Postfault operation of five-phase induction machine with minimum total losses under aingle open-phase fault

Five-phase induction machines (FPIM) have attracted notable interest in safety critical applications as well as wind energy generation systems. This is largely due to their additional degrees of freedom that retain the machine starting/running steadily under fault conditions. In the available literature, postfault operation of multiphase machines is typically implemented using two techniques: minimum losses (ML) or maximum torque per ampere (MT) strategies. The optimization embedded into the control strategy, however, mostly addresses minimization of the stator copper loss, while the effect of the rotor loss and core loss are discarded in the optimal current calculation. This paper revisits postfault operation of the FPIM under single open phase fault (1OPF) by including the effect of both rotor loss and core loss on the machine's optimal current calculation over the full achievable loading range. The proposed searching algorithm, which combines the advantages of both MT and ML techniques, attempts to minimize the total machine losses induced by the current components of both the fundamental \alpha \beta and the secondary xy subspaces. The theoretical findings have been experimentally validated using a 1.5Hp five-phase prototype system

General online current-harmonic generation for increased torque capability with minimum stator copper loss in fault-tolerant multiphase induction motor drives

This article proposes a current-reference generation method, including current harmonic injection (CHI) for enhancing the torque capability of multiphase induction machines (IMs) with negligible space-harmonic effects, which is useful, e.g., during transient overload in electric vehicles. The admissible torque is increased because the harmonics reduce the phase-current peaks so that the instantaneous peak-current constraint of the drive, usually associated with the converter ratings, is respected. The harmonics are injected in the so-called x−y subspaces of the IM, which do not produce torque, so that no torque ripple is introduced. The optimum harmonics are found online for each load, making it possible to minimize the stator copper loss (SCL) per torque in the entire torque range, ensuring full-range minimum loss (FRML). The method is suitable for healthy operation or open-phase faults and for multiphase machines of any phase number and with either symmetrical or asymmetrical windings. Compared with FRML methods without CHI, higher torque is achieved. Although some techniques were available for increasing the torque capability by CHI, FRML was not attained, laborious off-line optimization was needed, or they were only suitable for specific drives or healthy conditions, unlike the proposal. Experimental results with a symmetrical six-phase IM are providedXunta de Galicia | Ref. ED431F 2020/07Xunta de Galicia | Ref. GPC-ED431B 2020/03Information Technology Industry Development Agency | Ref. ARP2020.R29.7Agencia Estatal de Investigación | Ref. PID2019-105612RB-I00Agencia Estatal de Investigación | Ref. RYC2018-024407-

DC-signal injection for stator-resistance estimation in symmetrical six-phase induction motors under open-phase fault

Multiphase machines are often chosen due to their enhanced fault tolerance. Six-phase ones are especially convenient because they may be fed by off-the-shelf three-phase converters. In particular, those with symmetrical windings offer superior postfault capabilities. On the other hand, estimation of the stator resistance is important for purposes such as thermal monitoring and preserving control performance. Resistance estimation by dcsignal injection provides low sensitivity to parameter deviations compared with other techniques. It has previously been shown that the dc signal can be added in the non-torque-producing (xy) plane of a six-phase machine to avoid the torque disturbances that typically arise in three-phase machines. However, extending this method to the case of an open-phase fault (OPF) is not straightforward, because of the associated current restrictions. This paper addresses dc-signal injection in a symmetrical sixphase induction motor with an OPF. It is shown that, in contrast to healthy operation, the postfault dc injection should be carefully performed so that minimum copper loss, peak phase current and zero-sequence braking torque are achieved. A solution that attains optimum performance in all these aspects simultaneously is proposed. Adapted controller and resistance estimation are also presented. Experimental results confirm the theoryMinistry of Higher Education, Malaysia | Ref. Scheme FP090-2020Science, Technology and Innovation Funding Authority | Ref. 37066Xunta de Galicia | Ref. GPC-ED431B 2020/03Xunta de Galicia | Ref. ED431F 2020/07Agencia Estatal de Investigación | Ref. RYC2018-024407-IAgencia Estatal de Investigación | Ref. PID2019-105612RB-I0

A carrier-based overmodulation strategy with minimum voltage distortion for symmetrical n-phase induction motor drives

Multiphase machines have significant benefits over their three-phase counterparts. One of them is the possibility of further exploiting the dc-link voltage by injecting non-torquerelated voltage harmonics in the xy planes. This allows reaching higher modulation indices in the overmodulation (OVM) region of pulsewidth modulation (PWM) while avoiding torque ripple in motors with negligible space harmonic effects. The most prominent OVM techniques are those based on the instantaneous minimization of the xy voltage. These methods were devised just for five-phase drives and have the drawback that they lack generality. This article overcomes this limitation by proposing an OVM strategy for symmetrical induction motor drives with any odd phase number n. The proposal achieves the minimum voltage distortion, and consequently, it greatly mitigates the xy current distortion. Indeed, it attains the lowest current distortion compared with the existing techniques. It relies on carrier-based PWM in its design and implementation stages, and accordingly, it is much simpler than previous methods. Furthermore, the inverter switching frequency (loss) is substantially reduced. Theoretical assessments are given for drives with odd n between 5 and 11. The experiments are performed with a symmetrical nine-phase induction motor with a single neutral pointAgencia Estatal de Investigación | Ref. RYC2018-024407-IAgencia Estatal de Investigación | Ref. PID2022-136908OB-I00Xunta de Galicia | Ref. ED431F 2020/07Xunta de Galicia | Ref. GPC-ED431B 2020/0

Assessment of predictive current control of six-phase induction motor with different winding configurations

Asymmetrical six-phase (A6P) induction motor-based drives can be considered as a well-established employed technology in high-power safety-critical industry sectors. Of the different control techniques proposed for multiphase machines, model predictive control (MPC) has recently been favored thanks to its simplicity, rapid dynamic response, and flexibility to define new control objectives. One of the main operating challenges when employing MPC to A6P induction machine is the poor quality of the phase current waveform due to the relatively low impedance of the secondary xy subspace. Although different controller structures have been introduced in the available literature to mitigate this problem, most of the available proposals, if not affecting the dc-link voltage utilization, will likely add to the control complexity. From the stator winding layout perspective, this paper attempts to investigate the effect of different winding configurations of six-phase stators with isolated neutral arrangements on the performance of predictive current control (PCC). This study shows that the winding configuration affect the mapping of the 64 available voltage vectors to the αβ and xy subspaces, the induced current ripples, and the required weighting factor employed in PCC. The theoretical findings have experimentally been validated using a 1kW twelve-phase machine that can externally be reconnected to form any of the three available six-phase winding configurations

Open-phase-tolerant online current references for maximum torque range and minimum loss with current and torque-ripple limits for n-phase nonsaliente PMSMs nonsinusoidal back EMF

Multiphase permanent-magnet synchronous machi- nes (PMSMs) with nonsinusoidal back-electromotive force (back EMF) offer high fault tolerance and torque density for electric vehicles. Most current-reference generation methods either minimize stator copper loss (SCL) or maximize achievable torque. Optimization of both goals is accomplished by full-torque-range minimum-loss (FRML) strategies, but so far just for sinusoidal back EMF. Thus, FRML for nonsinusoidal back EMF should be sought. Moreover, many methods are only suitable for healthy conditions or specific machines, harmonics, or open-phase-fault (OPF) scenarios. In addition, the torque range may be extended by permitting torque ripple or (transiently) greater rms current, but this approach is not general nor FRML yet. This article proposes online FRML current-reference generation for multiphase PMSMs with nonsinusoidal back EMF: nonsinusoidal-back-EMF FRML (NSBE-FRML). When the torque reference is feasible, minimum SCL is attained while maximizing the achievable torque (i.e., FRML). For higher torque references, the instantaneous torque deviation is minimized, and the torque reference is saturated in consecutive samples limiting the torque ripple to a prespecified threshold. Furthermore, the rms current is limited after transient overload by automatically decreasing the torque reference. The NSBE-FRML is suitable for any harmonics, healthy/OPF conditions, and multiphase PMSMs with negligible saliency ratio. Experiments are performed with a six-phase PMSM.Information Technology Industry Development Agency | Ref. ARP2020.R29.7Xunta de Galicia | Ref. ED431F2020/07Xunta de Galicia | Ref. ED431B2020/03Agencia Estatal de Investigación | Ref. RYC2018-024407-IAgencia Estatal de Investigación | Ref. PID2019-105612RB-I0

Hysteresis current control for six-phase induction motor drives with reduced ripple and improved tracking based on subspace decomposition and restrained voltage vectors

Six-phase induction motor (6PIM) drives offer enhanced fault tolerance and reduced per-phase ratings. Hysteresis current control (HCC) is attractive for 6PIMs because it is simple, robust and fast. HCC is conventionally implemented so that each leg voltage is directly set based on the respective phase-current error. However, this approach does not consider that, in multiphase drives, phase voltages and currents are related through a combination of equivalent impedances corresponding to various subspaces. In general, there is a notable dissimilarity between these impedances, being typically small for secondary ( xyxy ) subspaces. This can cause large current distortion and poor reference tracking. This article proposes an improved HCC for 6PIM drives. Instead of directly inputting the per-phase current error to the hysteresis comparator and directly applying the switching states chosen by it, the input and output components associated with different subspaces are segregated. The input and output xyxy components are nullified in open loop so that the xyxy impedance no longer affects the HCC behavior, even if low. This prevents the disrupting xyxy currents, ensures effective tracking of the torque/flux-producing αβαβ reference current, and enables reconfiguration-less fault tolerance. Experiments using 6PIMs with different winding configurations corroborate the significant advantages of the proposal.Xunta de Galicia | Ref. ED431F2020/07Xunta de Galicia | Ref. ED431B2020/03Agencia Estatal de Investigación | Ref. RYC2018-024407-IAgencia Estatal de Investigación | Ref. PID2022-136908OB-I0

Online control strategy for tolerating resistance asymmetry with minimum copper loss in the full torque range for symmetrical six-phase AC drives

Multiphase drives exhibit remarkable advantages (e.g., fault tolerance) over three-phase ones. Six-phase drives are particularly attractive, given their moderate complexity and suitability for off-the-shelf three-phase converters. Regarding the stator winding arrangement, the symmetrical one offers superior postfault capabilities in most scenarios. On the other hand, resistance asymmetry in the stator phases or connections may arise due to different causes. The conventional full-range minimum-loss strategy (FRMLS) generates stator-current references under open-phase (infinite resistance) faults so that torque ripple is prevented while minimizing the losses for each torque ( d - q current) value and maximizing the torque range; however, this method is unsuitable for unequal resistances of finite value. This article proposes an FRMLS for setting the current references to reach these goals in symmetrical six-phase drives with any resistance asymmetry. The optimum references are found online depending on the resistances, without lookup tables. The phase currents are individually limited by an iterative algorithm, so that minimum stator copper loss (SCL) is achieved over the maximum admissible torque range. In this manner, unlike the conventional FRMLS, minimum SCL and maximum torque range are attained even for finite resistance imbalance. The currents in phases affected by high resistances are suitably reduced. Experimental results are providedXunta de Galicia | Ref. ED431F 2020/07Xunta de Galicia | Ref. GPC-ED431B 2020/03Agencia Estatal de Investigación | Ref. RYC2018-024407-IAgencia Estatal de Investigación | Ref. PID2019-105612RB-I00Ministry of Higher Education, Malaysia | Ref. Scheme FP090-2020Science, Technology and Innovation Funding Authority | Ref. 3706

Symmetrical nine-phase drives with a single neutral-point: common-mode voltage analysis and reduction

Power converters generate switching common mode voltage (CMV) through the pulse width modulation (PWM). Several problems occur in the drive systems due to the generated CMV. These problems can be dangerous to the insulation and bearings of the electric machine windings. In recent years, many modulation methods have been developed to reduce the CMV in multiphase machines. Symmetrical nine-phase machines with single-neutral are considered in this paper. In this case, conventional PWM uses eight active vectors of different magnitudes in combination with two zero states in a switching cycle, and this generates maximum CMV. This paper proposes two PWM schemes to reduce the CMV in such a system. The first scheme is called active zero state (AZS). It replaces the zero vectors with suitable opposite active vectors. The second scheme uses ten large active vectors during switching and is called SVM-10L. Compared with conventional strategies, the AZS reduces the peak CMV by 22.2%, and the SVM-10L reduces the peak CMV by 88.8%. Moreover, this paper presents a carrier-based implementation of the proposed schemes to simplify the implementation. The proposed schemes are assessed using simulations and experimental studies for an induction motor load under different case studies