The fault tolerance of electric drives is highly appreciated at industry for security and economic reasons, and the inherent redundancy of six-phase machines provides the desired fault-tolerant capability with no extra hardware. For this reason some recent research efforts have been focused on the fault-tolerant design, modelling and control of six-phase machines. Nevertheless, a unified and conclusive analysis of the post-fault capability of six-phase machine is still missing. This work provides a full picture of the post-fault derating in generic six-phase machines and a specific analysis of the faulttolerant capability of the three mainstream six-phase induction machines (asymmetrical, symmetrical and dual three-phase). Experimental results confirm the theoretical post-fault current limits and allow concluding which is the best six-phase machine for each fault scenario and neutral arrangement.Index Terms Six-phase drives, fault-tolerance, field oriented control.

Abstract One of the most attractive features of multiphase machines is the fault-tolerant capability due to the higher number of phases. Different post-fault control strategies based on hysteresis, PI-resonant and predictive techniques have been recently proposed. They all proved their capabilities to withstand fault situations and to preserve the fundamental component of the air-gap field, while achieving minimum losses, maximum torque per-ampere and reducing torque vibrations. Nonetheless, due to their recent introduction, no thorough study has yet appeared comparing the performance of these controllers. In this paper two open-phase fault-tolerant control schemes are experimentally compared in a real five-phase induction machine. The controllers being compared are based on PI-resonant and predictive control techniques, respectively. The experiments include pre-and postfault situations. Obtained results show that both control methods offer nearly the same performance. When compared, predictive control provides faster control response and superior performance at low speed operation but is found to be less resilient to fault detection delays and to have higher current ripple. Regarding the controller implementation, it is shown that the transition from preto post-fault operation involves modelling the non-linear effects observed when an open-phase fault occurs for the predictive controller, and the proper retuning of the PI trackers for the PIresonant controller, in order to ensure post-fault operation.

The most serious and recent competitor to the standard field oriented control (FOC) for induction motors (IM) is the finite control set model predictive control (FCS-MPC). Nevertheless, the extension to multiphase drives faces the impossibility to simultaneously regulate the flux/torque and the secondary current components (typically termed 𝒙-𝒚 in literature). The application of a single switching state during the whole sampling period inevitably implies the appearance of 𝒙-𝒚 voltage/currents that increase the system losses and deteriorate the power quality. These circulating currents become intolerably high as the per unit 𝒙-𝒚 impedance and the switching frequency diminish. Aiming to overcome this limitation, this work suggests the integration of virtual voltage vectors (VVs) into the FCS-MPC structure. The VVs ensure null 𝒙-𝒚 voltages on average during the sampling period and the MPC approach selects the most suitable VV to fulfill the flux/torque requirements. The experimental results for a six-phase case study compare the standard FCS-MPC with the suggested method, confirming that the VVbased MPC maintains the flux/torque regulation and successfully improves the power quality and efficiency.

Achieving a self-reconfigurable fault-tolerant control in multiphase machines requires a fast fault detection and localization. Most fault detection techniques inherit the three-phase approach by defining fault indices in a per-phase basis. A recent approach suggests an alternative fault detection mechanism based on vector space decomposition (VSD) variables, but the study is limited to open-phase faults (OPFs) for a sixphase drive that is regulated under field oriented control (FOC). It is known however that i) the open-switch faults (OSFs) in the converter are more likely than the OPF in the machine and ii) the drive performance in the event of an open-circuit fault is more critical when model predictive control (MPC) is used. This work extends the study of the VSD fault detection method to multiphase machines with different number of phases (five), control strategy (MPC) and type of faults (OPF and OSF). Although experimental results show that MPC misbehaves after the fault occurrence, the fast detection provided by the VSD approach allows a satisfactory transition to post-fault mode of operation.

an alternative to traditional field-oriented control (FOC) methods for three-phase drives. The conventional DTC scheme has been successfully extended to multiphase drives in recent times, using hysteresis regulators to independently track the desired torque and flux in symmetrical five-phase induction machines (IM). The fault-tolerant capability of multiphase drives is an interesting intrinsic advantage for safety-critical applications, where recent research has demonstrated the effectiveness of FOC schemes to perform ripple-free post-fault operation. In spite of the utility of DTC methods in normal operation of the multiphase machine, no extension to manage the post-fault operation of the drive is found in the literature. In this paper, a novel fault-tolerant DTC scheme is presented. The performance of the proposed method is experimentally validated in a five-phase IM drive considering an open-phase fault condition. Provided tests analyze steady and transient states, including the transition from pre-to post-fault operation. Obtained results prove the interest of the proposal, which ensures the open-phase fault-tolerant capability of DTC controlled five-phase IM drives. Index Terms-Direct torque control (DTC), fault-tolerant capability, multiphase induction motor (IM) drives, open-phase fault operation.