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Even though multiphase machines have inherent fault-tolerant capabilities due to their additional degrees of freedom, fault diagnosis still plays an important role to continuously monitor these machines and, thus, prevent critical failures. High-resistance connection (HRC) is one of the most common electric faults in electrical machines and is caused by damaged power connections. These faults create a current unbalance, increasing both the machine losses and torque pulsations, and can eventually lead to open-phase faults (OPFs). This article proposes an online fault diagnostic method to detect HRCs and OPFs in six-phase permanent magnet machine drives. The proposed method is intended for application in electric drives controlled by a predictive current control strategy and does not require the installation of extra hardware. Two variants of the fault diagnostic algorithm are presented in this work: one based on the analysis of the secondary components of the reference current errors and the other variant based on the prediction current errors. Experimental results demonstrate the effectiveness and sensitivity of the proposed diagnostic method for the detection of these types of fault. [ABSTRACT FROM AUTHOR]

This article presents a current limitation technique for a multiphase bearingless machine featuring a combined winding system. This winding structure allows the machine to produce both suspension force and motoring torque. The main challenges with the combined winding configuration consist of decoupling the suspension force and torque generation and designing a proper current limitation algorithm. The former topic has been already tackled and presented in previous publications, while the latter will be addressed in this article. In particular, the proposed suspension force control technique will allow to prioritize either suspension force or torque generation. In this article, the priority is given to the rotor levitation, hence the suspension force rather than the torque is essential. Finally, simulation results and experiment validation on a $\text{1.5}\,\text{kW}/3000\,\text{r/min}$ prototype machine are provided. [ABSTRACT FROM AUTHOR]

This article presents three-phase and single-phase open-circuit fault-tolerant control strategies of radial suspension force and torque generation for bearingless multisector surface mounted permanent magnet synchronous machines. A combined winding topology is installed in the stator so that each phase contributes to both suspension force and motoring torque production. Indeed, a triple sectored three-phase winding is employed. Each three-phase winding is supplied by a power converter to independently control the suspension force vector and torque over each sector. An analytical formulation of the force and torque generation is described considering both three-phase open-circuit and single-phase open-circuit fault conditions. The analytical models of the machine are presented in a generalized matrix form so that they can be applied to any machine with a multisector winding structure. The analytical models are then used to derive the control algorithms that allow us to control the bearingless machine when a whole three-phase winding or one phase of one or two sectors are open circuited. The theoretical analysis is verified with both finite elements analysis and experiment tests. This system shows a good open-circuit fault-tolerant capability. [ABSTRACT FROM AUTHOR]

Metal industries combine several conditions that match with the advantages of multiphase motor drives, such as high power and high reliability. High power usually means cumbersome multilevel converters, with very expensive high-blocking voltage semiconductors. By adding more phases, it is possible to reduce the motor drive voltage levels and to use two-level converters with more common and less expensive low-blocking voltage power modules. High reliability means lower probability of unexpected production interruption, which is achieved by the fault-tolerant characteristic of the multiphase motor drive. In this article, a wire rod plant is depicted and the replacement of a traditional three-phase motor drive by a nine-phase one is discussed. Experimental results showing the effects of faults in multiphase drive on torque and current waveforms are presented, including their mitigation by the use of a post fault control method. This article also includes reliability calculations for the nine-phase motor drive in order to compare its lifetime with that of the three-phase conventional alternative. [ABSTRACT FROM AUTHOR]

Multiphase drives are convenient for high-power/high-current applications as they allow the reduction of the phase current for given rated power and phase voltage. Due to their redundant structure, the multiphase drives have intrinsic open-phase fault-tolerant operation capability. This situation may happen when one or more power electronic units are turned off after a fault event, and the drive configuration allows phase disconnection. In this case, the healthy machine phases can be overloaded to keep the torque constant and without any pulsations. The goal of the work is the evaluation of the thermal parameters of the stator windings of multiphase machines to be used in the analysis of both short thermal transients and steady-state operation, during normal and open-phase faults. The approach has general validity and can be applied to any ac multiphase machine having a distributed winding configuration. The prototype used for the experimental tests is an asymmetrical 12-phase induction machine, having four 3-phase stator sets with isolated neutral points. The stator windings thermal model is obtained experimentally, by considering the mutual heat exchange phenomena among the windings when one or more winding sets are disconnected. This thermal model allows at evaluating the proper machine overloading for short thermal transients and steady-state operation in case of open-phase faults. [ABSTRACT FROM AUTHOR]

Short-time thermal transient identification method was successfully adopted to evaluate the winding thermal parameters of induction motors for industry applications. In this work, the modeling approach and the identification methodology are extended to the more sophisticated case of multiple three-phase machines. The generalized model takes into consideration the mutual heat exchange between the windings as well as the possible causes of temperature mismatch. A complete procedure to evaluate the parameters of the modified model is provided, supported by experimental validation on a 7.5 kW machine with two three-phase windings in contact at slot level. The method covers any type of multiple three-phase machines, whatever the thermal promiscuity of the winding sets: From deep coupling as the ones presented, to the case where only the end-turns are in contact, to the completely decoupled case. The proposed technique can be useful for the machine design and for real-time temperature monitoring during operation. [ABSTRACT FROM AUTHOR]

Multiphase machines have several benefits compared with three-phase machines. Their inherently higher reliability makes them attractive in several applications. For instance, the nine-phase motor can operate with up to six open phases in its stator but with torque ripple production. However, the modeling and control schemes for a balanced motor cannot be used since now the stator is unbalanced. In order to allow the multiphase machine to work magnetically balanced even in open-phase conditions, an adequate dq current control should be employed. To achieve this control scheme, it is necessary to have a synchronous-frame modeling of a nine-phase induction motor under open-phase faults, which is developed here. Using it, a synchronous-frame control is derived, still using proportional controllers in the dq current loops. This is possible due to the use of an appropriate voltage decoupling considering the fault condition. Hence, the unbalanced multiphase induction motor can be controlled very similarly as a balanced one, additionally resulting in the torque ripple minimization. Experimental results characterize the operation with open phases. Simulation and finite-element results show the success of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Multiphase machines have several advantages when compared to three-phase machines. Reliability is one of the most important of those advantages, since the multiphase machine can operate with one or more phases open-circuited. However, the traditional models and control of a balanced induction machine can no longer be used under such open-phase conditions. This paper addresses important issues in the modeling and control of nine-phase induction machines operating under open-phase faults. The development of the proposed model is presented in detail, including an algorithm to guarantee the proper representation of isolated neutral motors. Simulation results illustrate the postfault behavior of the nine-phase motor due to an open phase, including the elimi-nation of torque ripple by applying the proper control methods. Experimental results from a laboratory test setup illustrating the operation of a nine-phase induction motor under one open-phase fault are presented and used to validate the proposed model. [ABSTRACT FROM AUTHOR]

The next big change in various industry applications is associated with power dense motors, such as permanent magnet synchronous motors (PMSMs). Utilization of multiphase PMSM can offer an additional fault-tolerant capability to their applications. Combining both advantages, this paper proposes a simple yet robust current control strategy for a five-phase PMSM under normal operation and in a case of a loss of one phase. Traditional linear current controllers successfully applied to reference tracking of a healthy motor may become less efficient during postfault operation due to additional unmodeled dynamics. This paper proposes a modified angular transformation to a special rotating frame in which the postfault permanent magnet flux linkage remains unchanged and the motor model remains decoupled. Following this, a nonlinear current control scheme based on sliding mode control is proposed, which successfully treats model inaccuracies and provides good dynamic performance and tracking accuracy. Feasibility of the proposed control strategy is experimentally validated on a laboratory scale PMSM motor with a digital signal processor/field programmable gate array based drive. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic model suitable for accurate cosimulation of fault-tolerant permanent-magnet motor drives featuring independent-phase structure. The model is developed in a circuital form where the usual inductive parameters and back electromotive force coefficient are replaced by current and rotor position dependent functions, so that the exact electromagnetic nature and geometry of the machine are accounted over the large flux-current operating range. The model functions are precomputed by a finite element method analysis of a single phase of the machine, once the magnetic independence among the phases has been verified. Then, the circuital model is solved by a dynamical simulator which implements also the drive system, converter, and control, following on the offline cosimulation approach. The proposed model is validated by experiments carried on a fault-tolerant five-phase permanent-magnet motor-drive for aeronautical application, controlled by the brushless dc technique. The results show that the modeling solution is capable to simulate the motor dynamics with a high degree of accuracy, and can be used for an effective rapid prototyping of fault-tolerant drives. [ABSTRACT FROM AUTHOR]

Two series-connected two-motor drive systems with a single inverter supply have been proposed recently. The first one consists of two five-phase machines supplied from a single five-phase inverter, whereas the second one comprises one symmetrical six-phase machine, a three-phase machine, and a single six-phase inverter. It has been verified experimentally that, by introducing an appropriate phase transposition in the series connection, completely independent dynamic control of the two machines can be realized using principles of vector control. Detailed dynamic models in the stationary common reference frame have also been reported for the two drive systems. These are taken here as the starting point, and equivalent circuit steady-state representation of the series-connected two-motor drive systems is developed. The equivalent circuits can then be used to evaluate steady-state characteristics of the drives in the same manner as it is done for three-phase induction machines, as well as for the inverter design. The dc-link voltage required for operation of the two series-connected machines is investigated next. It is shown that the specific winding connections lead to a dc voltage requirement that is smaller than it would have been had the windings of the two machines been connected directly in series. Experimental results, obtained during steady-state operation of the series-connected five-phase and six-phase two-motor drives, are presented. Developed equivalent circuit representation is finally verified by comparing calculated and experimentally obtained values for various voltages of the two-motor drives. [ABSTRACT FROM AUTHOR]

The paper shows how the techniques of generalized harmonic analysis may be used to simulate the steady-state behavior of a multiphase cage induction motor with any form of open-circuit or short-circuit fault in the stator winding. The analytical model is verified using a four-pole machine with a 48-slot stator. Each coil of the stator winding of this machine is brought out to a patchboard that enables the stator to be configured for single-phase, two-phase, three-phase, four-phase, six-phase, or 12-phase excitation. Experimental results are compared with computer predictions for a six-phase machine with both open-circuit and short-circuit faults. [ABSTRACT FROM AUTHOR]

This paper analyzes a six-phase two-motor drive, consisting of a six-phase voltage-source inverter (VSI), a six-phase induction machine, and a three-phase induction machine. Stator windings of the two machines are connected in series in an appropriate manner. This enables full decoupling of the dynamics of the two machines by means of vector control. Detailed mathematical modeling of the system is performed and the set of d-q equations, describing the two machines connected in series to a six-phase VSI, is developed. The resulting model clearly shows the possibility of independent vector control of the two machines, although a single VSI is used as the supply. An experimental rig is constructed and existence of the decoupled dynamic control of the two machines supplied from a single VSI is fully verified by extensive experimental testing. [ABSTRACT FROM AUTHOR]