Five-phase Permanent-magnet Machine Research Articles

Single-Phase Short-Circuit Fault Tolerant Control for Five-Phase Permanent Magnet Machines With Copper Loss Reduction

Single-Phase Short-Circuit Fault Tolerant Control for Five-Phase Permanent Magnet Machines With Copper Loss Reduction

Comparative Investigations of Inverter Short-Circuit Fault and Winding Terminal Short-Circuit Fault in Open-End Winding Five-Phase PM Machine System

The open-end winding multi-phase permanent-magnet (PM) machine driven by full-bridge inverter is a potential choice for safety-critical application fields. Based on the open-end winding multi-phase PM machine system, comparative investigations of the inverter short-circuit (ISC) fault and winding terminal short-circuit (WSC) fault are carried out to indicate the special characteristics of the ISC in aspects of short-circuit (SC) current characteristics and post-fault performances. The analytical prediction of the SC current under ISC fault is investigated based on the results of WSC conditions. The maximum SC current estimation, SC current rms value variation, and torque harmonic component distribution under the two fault conditions are further investigated. The results can be used for ISC fault diagnosis, PM demagnetization evaluation, and post-fault performance prediction of ISC fault. The post-fault hardware reconfiguration method is further provided to alleviate the effect of ISC fault and reduce the difficulty of compensation.

Torque Ripple Reduction Method in a Multiphase PM Machine for No-Fault and Open-Circuit Fault-Tolerant Conditions

This paper presents a method that has been developed to reduce the torque ripples under healthy and open-circuit fault-tolerant (OCFT) conditions for a multiphase permanent magnet (PM) machine. For smooth torque, both the phase current and the back electromotive force (back-EMF) should be purely sinusoidal. To improve the torque in a multiphase machine, higher-order current harmonics are injected, which are related to the harmonics in the back-EMF. For this reason, generally, multiphase machines are designed with higher-order back-EMF harmonics. However, these harmonics produce ripples in the torque. In light of this, a torque ripple cancellation method has been developed that first determines an additional current component from the harmonic content of the back-EMF and then injects these additional components to cancel the torque ripple. It has been found that this new torque ripple cancellation method works for both faultless and faulty conditions in a five-phase PM machine. The method has been validated using Finite Element Analysis, and the results are presented in this paper.

Open-Circuit Fault-Tolerant Control of Multi-Phase PM Machines by Compensating the d-q Axes Currents

This paper presents a novel method to control sinusoidal distributed winding or sinusoidal back electromotive force (back-EMF) multi-phase permanent magnet (PM) machines under open-circuit fault conditions. In this study, five different fault conditions are considered: single-phase, adjacent double-phase, non-adjacent double-phase, adjacent three-phase, and non-adjacent three-phase open circuit conditions. New current sets for the remaining healthy phase under open-circuit fault conditions are obtained by compensating the direct-quadrature (d-q) axes currents. For this purpose, an iterative method has been used to get the new set of currents. D-q axes currents, due to faulty phase/phases, are shared to the healthy phases to obtain the same d-q axes currents as in the healthy condition. Therefore, the same torque is produced as in the healthy condition. The developed method is simulated in MATLAB/Simulink by using a d-q modelled sinusoidal back-EMF five-phase machine. A vector control block diagram has been designed to run the machine under healthy and faulty conditions. The machine model has been run successfully under fault tolerant conditions. Additionally, a finite element analysis (FEA) has been undertaken to simulate the five-phase PM model machine by using MagNet software. Open-circuit fault-tolerant control currents are fed into the coils of the machine model. Satisfactory torque results have been obtained. Because the model five-phase PM machine includes higher order back-EMF harmonics, especially the third harmonic, torque has ripple due to interaction between the fault-tolerant control currents and the higher order back-EMF harmonics.

Research on torque ripple under healthy and open-circuit fault-tolerant conditions in a PM multiphase machine

In this paper, research into torque ripple production has been undertaken for both the healthy and open-circuit fault- tolerant conditions of a five-phase permanent magnet (PM) machine by using the instantaneous power (I-Power) approach. When only the fundamental component of the phase currents is applied to the phase windings, it has been shown that the 9th and 11th harmonics of the back-electromotive force (back-EMF) causes torque ripples in a five-phase PM machine and its frequency is ten times the frequency of the fundamental phase currents. When the combined fundamental and third harmonic components of the phase currents are applied to the phase windings, it has been shown that the 7th and 13th harmonic of the back-electromotive force (back-EMF) causes additional torque ripples in a five-phase PM machine .These torque ripples under fault-tolerant conditions have been analyzed analytically, as well. It has been proven that there are interactions between the fundamental component of current and the third harmonic component of the back-EMF and vice versa. These interactions cause torque ripples. A finite element analysis (FEA) model of the five-phase PM machine has been done to validate the analytical results.

Multiple Open-Switch Fault Diagnosis for Five-Phase Permanent Magnet Machine Utilizing Currents in Stationary Reference Frame

Apart from the passive fault-tolerant control strategies, any active fault-tolerant strategy requires a timely fault diagnosis signal to trigger the control reconfiguration under asymmetric condition. This article proposes a fault diagnosis method where the inherent relationship between fundamental and third harmonic stationary reference currents is established by declaring two new groups of current-based variables. Then two distinct fault characteristics of current performance in the transient conversion from healthy to faulty operation are derived. Based on the fault characteristics, a fast, robust and easy-implemented diagnosis algorithm is proposed based on currents in stationary reference frame to address the multiple open-switch fault diagnosis in five-phase permanent magnet machines. In order to verify the effectiveness of the proposed algorithm, a series of experiments have been conducted and the results indicate the strong robustness even under load variation and the valid diagnosis to each type of faults are performed. Moreover, the proposed method possesses favorable generality and can be extended to other multiphase machines.

Multi-Harmonic Currents Control Strategy for Five-Phase Permanent Magnet Machine with non-sinusoidal back-EMF

This paper describes an optimal torque per peak current control method for a five-phase permanent magnet (PM) machine considering both 3rd and 5th harmonic currents. These optimal ratios to the fundamental component are analytically derived to maximize the output torque. It is found that except for the 3rd harmonic current contributing to the output torque, the 5th harmonic current can also produce the additional positive torque. However, the 5th harmonic is zero sequence component for the five-phase machines, which does not exist in the phase windings. Hence, the neutral point is required to connect the middle point of the DC link capacitors for constructing flowing path. The conventional vector space decomposition (VSD) control is extended to zero sequence sub-plane, which can quantitatively control 5th harmonic current. For a prototype five-phase PM machine, the average torque can be increased by 21.4% with 3rd harmonic current injection. Meanwhile, 10.7% additional positive torque is achieved together with 3rd and 5th harmonic injection. The torque ripple remains similar to that without harmonics injection. Finally, the experiments are given to demonstrate the theoretical analysis.

Multiphase PM Machines With Halbach Array Considering Third Harmonic Flux Density

For the five-phase permanent magnet (PM) machines with conventional Halbach array rotor, although the third harmonic current is injected into the phase current, the torque density hardly increases due to the absence of the third harmonic air gap flux density. Hence, a Halbach array considering third harmonic air gap flux density is proposed in this paper, and the topology is described and compared with the conventional Halbach array. The magnetizing angle of the proposed Halbach array and the stator teeth width are optimized based on the nonlinear programing method. The electromagnetic performances of the five-phase PM machines with the proposed Halbach array, including open-circuit flux density, back electromotive force, flux density under the rated load, cogging torque, average torque, torque ripple, loss, and efficiency, are compared with the PM machine with conventional Halbach array by finite-element method. It is demonstrated that the torque density and efficiency of the PM machine with the proposed Halbach array are higher than that of the PM machine with conventional Halbach array. Finally, the five-phase 10-slot/8-pole PM machines with proposed Halbach array is manufactured and tested to verify the analysis.

Open-Circuit Fault-Tolerant Control of Five-Phase PM Machine Based on Reconfiguring Maximum Round Magnetomotive Force

This paper investigates the open-circuit fault-tolerant current control for a five-phase fault-tolerant permanent-magnet (PM) machine used for electric vehicles. By relieving the common constraint of zero neutral point current in the existing fault-tolerant control strategies and reconfiguring the maximum round rotating magnetomotive force under different fault conditions, the new current sets, which enable the five-phase PM machine to output the maximum smooth torque, are obtained by the analytical method. Compared with the existing fault-tolerant control strategy, larger torque and lower torque ripple can be obtained with the proposed fault-tolerant control strategy, and the five-phase PM machine can safely operate with the loss of up to three phase windings. The proposed fault-tolerant control strategy is verified by both finite-element analysis and experimental results. The developed fault-tolerant current control strategy can be generalized into any multiphase PM machines.

PWM Ripple Currents Based Turn Fault Detection for Multiphase Permanent Magnet Machines

Most permanent magnet (PM) machines are driven by inverters with pulse-width modulation (PWM) voltages. The currents contain high-frequency (HF) components which are inversely proportional to machine inductance. The HF PWM ripple currents can be used to detect a turn fault that gives rise to changes in inductance. The features of these HF components under turn fault conditions are analyzed. A bandpass filter is designed to extract the selected sideband components, and their root-mean-square (rms) values are measured. The rms values in all phases are compared. It is shown that the rms ripple current ratios between two adjacent phases provide a very good means of detecting turn fault with high signal-to-noise ratio. The detection method can identify the faulted phase and tolerate inherent imbalance of the machine, and is hardly affected by transient states. The method is assessed by simulations and experiments on a five-phase PM machine.

Evaluation of Combined Reference Frame Transformation for Interturn Fault Detection in Permanent-Magnet Multiphase Machines

This paper focuses on modeling and experimental validation of a diagnostic fault classification procedure for interturn fault detection in permanent-magnet (PM) multiphase machines designed for fault-tolerant electric drives. The diagnostic procedure is based on the symmetrical component theory and relies upon the combined space vector vector D that gathers information from the two original space vectors obtained with different reference frames. The diagnostic index effectiveness and robustness were also investigated against other fault types such as rotor eccentricities and magnet damage to assess its discrimination capability. The proposed procedure was experimentally evaluated for the interturn fault case on a five-phase PM machine. Experiments were carried out at different speed and load levels, with increasing numbers of short-circuited turns. Both simulation and experimental results demonstrated the feasibility of the proposed diagnostic method.

Global Fault-Tolerant Control Technique for Multiphase Permanent-Magnet Machines

In this paper, a global fault-tolerant control (FTC) technique is proposed for multiphase permanent-magnet (PM) machine drives. The goal of the proposed FTC is to find a general closed-form solution for healthy phase currents under steady-state post fault conditions. Healthy phase currents are found through an optimization problem to produce ripple-free output torque with minimum ohmic losses. A comprehensive FTC approach should be able to provide fault-tolerant currents for multiphase machines with any number of phases. In addition, it needs to find currents based on fault type (open-circuit/short-circuit), fault locations [phase(s) and/or line(s)], connection of stator windings, and even different control objectives. An important feature of the proposed method is its flexibility and simplicity in dealing with all possible fault conditions. The proposed method is a great tool to evaluate fault-tolerant capability of different drive systems in terms of maximum available ripple-free torque and copper losses. Due to its simplicity and flexibility, it is also well-suited for real-time implementation. A five-phase PM machine is used as an example to investigate the validity of the proposed solutions through finite-element analysis and experimental tests.

Torque Improvement of Five-Phase Surface-Mounted Permanent Magnet Machine Using Third-Order Harmonic

This paper presents optimal third harmonics in both permanent magnet (PM) shaping and current waveform to improve the output torque of five-phase surface-mounted PM (SPM) machines. The optimal value of third harmonic injected into the sinusoidal PM shape and current waveform for maximum torque improvement is analytically derived and validated by both finite element (FE) analyses and experiments. It is found that the optimal third harmonic is 1/6 of the fundamental one for both PM shape and current waveform. For the five-phase SPM machines having rotors without shaping, Sine shaping, or Sine shaping with third harmonic injected, the electromagnetic performance including the back EMF waveform, cogging torque, average torque, torque ripple, copper loss, iron loss, impact on power inverter, and demagnetization withstand capability are compared. It is demonstrated that, although the copper loss and iron loss increase due to additional third harmonic in the winding current and magnet shape, the average torque with optimal third harmonics injected in PM shaping and current waveform can be improved by >30% while the torque ripple and remains similar to that of the one with Sine shaping. In addition, this will reduce the dc bus voltage while maintaining the machine torque density. Furthermore, the machine with Sine and Sine+3rd rotors presents much better demagnetization withstand capability performance than conventional rotor.

Fault-Tolerant Operation of Multiphase Permanent-Magnet Machines Using Iterative Learning Control

Fault-tolerant control (FTC) techniques for multiphase permanent magnet (PM) motors are usually designed to achieve maximum ripple-free torque under fault conditions with minimum ohmic losses. A widely accepted approach is based on flux distribution or back EMF (BEM) model of the machine to calculate healthy phase currents. This is essentially an open-loop technique where currents are determined (based on motor fault models) for each fault scenario. Therefore, it is highly model dependent. Since torque pulsation due to open-circuit faults and short-circuit faults are periodic, learning and repetitive control algorithms are excellent choices to minimize torque ripple. In this paper, iterative learning control (ILC) is applied as a current control technique for recovering performance in multiphase PM motor drives under fault conditions. The ILC-based FTC needs torque measurement or estimation, but avoids the need for complicated fault detection and fault diagnosis algorithms. Furthermore, BEM-based FTC and ILC-based FTC are proposed that initiates the learning from a model-based approximate guess (from the BEM method). Therefore, this method combines the advantages of both model information as well as robustness to model uncertainty through learning. Hence, the proposed method is well suited for high-performance safety critical applications. Finite element analysis and experimental results on a five-phase PM machine are presented for verification of the proposed control schemes.

High-speed functionality optimization of five-phase PM machine using third harmonic current

Purpose– The purpose of this paper is to apply some surrogate-assisted optimization techniques in order to improve the performances of a five-phase permanent magnet machine in the context of a complex model requiring computation time.Design/methodology/approach– An optimal control of four independent currents is proposed in order to minimize the total losses with the respect of functioning constraints. Moreover, some geometrical parameters are added to the optimization process allowing a co-design between control and dimensioning.Findings– The optimization results prove the remarkable effect of using the freedom degree offered by a five-phase structure on iron and magnets losses. The performances of the five-phase machine with concentrated windings are notably improved at high speed (16,000 rpm).Originality/value– The effectiveness of the method allows solving the challenge which consists in taking into account inside the control strategy the eddy-current losses in magnets and iron. In fact, magnet losses are a critical point to protect the machine from demagnetization in flux-weakening region.

A Generalized Fault-Tolerant Control Strategy for Five-Phase PM Motor Drives Considering Star, Pentagon, and Pentacle Connections of Stator Windings

In this paper, a generalized optimal fault-tolerant control (FTC) technique is proposed for open-circuit faults in five-phase permanent-magnet (PM) machine drives. The proposed FTC technique considers pentagon and pentacle connections of stator windings in addition to conventional star connection. Open-circuit faults in stator windings of the machine as well as switches of the inverter are analyzed. The optimization objective is to produce ripple-free electromagnetic torque with minimum ohmic losses under open-circuit fault conditions. This is achieved by a simple closed-form equation to calculate reference optimal currents in all cases where a lookup table is utilized to represent the effect of fault type. The proposed FTC technique is easily extended to PM machines with higher number of phases. Experimental results for a five-phase PM machine drive are provided to demonstrate the proposed FTC strategy. In addition to single and double faults, which are analyzed for five-phase machine with star connection, triple-phase faults are also considered in pentagon and pentacle connections.

Impact of Neutral Point Current Control on Copper Loss Distribution of Five Phase PM Generators Used in Wind Power Plants

Efficiency improvement under faulty conditions is one of the main objectives of fault tolerant PM drives. This goal can be achieved by increasing the output power while reducing the losses. Stator copper loss not only directly affects the total efficiency, but also plays an important role in thermal stress generations of iron core. In this paper, the effect of having control on neutral point current is studied on the efficiency of five-phase permanent magnet machines. Open circuit fault is considered for both one and two phases, and the distribution of copper loss along the windings are evaluated in each case. It is shown that only by having access to neutral point, it is possible to generate less stator thermal stress and more mechanical power in five-phase permanent magnet generators. Wind power generation and their applications are kept in mind, and the results are verified via simulations and experimental tests on an outer-rotor type of five-phase PM machine.

An Improved Performance Direct-Drive Permanent Magnet Wind Generator Using a Novel Single-Layer Winding Layout

Direct-drive permanent magnet (PM) generators have become a strong contender in medium and large rating wind energy conversion systems as they not only provide higher efficiency and annual energy production, but also reduce the operational and maintenance cost. PM generators with nonoverlap single-layer windings provide a cost-effective design variation that eases manufacturing, reduces torque ripples, enhances voltage quality, and provides fault tolerant capability. The performance of such machines depends mainly on the proper selection of the pole and slot numbers, which results in negligible coupling between phases. The preferred slots per phase per pole (SPP) ratios eliminate the effect of low order harmonics in the stator magnetomotive force (MMF), and thereby the vibration and stray loss are reduced. This paper proposes a new three-phase winding configuration based on the 20 slots/18 poles five-phase PM machine. The proposed design is compared with the well-known 24 slots/20 poles three-phase PM machine. The comparison shows that the proposed generator offers reduced torque ripples, improved output voltage quality, and less core loss for the same machine volume.

Performance evaluation of a five-phase modular external rotor PM machine with different rotor poles

The performance of fault-tolerant modular permanent magnet (PM) machines depends on the proper selection of the pole and slot numbers which result in negligible coupling between phases. The preferred slot and pole number combinations eliminate the effect of low order harmonics in the stator magneto motive force and thereby the vibration and stray loss are reduced. In this paper, three external rotor machines with identical machine dimensions are designed with different slots per phase per pole (SPP) ratios. A simulation study is carried out using finite element analysis to compare the performance of the three machines in terms of machine torque density, ripple torque, core loss, and machine efficiency. A mathematical model based on the conventional phase model approach is also used for the comparative study. The simulation study is extended to depict machine performance under fault conditions.

Wide Operational Speed Range of Five-Phase Permanent Magnet Machines by Using Different Stator Winding Configurations

The winding changeover technique is an effective method for extending the operational speed range of permanent magnet synchronous machines (PMSMs). Unlike three-phase machines which could be wound in wye or delta connections, there are three possible connections, i.e., star, pentagon, and pentacle for five-phase electric machines. In this work, the effects of these three winding connections on a five-phase PMSM performance are studied. First, based on the mathematical model and finite-element analysis, the effects of different stator winding connections on torque-speed and efficiency characteristics are investigated. Then, the complete electric machine drive system including the winding changeover mechanism is simulated to present its dynamic performance. Finally, experimental results are provided to show the transient and steady-state operation of a five-phase PMSM with star, pentagon, and pentacle winding connections.