Open-phase Fault Research Articles

Discrimination Between Eccentricity and Interturn Faults Using Current or Voltage-Reference Signature Analysis in Symmetrical Six-Phase Induction Machines

Two common defects in induction machines (IMs) are eccentricity and interturn faults, which should be diagnosed to prevent performance degradation and further damage. A popular fault-detection approach is the current signature analysis (CSA), because of its simplicity and nonintrusiveness. Under closed-loop control, it is combined with analogous voltage-reference (VR) signature analysis (VRSA). However, by using these methods in three-phase IMs, it is difficult to discriminate between these faults, which cause similar symptoms. Multiphase machines provide remarkable advantages such as inherent tolerance to open-phase faults. Six-phase IMs are particularly attractive since they allow adopting three-phase converters. Among them, those with symmetrical spatial arrangement of the stator phases offer superior fault tolerance. Nonetheless, the distinction between eccentricity and interturn failures in these IMs has not been addressed so far. This article studies the discrimination between eccentricity and interturn faults in symmetrical six-phase (S6) IMs by CSA or VRSA. It is shown that, conversely to three-phase IMs and most other multiphase IMs, in S6 ones, these two types of failures can be easily distinguished: interturn faults considerably alter the currents or VRs in the so-called <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{x}$</tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{y}$</tex-math></inline-formula> plane, whereas eccentricity leads to current/voltage symptoms only in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\alpha} _{\mathbf{1}}$</tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\beta} _{\mathbf{1}}$</tex-math></inline-formula> plane. Experimental results confirm the theory.

Steady-State Vibration Level Measurement of the Five-Phase Induction Machine during Third Harmonic Injection or Open-Phase Faults

Multiphase electric machines are increasingly used in various industries and for electromobility. Complex systems have been developed for the control and powering of multiphase machines, which require verification. The quality of control and the power supply of electric machines is usually evaluated by analyzing various electrical parameters. On the other hand, taking into account the fact that a motor is an electrical-mechanical object, its full diagnostics should also include the analysis of vibration signals to verify the operation of the motor as a mechanical device. In this paper, a sensorless control algorithm was studied and applied to a 5-phase induction motor. Various scenarios were considered; in particular, the operation of the studied motor in the absence of one or two phases and in the case of the introduction of the third harmonic to increase the torque was analyzed. In the scenarios considered, the motor was connected to another machine and operated with no load as well as with a preset load. The results obtained were analyzed in the time and frequency domain and were related to the standards used.

Open-Phase Fault-Tolerant Control Strategy for Dual Three-Phase Permanent Magnet Synchronous Machines Without Controller Reconfiguration and Fault Detection

Due to high power-transforming capability and excellent fault-tolerant performance, dual three-phase permanent magnet synchronous machines (DT-PMSMs) are widely used in high-reliability applications. Concerning on the remedial strategy for DT-PMSMs under open-phase faults, the conventional method is to reconfigure the controller or the current references in order to obtain optimum postfault performances. Two different control schemes should be separately designed for normal and fault conditions, respectively. Meanwhile, related fault detection measures need to be adopted and the inherent fault detection delay will further result in transient performance deterioration. In order to solve these problems, this article proposes a control strategy, which can achieve natural fault-tolerant capability of DT-PMSMs without additional fault detection algorithm. With the proposed control method adopted, reconfiguration of the control structure and modification of the current command are not required. Thus, the control scheme is simplified. The transition from the healthy to the postfault operation can be improved, meanwhile, the steady-state performances in prefault and postfault situations can be guaranteed. The proposed strategy can be generally applied to single open-phase faults and multiple open-phase faults in DT-PMSMs. Experimental validations are presented on a 1 kW DT-PMSM platform to prove the effectiveness of the proposed strategy.

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 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{d}$</tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{q}$</tex-math></inline-formula> 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 provided.

Wide-Speed Range Sensorless Control of Five-Phase PMSM Drive under Healthy and Open Phase Fault Conditions for Aerospace Applications

This paper presents a speed sensorless control of a five-phase PMSM in healthy operation and under the Open-Phase Fault on any phase of the machine. The solution is recommended for mission-critical applications requiring high reliability capacities, such as Aerospace applications. An adapted Active Fault Tolerant Control is proposed with the aim of obtaining electromechanical torque as close as possible to that normally developed by a machine working in healthy condition. In instances of a loss of power to one phase of the machine, a reconfiguration of the control law is performed to ensure the continuity of service and to maintain acceptable control performances without requiring a hardware rearrangement of the power architecture. The motor rotation speed and position, required for the Field Oriented Control (FOC) of the stator currents, are estimated using a Back-Electromotive Forces (Back-EMF) observer based on a mathematical model of the motor and implemented in the stator diphase reference frame. Different electrical models that describe the behavior of the five-phase machine in the normal and degraded operations are given. Experimental results on a 1.25 kW synchronous PM machine are shown to confirm the effectiveness of the motor control.

A Generalized Fault Tolerant Control Based on Back EMF Feedforward Compensation: Derivation and Application on Induction Motors Drives

In this paper, a fault-tolerant three-phase induction drive based on field-oriented control is studied, and an analytical approach is proposed to elucidate the limitations of FOC in flux-torque regulation from the controller perspective. With an open-phase fault, the disturbance terms appear in the controller reference frame and degrade the controller performance when operating in a d-q plane with DC quantities. In addition, the hardware reconfiguration, which is essential to operate faulted three-phase drives, causes substantial change in the way the control parameters vd, vq are reflected onto the machine terminals. An accurate understanding of the feedforward term, by considering the open-phase fault and the hardware modifications, is provided to re-enable the FOC in presence of an open-phase fault. Furthermore, the concept of feedforward term derivation is generically extended to cover multiphase induction drives encountering an open-phase fault whereby no hardware reconfiguration is intended. The proposed method is explained based on a symmetrical six-phase induction and can be extended to drives with a higher number of phases. The effectiveness of the proposed derivation method, which is required to form a feedforward fault-tolerant controller, is verified and compared through the simulation and experiment, ensuring smooth operation in postfault mode.

Analysis and Fault-Tolerant Control for Dual-Three-Phase PMSM Based on Virtual Healthy Model

Dual-three-phase permanent magnet synchronous machines (DTP-PMSMs) are famous for their fault-tolerant capability. However, the complex modeling, high copper loss, and torque ripple under postfault operation limit their further application. In this article, a fault-tolerant control (FTC) strategy is developed for DTP-PMSMs under the open-phase fault (OPF) with straightforward modeling and smooth output torque. The virtual healthy DTP-PMSM model, where the coordinate transformation, the modulation strategy, and the controller structure remain unchanged under OPF, is adopted in the proposed FTC scheme. And the current references are derived in sinusoidal waves with minimum copper loss. The inaccurate transmission of control signals under OPF is also focused on. Comprehensive theoretical analysis shows the relationship between the controller output voltage and the actual stator voltage should be considered in the proposed FTC strategy; otherwise, distortion in torque and current will be introduced. The voltage compensation is utilized to compensate for the voltage difference and ensure the smooth torque output. Besides, a quasi proportional resonance controller is designed to further suppress the residual torque ripple. The proposed strategy will not induce complex implementation and heavy computation burden. The simulation and experimental results prove the analysis and the effectiveness of the proposed strategy.

Current Prediction-Based Fast Diagnosis of Electrical Faults in PMSM Drives

Open-phase fault (OPF) and open-switch fault (OSF) are two common electrical faults in the permanent magnet synchronous machine (PMSM) drives. To locate the faults rapidly and relieve the fault damage, a current prediction-based diagnosis scheme is proposed for OPF and OSF in three-phase PMSM drives. The proposed diagnosis scheme consists of three steps, namely, determining fault occurrence, locating the faulty phase, and identifying the specific fault. The general current deviations between dq-axis predicted currents from the normal model and dq-axis feedback currents are used to determine the fault occurrence. The general current deviations between dq-axis feedback currents and dq-axis predicted currents from different faulty models are used to locate the faulty phase. Then, an intervention-based diagnosis is developed to identify the specific fault. In this way, the specific fault can be quickly located within several switching periods. Finally, the effectiveness of the proposed diagnosis scheme is verified by the experimental results.

Fault Tolerant Maximum Torque Per Ampere (FT-MTPA) Control for Dual Three-Phase Interior PMSMs Under Open-Phase Fault

For the dual three-phase interior permanent magnet synchronous machines (DT-IPMSMs), open-phase fault can cause current unbalance and degrade the drive performance. This article proposes a fault tolerant maximum torque per ampere (FT-MTPA) control for DT-IPMSMs, which can maximize the ratio of the average torque to the stator current and minimize the fault-induced torque ripple. In the proposed approach, optimal FT-MTPA solution is derived and theoretically proven, and current rms is considered as one design constraint to ensure the equivalent loss to the healthy condition. The proposed FT-MTPA control ensures the smooth switching between fault tolerant control and healthy control without inducing noticeable torque ripple. Compared with existing methods, the proposed approach is computation-efficient and fast in achieving the FT-MTPA control, which is critical to practical applications with fast changing loads. Moreover, this article derives the optimal solution from the faulty MTPA model with fault induced terms considered to ensure high torque. Experiments and comparisons with existing methods are conducted to evaluate the proposed approach on a laboratory DT-IPMSM.

Enhanced Natural Fault-Tolerant Model Predictive Current Control in Nine-Phase Motor Drives Under Open-Phase Faults

For the open-phase fault (OPF) tolerant operation of multi-phase motors based on model predictive current control (MPCC), the fault diagnosis and control scheme reconfiguration are difficult to avoid, which leads to the increase of control complexity. A natural fault-tolerant (NFT) MPCC method based on virtual voltage vectors (V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula> ) can simplify this procedure and realize the fault-tolerant operation by using a unified control algorithm during pre- and post-faults. However, there is no effective control scheme to deal with the V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula> shifting after the fault, which will lead to the control errors. To overcome this problem, an enhanced NFT-MPCC strategy is proposed in this paper. Firstly, this paper analyzes the causes of the control error in NFT, and elaborates the influence of V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula> shifting on the current tracking performance. Then, a duty ratio optimization based on an PI regulator is proposed for the first time. After the fault occurs, the PI regulator automatically corrects the duty ratio calculation result to eliminate the control error caused by vector shifting, and the proposed method is versatile to different OPF scenarios. Compared with existing NFT-MPCC technologies, the experimental results show that the proposed method significantly suppresses the torque ripple in post-fault, and provides a satisfactory control performance before and after fault without the need of software reconfiguration.

Electromagnetic Performance Prediction for the Symmetrical Dual Three-phase Surface-mounted PMSM under Open-phase Faults Based on Accurate Subdomain Model

The paper presents an accurate analytical subdomain model for predicting the electromagnetic performance in the symmetrical dual three-phase surface-mounted permanent magnet synchronous machine (PMSM) under open-phase faulty conditions. The model derivations are extended from previous accurate subdomain models accounting for slotting effects. Compared with most conventional subdomain models for traditional three-phase machines with nonoverlapping winding arrangement, the subdomain model proposed in this paper applied for the 24-slot/4-pole dual three-phase machine with symmetrical overlapping winding arrangement. In order to investigate the postfault electromagnetic performance, the reconfigured phase currents and then current density distribution in stator slots under different open-circuit conditions are discussed. According to the developed model and postfault current density distribution, the steady-state electromagnetic performance, such as the electromagnetic torque and unbalanced magnetic force, under open-circuit faulty conditions are obtained. For validation purposes, finite element analysis (FEA) is employed to validate the analytical results. The result indicate that the postfault electromagnet performance can be accurately predicted by the proposed subdomain model, which is in good agreement with FEA results.

Universal Control Scheme of Dual Three-Phase PMSM Drives With Single Open-Phase Fault

The fault-tolerant control method of dual three-phase permanent-magnet synchronous motor (PMSM) drives has gained more and more attention in some applications, such as aircraft system and offshore wind energy system. In this letter, the reason for the torque ripple caused by the standard control method has been investigated in view of the frequency domain after the occurrence of a single open-phase fault. Based on the theoretical analysis, a universal control scheme has been proposed for the dual three-phase PMSM drives with two isolated neutral points, which can be implemented under either healthy condition or fault condition. The key is to modify with a closed-loop controller on harmonic subspace with the notch filter. The proposed method not only realizes the seamless transition between two operation conditions without fault diagnosis but also avoids the reconfiguration of the control structure. Moreover, the system optimization of the minimum copper loss under postfault condition has been integrated into the proposed method. The experimental results are presented to illustrate the effectiveness of the proposed method.

Simulation and experimental study of power flow in a seven‐phase transformer under fault conditions

Fault analysis is necessary to design a protection system and circuit breakers for a power system network. The multiphase system is a new technology so very little work is found for the fault analysis of a multiphase system contrary, unlike a three-phase system. The faults in a seven-phase system may be either symmetrical or non-symmetrical. A non-symmetrical fault requires studying the sequence components of a seven-phase system using Fortescue transformation. In the paper, non-symmetrical fault analysis of the seven-phase system has been analysed. For this, a new definition of sequence components of an unbalanced seven-phase system has been proposed in this paper. Then the proposed sequence components are applied to different faults studies at the input and output sides of a seven-phase transformer. Further, the effect of the construction of a seven-phase transformer, that is, core type and shell type on its output is studied. Moreover, the effect of open-phase input is analysed using simulation and hardware results. From the analysis, it is found that in certain configurations of a seven-phase transformer, the seven-phase output is produced even though open-phase fault is present at the input while in other configurations, seven-phase is not produced in the case of open-phase fault.

A Unified Fault-Tolerant Strategy for Multiphase Machine With Minimum Losses in Full Torque Operation Range Based on Closed-Form Expressions

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.

Open-Phase Fault Modeling for Dual Three-Phase PMSM Using Vector Space Decomposition and Negative Sequence Components

Post-fault modeling for dual three-phase permanent magnet synchronous motors (DTP-PMSMs) under the open-phase fault is presented in this article. By using the positive and negative sequence components for the dual three-phase system, the stator currents under the open-phase fault can be treated as a combination of the positive and negative sequence currents. By artfully constructing the negative sequence components in terms of the different initial angles and amplitudes, the single-phase open-circuit fault and certain dual-phase open-circuit fault can be represented. Based on the conventional and modified vector space decomposition (VSD), only the positive sequence components are projected into the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> subspace, namely, the torque-producing subspace, and the negative sequence components are transformed into the harmonic subspace. With the conventional proportional–integral (PI) current regulators, the dc components of the positive and negative sequence currents can be readily controlled under the open-phase fault. Experiments based on a DTP-PMSM prototype verify the effectiveness of the proposed open-phase fault modeling using VSD and negative sequence components.

Noise and Vibration Prediction of a Six-Phase IPMSM in a Single Open-Phase Failure Under a Negative Sequence Current Compensated Fault Tolerant Control Mode

Due to a multiphase configuration, six-phase interior permanent magnet synchronous machines (IPMSMs) exhibit the inherent reliability and fault-tolerant capabilities. Using a fault-tolerant control (FTC) method, the machine is still able to deliver the desired torque under the open-phase fault with a potential degradation in its efficiency and vibration behavior. The vibroacoustic characteristics of the IPMSM under the faulty operation during the FTC mode have not been studied in the literature. To fill this knowledge gap, the simulation and experiment-based noise and vibration (NV) analysis of a six-phase IPMSM in a single open-phase condition using a negative sequence current-compensated FTC mode is studied in this article. First, the whole procedure of the NV analysis for the IPMSM model is introduced. The calculation of the electromagnetic (EM) force is then studied to explore the main excitation of the vibration in the system. After this, the NV synthesis is carried out and the simulation results of the NV under a negative sequence current-compensated FTC mode are obtained. It is found that the 28th order of the flux density spatial harmonics component influences vibration the most. Finally, the NV predictions in both healthy mode and controlled faulty mode are verified by experiments.

Modeling and Diagnosis of Induction Machines Operating under Open-Phase Fault

Modeling and Diagnosis of Induction Machines Operating under Open-Phase Fault

A Phase-Domain Model of Dual Three-Phase Segmented Powered Linear PMSM for Hardware-Assisted Real-Time Simulation

In this article, a hardware-assisted real-time simulation phase-domain (PD) model of a dual three-phase segmented powered linear permanent magnet synchronous machine (SP-LPMSM) used in electromagnetic drive is established. The model is able to consider the end-effect, saturation effect, coupling effect, and open-phase fault conditions, which are specially occurs in the segmented structure in SP-LPMSM. The unbalanced inductances and saturation effect caused by the segmented structure are investigated and a simplified look-up table for inductances is introduced. Additionally, to accurately evaluate the coupling effect, the permanent magnet (PM) flux linkage is decoupled into the product of unit PM flux linkage and coupling coefficient. In order to avoid the numerical impulse caused by the derivative in the progress of calculating back-EMF, sigmoid function is adopted to express the coupling coefficient. Meanwhile, to make the PD-model compatible with the open-circuit fault conditions, incidence matrices and current constraint matrices are introduced to unify the PD-models with different winding connection types and open circuit faults into one general form. The PD-model realized in field-programmable gate arrays (FPGA) based hardware is validated in comparison with finite-element analysis results.

Cost-Efficient Fault-Tolerant Scheme for Three-Phase Surface-Mounted Permanent Magnet Synchronous Machines Fed by Multifunctional Converter System Under Open-Phase Faults

Considering the innately connected neutral point in a multifunctional converter system (MFCS) fed three-phase surface-mounted permanent magnet synchronous machine (SPMSM) drive, this article develops its fault self-tolerance toward open-phase faults (OPFs) without assisted hardware. First, the operating principle of the MFCS fed SPMSM drive is explained by building a zero-sequence model. Second, to achieve fault-tolerant operations, an unconventional current vector trajectory is proposed, aiming at maintaining the torque and average input power invariable. The shape of the proposed trajectory tends to be like a heart rather than a circle with the rise of motor speed. Moreover, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{d}$</tex-math></inline-formula> -axis and zero-sequence currents vary with the position of rotor. By considering the limits of inverter voltage and current, the postfault output capability of the drive is explicitly analyzed. Third, a new fault-tolerant control strategy is thus proposed, in which the OPF is detected by comparing the deviation between predictive and actual currents. To track the time-varying current references, a differential flatness-based current controller is further developed. Finally, experimental results are given to verify the effectiveness.

Voltage Balancing of the DC-Link Capacitors in Three-Level T-Type Multiphase Inverters

This article illustrates an algorithm to balance the voltages across the dc-link capacitors of a three-level multiphase inverter feeding a star-connected load with an odd number of phases. The proposed strategy keeps the dc-link capacitors balanced in any operating condition and minimizes the low-frequency voltage oscillations, even during open-phase faults and with non-sinusoidal output currents. Finally, the article identifies the operating region where the voltage oscillations of the dc-link capacitors can be canceled. The developed algorithm can maintain constant voltages even if the capacitors are unbalanced, and its performance is compared to that of other carrier-based algorithms. Experimental tests are carried out on a five-phase induction motor in healthy and faulty conditions.