Flux-switching Machine Research Articles

Investigation of design methodology for non‐rare‐earth variable‐flux switched‐flux memory machines

This study presents a design methodology for a newly emerged switched-flux memory machine with low-cost non-rare-earth. The adoption of aluminium–nickel–cobalt magnet with low-coercive force enables the flexible air-gap flux control without unnecessary excitation loss. Hence, the excellent flux variability and high-efficiency sustainability within a wide-speed range can be achieved. Since geometric constraints exist among major parameters, a design trade-off should be established to facilitate the feasible design. Therefore, a simplified magnetic circuit is modelled to analytically determine the relationships among various parameters, and the generic design considerations including magnet grade selection and stator/rotor-pole combination are addressed, respectively. The influences of design parameters on back-electromotive force and torque capability are evaluated to aid the performance optimisation. A machine prototype is then fabricated to experimentally validate the theoretical analyses.

Novel Hybrid-Excited Switched-Flux Machine Having Separate Field Winding Stator

A dual-stator hybrid-excited switched-flux (DS-HESF) machine, which is based on the conventional SF permanent magnet machine but employs a secondary stator to accommodate the field windings, is proposed in this paper. The DS-HESF machine inherits the robust rotor and benefits from a better space utilization. The topology and operating principle of the proposed machine are introduced in detail. Based on the 2-D finite-element analysis, the comparison between the DS-HESF machines having different rotor pole numbers is carried out, accompanied with a conventional HESF machine as a baseline. The results reveal that the DS-HESF machines exhibit not only higher torque density but also a better flux regulation capability. The prototype is manufactured and tested to validate the predictions.

Modeling Torque Characteristics and Maximum Torque Control of a Three-Phase, DC-Excited Flux-Switching Machine

Three-phase, dc-excited flux-switching machines have been widely researched in recent years. They use dc field windings to produce a field that interacts with the flux produced by the armature windings to generate torque. Numerous studies have discussed designing this type of machine; however, only a few studies have investigated controlling their maximum torque. This paper examines the torque characteristics of a three-phase, 24-slot, 14-pole, outer rotor, dc-excited flux-switching machine, and presents a control strategy for achieving maximum torque output. This machine is designed for traction applications. The mathematical model was derived from the finite-element analysis results. In addition to the theoretical analysis, the proposed control strategy was also verified experimentally.

Hybrid-Excited Switched-Flux Hybrid Magnet Memory Machines

The memory machine concept is recently extended to switched-flux (SF) machines, forming a series of SF hybrid magnet memory machines (SF-HMMMs). They exhibit the merits of good demagnetization withstand capability and effective flux adjustability. Nevertheless, the torque density is inevitably compromised due to the geometric conflict within the stationary part. Meanwhile, the inactivated dc coil, excluding online magnetization transients, results in system redundancy. Hence, in this paper, a new hybrid-excited (HE) concept is developed and implemented in a partitioned-stator SF-HPMM (PS-SF-HMMM). Thereby, the distinct synergies of a dual-magnet memory machine and an HE machine are achieved. With slight compensated field excitation, the torque can be improved at low-speed operation. Meanwhile, the high-speed constant-power region can be further extended without sacrificing high efficiency. The stator/rotor pole numbers are optimized first. The operating mechanism and optimal stepwise HE implementation over a whole operating envelop are then addressed. In addition, a comparison between PS-SF-HMMM with hybrid-excitation and its pure HE counterpart is established. Finally, both the finite-element simulation and experiments are carried out to verify the utility of the proposed HE concept.

Flux switching alternators for small wind generation

This paper considers the application of flux switching alternators for small wind generation. First, after a brief presentation of the small wind energy systems, a description and the fundamentals of flux switching machines is given, then the state of art of flux switching generators is presented. Finally a critical assessment is shown, considering the main advantages and drawbacks of this type of machines as alternator for use in small wind generation systems.

Novel switched flux machine with radial and circumferential permanent magnets

Purpose– The paper purposes a novel SFPM machine topology with radial and circumferential permanent magnets (PMs). The paper aims to discuss this issue.Design/methodology/approach– In order to reduce the flux leakage in the stator-outer region and consequently achieve higher magnetic material utilization in switched flux permanent magnet (SFPM) machine, a novel topology with radial and circumferential PMs is proposed. This topology (SFRCPM) has the same structure as conventional SFPM (CSFPM) machine except of the additional set of radially magnetized PMs located around the back iron and surrounded by a laminated ring frame. Using finite element analysis (FEA) the influence of the design parameters on the performance is investigated in order to obtain an effective optimization procedure. Internal and external rotor SFRCPM machines with either NdFeB or ferrite magnets are investigated, optimized and compared with the CSFPM machine having the same size, copper loss and stator/rotor pole combination.Findings– It is concluded that comparing SFRCPM with its CSFPM machine counterpart, internal rotor SFRCPM machine can achieve high PM flux-linkage per magnet volume, however reduced slot area leads to low output torque, whereas external rotor SFRCPM machine can produce higher torque and torque per magnet volume.Originality/value– This paper proposes a novel SFPM machine topology.

Switched-flux machines with hybrid NdFeB and ferrite magnets

Purpose – The purpose of this paper is to comparatively study the conventional, i.e. single magnet, and novel hybrid-magnet switched-flux permanent-magnet (HMSFPM) machines. Design/methodology/approach – The HMSFPM machines utilize two magnet types, i.e. low-cost ferrites and NdFeB. Thus, a set of magnet ratios (?), defined as the quotient of the NdFeB volume to the total PM volume, is introduced. This allows any desired performance and cost trade-off to be designed. Series- and parallel-excited magnet configurations are investigated using 2-dimensional finite element analysis. Findings – The torque of the HMSFPM machines is lower than the NdFeB SFPM machine but the flux-weakening performance is improved for similar machine efficiency. If the machine dimensions are unconstrained, the HMSFPM machines can have the same torque for reduced material costs and a moderate increase in machine dimensions. Ferrite SFPM machines have the lowest cost for the same torque but a significant increase in machine dimensions is required. Finally, the series-excited HMSFPM machine is the preferred over the parallel-excited HMSFPM machine because it has superior demagnetization withstand capability. Research limitations/implications – Mechanical and winding eddy current losses are not considered in the efficiency map calculations. Originality/value – The NdFeB SFPM, ferrite SFPM, series-excited HMSFPM, and the parallel-excited HMSFPM machines are compared for their electromagnetic performance, flux-weakening, PM demagnetization, efficiency, and material costs.

Influence of design parameters on inductances of e-core axial field flux-switching permanent magnet machines

Purpose – Axial field flux-switching permanent magnet machine (AFFSPMM) can be applied in the field of electric and hybrid electric vehicles because of short axial size, large torque density, and high-power density. The purpose of this paper is to improve the reliability of AFFSPMM itself, the design parameters have to be considered for attaining high self-inductances and reduced mutual-inductances. Design/methodology/approach – The original parameters of E-core AFFSPMM are designed with reference to a 600 W prototype, on the basis of which the 3-D model of the original AFFSPMM is established, and the inductances are calculated by finite element method. The influence of these parameters on the inductances, including combinations of stator and rotor pole numbers, spilt ratio, stator side tooth width, magnet thickness, and rotor pole width etc., are respectively investigated and analyzed under the constant copper loss. Findings – The relationships of rotor pole numbers and inductances are deduced on the condition of the fixed stator poles. It is found that the rotor pole numbers has significant effects on the ratio of mutual-inductance to self-inductance, and the self-inductance is mainly affected by the rotor pole numbers, the split ratio, the stator tooth width, and the rotor pole width. The asymmetry of back-EMF can be largely reduced by optimizing the rotor tooth width. In this paper, the static characteristics are compared and analyzed for the original and optimal 6/14-pole AFFSPMM. Meanwhile, the open-circuit and short-circuit fault are investigated by transient analysis. The results show that the optimized E-core AFFSPMM has good fault tolerance. Originality/value – The research of inductance characteristics for E-core AFFSPMM is valuable to design the fault-tolerant machine, by which the cost of control and manufacture can be largely saved.

Minimum‐copper‐loss control of hybrid excited axial field flux‐switching machine

Hybrid excited axial field flux-switching machine (HEAFFSM) with permanent magnets (PMs) and excitation windings in its stator is a novel hybrid excited flux-switching PM machine. The HEAFFSM combines the advantages of the hybrid excited synchronous machine and flux-switching PM machine. In this study, the topology and operating principles of the HEAFFSM are analysed, and the electromagnetic characteristics (including the winding inductances, flux, and cogging torque) are obtained by the three-dimensional finite-element method. On the basis of the vector control method, the mathematical model of the HEAFFSM is deduced, and the operating performance of the HEAFFSM across its entire operating range is investigated. A new minimum-copper-loss (MCL) control strategy for the HEAFFSM drive system, which can operate in both flux-enhancing and flux-weakening conditions, is proposed and compared with the traditional control. The experimental results demonstrate that the MCL strategy maximises the range of speed regulation while minimises the copper loss of the HEAFFSM drive system. Moreover, the flux-regulation capability and high power/torque density of the HEAFFSM are validated.

ANALYSIS OF COGGING TORQUE REDUCTION TECHNIQUES IN AXIAL FIELD FLUX-SWITCHING PERMANENT MAGNET MACHINE

An axial-field flux-switching permanent-magnet machine (AFFSPMM) is suitable to be used in wind power generation and electric vehicles. However, the cogging torque (Tcog) is large. To obtain high-performance AFFSPMM, the reduction of Tcog in AFFSPMM is investigated. The theoretical Tcog expression is deduced. Based on a 3-D finite-element method, the influences of the design parameters on the Tcog and the reduction methods of Tcog, such as rotor tooth skewing (RTS), rotor tooth circumferential pairing (RTCP), and rotor tooth notching (RTN), are analyzed. The design parameters, except the axial length of rotor, have influences on Tcog, and optimizing the design parameters can decrease Tcog. Tcog can be greatly reduced by RTS for AFFSPMM with a parallel stator tooth and fan-shape permanent-magnet structure. However, RTS is not effective in reducing Tcog for AFFSPMM with a fan-shape stator tooth and parallel PM (FSST-PPM) structure. Tcog greatly decreases by RTN and RTCP for an FSST-PPM stator structure. Moreover, the reduction effect of Tcog with RTN is better than that with RTCP.

Design and Comparison of Two Six-Phase Hybrid-Excited Flux-Switching Machines for EV/HEV Applications

Multiphase flux-switching permanent-magnet (FSPM) machines inherit the merits of conventional FSPM machines and the common advantages of multiphase machines, e.g., low torque pulsations, a high torque density, and a high fault-tolerant capability. When a hybrid excitation is applied, postfault armature currents can be reduced by applying a positive field current (PFC). Hence, in this paper, based on both 2-D and 3-D finite-element analyses (FEAs), two six-phase hybrid-excited flux-switching (HEFS) machines with E-core and C-core stator laminations are designed, respectively. Iron bridges are adopted in each machine, and the influences on the performance are investigated. In addition, alternative-tooth-wound armature windings are employed in the C-core machine to obtain physical and magnetic isolations between armature phases. A comprehensive comparison of the two designs is carried out. Then, more attention is paid to fault-tolerant operating capabilities when an open-circuit fault occurs to only one phase and two phases of armature windings, respectively. The PFC can be applied in the C-core machine under fault-tolerant operating conditions; thus, the postfault armature current can be reduced while maintaining the torque output. Overall, the torque output can be effectively maintained in each HEFS machine during postfault operations. Finally, the FEA-based studies are validated by experimental measurements on two HEFS machine prototypes.

Improved Sensorless Control of Switched-Flux Permanent-Magnet Synchronous Machines Based on Different Winding Configurations

In switched-flux permanent-magnet machines, winding coil connections can be rearranged into several types of machine winding configurations (MWCs), such as all poles wound, alternate poles wound, and single and dual three-phase windings. This paper investigates the influence of machine saliency on a sensorless control operation based on different MWCs. First, by injecting an HF carrier signal, the machine saliency level can be evaluated experimentally. The influence of saliency on an HF-signal-injection-based sensorless operation for different MWCs can be then investigated irrespective of any information of machine parameters. However, the secondary saliency caused by magnetic saturation or current harmonics may exist in a machine, due to which the sensorless control performance will deteriorate. Hence, an orthogonal signal-based position tracking observer is proposed to improve the accuracy of rotor position estimation rather than utilizing an existing position observer. Furthermore, experimental results confirm that a prototype machine exhibits different saliency characteristics and apparent saliency, in which the HF-injection-based sensorless control technique can be applied. Moreover, better steady and dynamic state performance can be achieved by the improved position estimator compared with using an existing position observer.

Development of a Magnetless Flux Switching Machine for Rooftop Wind Power Generation

This paper proposes and implements a new single-phase magnetless flux switching machine (FSM) for rooftop wind power generation. The key is to integrate an outer-rotor FSM and an inner-rotor FSM with shared stator yoke and stator drum windings. In such a way, this proposed machine can not only realize flux switching function to regulate magnetic flux effectively, but also incorporate the field and armature windings within one stator to reduce the copper losses and improve the overall machine efficiency. Meanwhile, the structure is compact and rotors can be directly driven by blades to capture the wind power. The machine design guidelines from both magnetic and electric aspects are illustrated, the machine characteristics are analyzed by using time-stepping finite-element method, and the system performances are evaluated based on mathematical modeling. Both simulation and prototyping test are conducted to verify the validity of the proposed machine and system.

Comparison of flux-switching machines with and without permanent magnets

Comparison of flux-switching machines with and without permanent magnets

Steady-State Characteristics Analysis of Hybrid-Excited Flux-Switching Machines with Identical Iron Laminations

Since the air-gap field of flux-switching permanent magnet (FSPM) machines is difficult to regulate as it is produced by the stator-magnets alone, a type of hybrid-excited flux-switching (HEFS) machine is obtained by reducing the magnet length of an original FSPM machine and introducing a set of field windings into the saved space. In this paper, the steady-state characteristics, especially for the loaded performances of four prototyped HEFS machines, namely, PM-top, PM-middle-1, PM-middle-2, and PM-bottom, are comprehensively compared and evaluated based on both 2D and 3D finite element analysis. Also, the influences of PM materials including ferrite and NdFeB, respectively, on the characteristics of HEFS machines are covered. Particularly, the impacts of magnet movement in the corresponding slot on flux-regulating performances are studied in depth. The best overall performances employing NdFeB can be obtained when magnets are located near the air-gap. The FEA predictions are validated by experimental measurements on corresponding machine prototypes.

Sub-Domain Model of Eddy Current Loss in Windings for Flux-Switching Permanent Magnet Machine under Open-Circuit Condition

This article shows a two-dimensional analysis model for predicting eddy current loss in a winding flux-switching permanent magnet under an open-circuit. The finite-element method is used for electrical machine design; however, this numerical method is always time consuming. Depending on the geometry, the mesh quality of the finite-element method should be high, and thus the finite-element method is more time consuming for special electrical machines. To solve this problem, a sub-domain model using Fourier analysis is improved for a flux-switching permanent magnet machine. It is obtained by solving Poisson and Laplace equations in polar coordinates for each domain, i.e., the magnet, air gap, rotor and stator slots, as well as external region. The model accounts for the influence of interaction between the stator slots and air gap, as well as rotor slots and air gap. Magnetic field distribution, flux linkage, back-electromotive force, cogging torque, and eddy current loss waveforms are derived from the analytical method and verified by finite-element analysis. To the best knowledge of the authors, such an analysis has not been carried out previously.

Analysis of High Torque and Power Densities Outer-Rotor PMFSM with DC Excitation Coil for In-Wheel Direct Drive

In recent years, flux switching machines (FSMs) have been an attractive research topic owing to their tremendous advantages of robust rotor structure, high torque, and high power capability suitable for intensive applications. However, most of the investigations are focusing on the inner-rotor structure, which is incongruous for direct drive applications. In this study, high torque and power densities of a new 12S-14P outer-rotor permanent magnet (PM) FSM with a DC excitation coil was investigated based on two-dimensional finite element analysis for in-wheel direct drive electric vehicle (EV). Based on some design restrictions and specifications, design refinements were conducted on the original design machine by using the deterministic optimization approach. With only 1.0 kg PM, the final design machine achieved the maximum torque and power densities of 12.4 Nm/kg and 5.93 kW/kg, respectively, slightly better than the inner-rotor HEFSM and interior PM synchronous machine design for EV.

Flux-Regulation Theories and Principles of Hybrid-Excited Flux-Switching Machines

A hybrid-excited flux-switching (HEFS) machine was proposed by reducing the permanent-magnet (PM) length of an original flux-switching PM machine and introducing a set of field windings into the saved space. According to the PM locations, three typical topologies of HEFS machines emerged, namely, PM-Top, PM-Bottom, and PM-Middle, respectively. The preliminary analysis indicates that different configurations of magnets and field windings have significant influences on the electromagnetic performance of HEFS machines. Hence, this paper is focused on the flux-regulation theories and principles of the three topologies of HEFS machines employing two typical magnet materials, namely, ferrite, and NdFeB, respectively. It is found that the field winding in the PM-Top machine exhibits reversal flux-regulation functions to that in the PM-Bottom one. The weakest flux-regulation capability is found in the PM-Middle one, since this can be considered as a combination of PM-Top and PM-Bottom ones. Overall, the highest magnet utilization and a strongest flux-regulation capability are exhibited in the PM-Bottom HEFS machine. The theoretical analysis is validated by both 2-D finite-element analysis-based predictions and experimental measurements on three prototype HEFS machines.

Investigation of an Improved Hybrid-Excitation Flux-Switching Brushless Machine for HEV/EV Applications

In this paper, based on 2-D finite-element analysis (FEA), the hybrid-excitation principle of a hybrid-excitation flux-switching (HEFS) machine is analyzed, and consequently, an improved topology is proposed to enhance the flux-regulation capability. These two machines are termed as the basic HEFS machine and the improved HEFS machine, respectively. In the basic HEFS machine, each permanent magnet (PM) is located at the middle of the corresponding slot and wound by a field winding coil. When field excitation is applied, it is found that the currents in the two field slots adjoining each PM exhibit reversal excitation functions, thus reversal flux-regulation functions. Hence, to improve the flux-regulation performances, an improved HEFS topology is proposed based on the basic one by reversing current direction in one of the field slots. Therefore, the improved topology shares identical structure with the basic one but has different field current directions in some field slots and thus different field winding accommodations. Comparisons of the FEA predictions indicated that both armature no-load flux-regulation capability and armature load torque capability in the improved HEFS machine are much better than those in the basic one. Finally, the FEA results are validated by experimental measurements on two prototyped HEFS machines.

Fault-Tolerant Control Using the GA Optimization Considering the Reluctance Torque of a Five-Phase Flux Switching Machine

This paper deals with the fault tolerance of a five-phase flux switching machine. Short-circuit currents calculation considering inductances variation is developed. Machine behavior (torque quality, copper losses, and homopolar current) under a single short-circuit phase fault, two consecutive and nonconsecutive phases short-circuited, is simulated with a two-dimensional finite elements (2-D FE) model and validated experimentally. Then, a new method is developed to improve its performances in faulty mode, by reconfiguring reference currents. In fact, an accurate torque model is established and then used in a genetic algorithm to optimize reference currents in faulty mode. In this approach of reference currents computation, the used algorithm has multiobjectives and multiconstraints, thereby allowing choosing the appropriate fault-tolerant current solution according to our application. The torque model is considered to be more accurate and closer to the 2-D FE results in both healthy and faulty modes. Then, a comparison of machine performances in healthy, degraded, and reconfigured modes is presented. Experimental results corroborate the analysis.