IM) as prototype. Various performance is provided for the proposed control strategy comparing with the traditional ones in terms of induced electromotive force (EMF) waveforms, harmonic currents and advance angles of air-gap flux. The simulated and experimental results validate the theoretical assumption and the effectiveness of the proposed method.

The traditional direct torque control (DTC) strategy utilizes the largest fourteen voltage vectors for a seven-phase induction machine (7PIM) which is directly extended from the three-phase counterparts. This method suffers from the issues of severe harmonic currents and torque ripple for multiphase ones. This paper firstly puts forward the concept of virtual vectors for a 7PIM. This can efficiently eliminate the third and fifth order harmonic currents. However, the DTC strategy based on the virtual voltage vector has insignificant effect on reducing torque ripple. The torque ripple caused by these virtual vectors in different torque and speed conditions is investigated. Analysis shows that there are some additional degrees of freedom while choosing the applied voltage vector. A novel sequential strategy integrated with duty ratio optimization algorithm is proposed to select the optimal voltage vector which can significantly alleviate the torque ripple. Experimental results verified that the proposed method can effectively reduce dynamic and steady-state torque ripple as well as harmonic current. This paper uses a 7PIM for an illustration, but it should be mentioned here that the proposed strategy is applicable for multiphase machines with any number of phases.

The development of multiphase drives has taken on an accelerated pace in the last decades, and fault-tolerance has been the subject of many classic studies. Traditional space vector pulsewidth modulation (SVPWM) would cause significant torque ripple in the case of open-circuit fault, which in turn degrades the fault-tolerant performance of multiphase machines. This work proposes a novel SVPWM fault-tolerant strategy for seven-phase induction machine under a single-phase open-circuit fault. A deeper insight has been provided into the reconstruction of voltage vectors without using reference current, ensuring the generality for various fault-tolerant principles. Subsequently, an eight-sector-based sector division method is proposed, and five specific nonzero voltage vectors and two zero-voltage vectors in each sector are determined to be candidates. Without confliction of switching state, the symmetric modulation module can still be utilized, which will minimize the modifications after fault occurrence. Besides, the close-loop control for harmonic planes can be fulfilled. Finally, experimental results have demonstrated the effectiveness of the proposed SVPWM fault-tolerant approach under both steady and transient states.

As the fault localization for multiphase machines is critical for fault isolation and active fault-tolerant strategies, reliable fault diagnosis strategy is essential to be proposed. After open-circuit fault occurs, harmonic current vectors will follow different trajectories according to different faulty phases. However, this event can only be evident in case of single phase fault occurrence. Thus, existing technique can only identify limited types of open-switch fault. This article proposes a simple and efficient open-switch fault detection strategy to extract fault features. Proposed strategy is able to locate any combinations of open-switch faults, which cannot be realized in the previous literature. The faulty phases are detected first to reduce computational burden and for avoidance of false alarms. Then the faulty switches are determined on the basis of the direction of current vectors in the harmonic plane. Taking account of open-switch faults in different phases, normal current vectors are reconstructed with newly defined auxiliary current variables, which can be utilized to detect the failure of multiple switches and result in enhanced robustness. Experimental results validate the effectiveness of the proposed multiple open-switch fault detection method in a seven-phase machine at 5.5 kW power level.