Wound-field Flux Switching Machine Research Articles

A Transient-State Lumped Parameter Thermal Model for Brushless Wound Field Switched Flux Machines

In this paper, the transient-state lumped parameter thermal model (LPTM) of the wound field switched flux (WFSF) machine is characterized for the first time. The modelling methodology for the LPTM is given in details, including the process of determining the parameters of the LPTM, based on which two LPTMs are compared with the experiments, i.e. the LPTM-I and LPTM-II without and with consideration of the practical winding position in the stator slot. It is found that the practical positions of the DC and AC coils in the slots is important to improve the accuracy of the LPTM, due to the existence of slot empty space caused by overlapped DC and AC windings. By considering the practical positions of the DC and AC coils in the slots, the maximum value of the absolute error between the LPTM model predicted temperature and the measured values of the AC winding are 3.4°C, 5.23°C, 3.18°C and 5.22°C under DC test, steady state test, cycling test-I and cycling test-II, respectively, whilst they are 3.95°C, 2.00°C, 2.28°C and 4.89°C for the DC winding temperature error. The average value of the absolute error between the LPTM model predicted temperature and the measured values of the AC winding are 0.88°C, 1.25°C, 0.85°C and 1.87°C under DC test, steady state test, cycling test-I and cycling test-II, respectively, whilst they are 1.08°C, 1.47°C, 1.06°C and 2.73°C for the DC winding temperature. The temperature of the WFSF machine during a 9-hour experiment can be predicted in 27.8s by the LPTM, which has a more than 1100 times faster calculation speed.

Analysis and Improvement of DC Winding Current Dynamics in Wound Field Switched Flux Machine

Analysis and Improvement of DC Winding Current Dynamics in Wound Field Switched Flux Machine

A Stable and Computationally Efficient Spatial Harmonic Model for Predicting the DC Winding Induced Voltage in WFSF Machine

In order to predict the DC winding induced voltage under pulse width modulation excitation in the wound field switched flux (WFSF) machine with consideration of the impact of magnetic coupling among <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis and the DC winding, lamination magnetic saturation and current harmonics in both AC and DC windings, the field-circuit coupling method based on the high-fidelity spatial harmonic (SH) model is applied in this paper, including the inductance-based SH model and flux-linkage-based SH model. However, it is found that the inductance-based SH model of the WFSF machine suffers from numerical instability in dynamic operation due to the high condition number of the inductance matrix caused by the magnetic coupling among <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis and the DC winding, resulting in prediction errors of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis flux-linkages, and that of the DC winding. To solve this problem, a flux-linkage-based SH model is proposed for the WFSF machine in this paper, and it shows that the proposed flux-linkage-based SH model is numerically stable during the dynamic operation. In addition, the calculation speed of the flux-linkage-based SH model is 147 times faster than the inductance-based one, due to the exemption of the calculation of the partial derivation equations. Finally, the analyzed 24-stator-slot/10-rotor-pole WFSF machine is built and tested to validate the flux-linkage-based SH model, and the experimental results agree well with those of the SH model.

Harmonics Orders Modelling of DC Winding Induced Voltage Pulsation in Wound Field Switched Flux Machines under PWM Excitation

Harmonics Orders Modelling of DC Winding Induced Voltage Pulsation in Wound Field Switched Flux Machines under PWM Excitation

Influence of PWM Excitation on DC Winding Induced Voltage Pulsation in Wound Field Switched Flux Machines

Influence of PWM Excitation on DC Winding Induced Voltage Pulsation in Wound Field Switched Flux Machines

Optimal Designs of Wound Field Switched Flux Machines with Different DC Windings Configurations

Wound field switched flux (WFSF) machines exhibits characteristics of the simple robust rotor, flexible flux-adjustable capability, and no risk of demagnetization. However, they suffer from a poor torque density compared with permanent magnet machines due to the saturation. Therefore, in this paper, two WFSF machines with single- and double-layer DC windings, respectively, are optimized for the maximum torque. The end-winding (EW) lengths differ in these two machines, which can affect the optimal design. Design parameters including the DC to armature winding copper loss ratio, slot area ratio and split ratio are optimized when two machines have the same copper loss and overall sizes. In addition, the influence of the flux density ratio, total copper loss, air-gap length and aspect ratio on the optimal split ratio is investigated using the finite element method and results are explained through the analytical model accounting for the saturation. It is discovered that the EWs have no effect on the optimal copper loss ratio, which is unity. In terms of the slot area ratio, the machine with single-layer DC windings prefers smaller DC slot areas than armature slot areas. In the WFSF machine with longer EWs, the optimal split ratio becomes smaller. Moreover, compared with other parameters, the flux density ratio can significantly affect the optimal split ratio.

Reduction of On-Load DC Winding-Induced Voltage in Partitioned Stator Wound Field Switched Flux Machines by Dual Three-Phase Armature Winding

Voltage induced in the dc winding by the armature field in wound field switched flux (WFSF) machines will induce voltage perturbation in the voltage supply and cause dc winding current ripple and field excitation fluctuation. This will challenge the dc voltage controller and deteriorate the control performance, especially when the load and/or the rotor speed are high. In this article, the dual three-phase armature winding design with a phase-shift of 30 electrical degrees is employed in the partitioned stator WFSF machines to reduce the <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -load dc winding-induced voltage. Analytical deductions and finite element (FE) results demonstrate that the dominant sixth-order-induced voltage harmonic can be suppressed to zero. Hence, the peak-to-peak induced voltage in the 12/11- and 12/13-pole partitioned stator WFSF machines can be reduced by 83.35% and 82.53%, respectively. Due to the improvement of winding factors, the torque densities are 4.85% and 5.60% higher, respectively. Prototypes are built and tested to verify the analytical and FE results.

Reduction of Open-Circuit DC Winding Induced Voltage and Torque Pulsation in the Wound Field Switched Flux Machine by Stator Axial Pairing of Tooth Tips

Stator axial pairing of tooth tips is proposed in this article to suppress the open-circuit dc winding induced voltage and torque pulsation in the wound field switched flux machine. In the proposed method, the stator is divided into two segments axially and these two segments have different arcs of tooth tip. The finite element analysis results show that the proposed technique can reduce the on-load torque ripple from 17.91% to 6.99%, reduce the peak-to-peak value of cogging torque by 90.66% from 5.14 N·m to 0.48 N·m, and reduce the peak-to-peak value of open-circuit dc winding induced voltage at 1500 r/min by 84.39% from 79.58 to 12.42 V. Although the efficiency of the paired machine is slightly reduced from 84.93% to 84.11%, the average electromagnetic torque does not deteriorate by more than 5%. The paired machine is prototyped and tested, and the experimental results validate the effectiveness of the proposed method.

Optimization and Performance Evaluation of Non-Overlap Wound-Field Converter-Fed and Direct-Grid Wind Generators

Non-overlap winding technology continues to remain relevant in the design of wound-field machines as an alternative for high torque density permanent magnet machines. In this paper, the finite element analyses-optimisation of two variants of the non-overlap wound-field machines viz., wound-field flux switching machine (WF-FSM) and phase shifting wound rotor synchronous machine (WRSM), are compared, first in terms of their performance for large-scale converter-fed wind generator drives, then experimentally as sub-scaled converter-fed versus direct grid-connected wind generators, respectively. This study is unique because there are no prior attempts to design, optimize and analyse on these machines for medium-speed wind power generation, as well as experiment on sub-scale prototypes for direct-grid and converter-fed operation. All investigations are contemplated in the medium-speed wind generator drivetrain which provides a tradeoff for generator efficiency and size. From the global optimisation of both machines at large-scale power levels, the torque per mass of the WF-FSM is found to be 50 % lesser compared to the WRSM. This is due to approximate volume, with closely matched optimal split and aspect ratios. In terms of the sub-scaled experimentation, both generators can easily vary their generated output power to match with varying wind resource, but direct grid-connected WRSM generator yields better efficiency performance compared to the WF-FSM converter-fed operating mode, given that the generator terminal voltage of the former is highly regulated.

Influence of rotor iron bridge position on DC-winding-induced voltage in wound field switched flux machine having partitioned stators

In this study, the influence of the position of the rotor iron bridge on the DC-winding-induced voltage pulsation in a partitioned stator wound field switched flux machine is investigated. Analytical and finite element (FE) analyses show that both the open-circuit and on-load DC-winding-induced voltages can be minimized by positioning the rotor iron bridge adjacent to the inner air gap closer to the DC winding. This is due to a smoother inner air-gap magnetic reluctance while maintaining the average electromagnetic torque at 92.59% of the maximum value. The analyzed machine with the rotor iron bridge adjacent to the inner air gap is prototyped, and the experimental results validate the analytical and FE results.

Enhancement of torque density in wound field switched flux machines with partitioned stators using assisted ferrites

In this paper, ferrites are applied in a partitioned stator wound field switched flux (PS-WFSF) machine to increase the air-gap flux density, and hence, the average electromagnetic torque and overload capability. Introducing short-circuited ferrites in the inner stator in the PS-WFSF machine can increase the open-circuit phase fundamental back-EMF and average electromagnetic torque at a 60 W copper loss by 2.33% and 3.77%, respectively. Moreover, the proposed PS-WFSF machine with ferrites can exhibit a better overload capability than conventional PS-WFSF machines without ferrites, e.g., a 7.36% torque increment can be achieved when the copper loss is 120 W. The torque increment mechanism is analyzed and verified using finite element (FE) analysis. Moreover, the demagnetization of the ferrites in the proposed machine under rated on-load and overload conditions is investigated. Both prototypes of the proposed PS-WFSF machine with ferrites and a conventional one without ferrite are built and tested to validate the analytical and FE analyses.

On the Electromagnetic Performance Prediction of Turbo Synchronous Condensers Based on Wound-Field Flux Switching Machine Design

The revival of the old synchronous condenser (SC) technology is presently occurring. Hence, this study is proposed on the electromagnetic design and evaluation of turbo wound-field flux switching machines (WF-FSMs) for SC operation, for the first time. Unlike classical wound-field rotating machines, WF-FSMs do not need brushless exciters while possessing a robust topology. A design process based on the frozen permeability technique in static 2-D finite element analysis script-based package, which enhances both iterative and analytic adjudications, is proposed to accurately predict the SC operation. To attain the specific design requirements (~300 MVAR, 16 kV), a global-optimized model (GOM) of the SC machine is obtained through a stochastic optimization process involving multiple objectives. Based on the GOM, simple torque ripple minimization techniques are implemented, from which a final benchmark design is selected. The proposed SC analytical design procedure is then tested and the expected V-curves clearly obtained. The whole design approach is further verified by experimenting on a small-scale prototype, which when synchronized to the grid, confirms the novelty and feasibility of the proposed SC design and operation.

Analysis of DC Winding Induced Voltage in Wound-Field Flux-Switching Machine With Air-Gap Field Modulation Principle

In this article, the direct current (dc) winding induced voltage in wound-field flux-switching machines is qualitatively analyzed with air-gap field modulation principle. Operation principle and effective harmonics of dc winding induced voltage are modelled and deduced. It is found that only air-gap field harmonics with specific spatial orders, which numerically equals to odd times of half stator poles, can contribute to dc winding induced voltage. Harmonic order of dc winding induced voltages under open-circuit only, armature reaction only, and on-load condition, are predicted for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> -phase WFFS machines with any feasible stator/rotor pole number combinations. Different armature winding configurations, including concentrated winding and distributed winding, are also analyzed and compared using the winding factor. Finite element analysis is employed to verify the prediction results. Finally, a prototype is built and tested for validation.

A Novel Stator Wound Field Flux Switching Machine With the Combination of Overlapping Armature Winding and Asymmetric Stator Poles

A novel 24/17-pole 3-phase stator wound field flux switching (SWFFS) machine with the combination of overlapping armature winding (OAW) and asymmetric stator poles (ASP) is proposed for higher torque density in this article. First, the air-gap modulation principle of the 24/17-pole OAW-SWFFS machine is analyzed to better understand its torque production mechanism. Second, since the torque capacity will be greatly restricted by the serious magnetic saturation which is introduced by the OAW configuration, two effective measures are proposed from two perspectives in this article to solve this problem. The first measure is to reduce the magnetic saturation directly by optimizing the area ratio of armature slot and excitation slot, and thus, the torque density can be improved effectively. The second measure is to further enhance the open circuit flux linkages, back electromotive force (EMF), and thus, the torque density by adopting the proposed ASP structure. Furthermore, the accurate ASP distances which are corresponding to the largest back EMF can be obtained effectively based on the air-gap field modulation theory. Finally, the proposed design is validated experimentally by a prototype machine. The simulated and experimental results show that about 25.7% higher torque density can be obtained when compared with 24/14-pole SWFFS machine.

Analysis and Reduction of DC Winding Induced Voltage Pulsation in Five-Phase Non-Overlapped Stator Wound Field Flux Switching Machine

DC winding induced voltage pulsation in Wound field flux switching (WFFS) machine is the main cause of the current ripples in DC winding. It may also deteriorates the DC power source and enhances the challenging issues to the control performances of the machine. Hence, reduction of this pulsation is very considerable parameter to analyze in WFFS machine's analysis. In this paper, DC winding induced voltage for five phase non overlapping stator WFFS machine is analyzed and reduced through various techniques. Three techniques i.e. rotor pole arc optimization, chamfering and rotor step skewing are analyzed and optimized to reduce the DC winding induced voltage. For optimization, krigging method based on genetic algorithm (GA) is performed due to compatibility with non-linear data and then results are verified by finite element (FE) analysis. By applying the chamfering and rotor pole arc optimization, 49.58% and 28.45% reduction in the peak to peak value of DC winding induced voltage pulsation is achieved and electromagnetic torque is maintained at 97.82% and 100% respectively while by applying the rotor skewing, 33.29% reduction in DC winding induced voltage pulsation is achieved and the torque is maintained at 98.37% for the five phase 10S-11P WFFS machine.

Cross-Coupling Inductance Parameter Estimation for More Accurate Performance Evaluation of Wound-Field Flux Modulation Machines

In this paper, a proposed numerical frozen permeability modeling procedure is fully derived to accurately estimate the parameters of two variants of wound-field flux modulation machines (WF-FMMs) namely: wound-field flux switching machine (WF-FSM) and DC-excited vernier reluctance machine (DC-VRM). During the procedure, the contribution of saturation to the variation of the machine parameters is investigated. The parameter variation effects on the machine performance is also investigated. It is shown that analysis of WF-FMMs is better understood by using the proposed procedure which computes accurately the machine parameters.

Analysis of Rotor Poles and Armature Winding Configurations Combinations of Wound Field Flux Switching Machines

The general rules of rotor pole and armature winding configurations combinations for 24-slot stator wound field flux switching machines (SWFFSMs) are investigated thoroughly in this article. Meanwhile, based on the virtual coil method, a novel general auxiliary evaluation variable, which is proportional to the fundamental amplitude of open-circuit flux-linkage of armature coil is proposed in this article. Hence, these SWFFSMs with larger open-circuit flux-linkage and back-EMF can be obtained effectively. In addition, the effect of rotor poles on the complementarity of armature windings is analyzed so that more sinusoidal and symmetrical back-EMF can be acquired. Furthermore, for these SWFFSMs with larger open-circuit back-EMF, the torque capability is restricted to a considerable extent due to the deep magnetic saturation caused by overlapping armature winding. Hence, a new method by optimizing the ratio of armature slot area and excitation slot area is proposed in this article to ensure that the higher torque density of selected SWFFSMs can be maintained. To verify the validity of analysis and methodology proposed in this article, seven representative SWFFSMs are chosen for verification at no load by the finite-element analysis (FEA) while a 24s/17p SWFFSM with the highest torque density is chosen for further verification at on-load conditions by the FEA and experiments.

Analysis and Suppression of Induced Voltage Pulsation in DC Winding of Five-Phase Wound-Field Switched Flux Machines

In wound-field (WF) switched flux (SF) (WFSF) machines, the DC winding induced voltage pulsation causes current ripple in the DC winding and challenges the DC power source, and deteriorates the control performance. In this paper, the induced voltage pulsation in DC winding of five-phase WFSF machines is analyzed and its reduction methods are proposed. The cycles per electric period of the open-circuit and armature reaction induced voltage pulsation in DC winding are derived analytically. Modifying the airgap permeance by optimizing the rotor pole arc or chamfering the rotor pole surface, and axial pairing of rotor segments having rotor pole with different arcs are used to suppress the induced voltage pulsation in DC winding, with >90% average torque maintained. Finite element results show that, by optimizing the rotor pole arc, the peak-to-peak value of the induced voltage pulsation in DC winding can be effectively suppressed to 59.59%, 30.67%, 29.99% and 43.35% for the 10-stator-pole five-phase WFSF machines with 8-, 9-, 11- and 12-rotor-pole rotors, respectively. By applying rotor pole surface shaping, the induced voltage pulsation in DC winding peak-to-peak value can be effectively suppressed to 61.76%, 45.47% and 40.21% for the 8-, 9- and 12-rotor-pole machines, respectively, while by applying axial pairing, it can be suppressed to 46.89%, 7.16%, 15.64% and 12.04%, respectively. The 10-stator-pole/12-rotor-pole WFSF machines having the original rotor, optimized rotor, chamfered rotor and axial paired rotor are prototyped and the experiments validate the analytical and finite element results.

Analysis and Reduction of On-Load DC Winding Induced Voltage in Wound Field Switched Flux Machines

DC winding induced voltage pulsation in wound field switched flux (WFSF) machines causes dc winding current ripple and field excitation fluctuation, challenges the dc power source, and deteriorates the control performance. Hence, reducing this pulsation is important in the design of a WFSF machine. In this paper, based on the analytical models, rotor skewing and rotor iron piece pairing are proposed and comparatively investigated by the finite-element (FE) method to reduce the on-load dc winding induced voltage in WFSF machines having partitioned stators and concentrated ac windings. FE results show that peak-to-peak value of the on-load dc winding induced voltage in the analyzed 12/10-pole partitioned stator WFSF (PS-WFSF) machines can be reduced by 78.42% or 77.16% by using rotor skewing or rotor pairing, respectively, while the torque density can be maintained by >90%. As for the 12/11-, 12/13-, and 12/14-pole PS-WFSF machines, by using rotor iron piece inner arc pairing, the on-load dc winding induced voltage can be reduced by 64.11%, 52.12%, and 76.49%, respectively, while the torque density can be maintained by more than 90%. Prototypes are built and tested to verify the analytical and FE results.

Comparative Study of Wound-Field Flux-Switching Machines and Switched Reluctance Machines

In this paper, comparisons between two permanent-magnet-free machines, namely a wound-field flux-switching machine (WFFSM) and a switched reluctance machine (SRM), are carried out. The topologies and operation principles of the WFFSM are illustrated first. Then, the effects of rotor skewing angle on static characteristics, e.g., open-circuit phase flux-linkage and back electromotive force due to field currents, electromagnetic torque, and torque ripple, have been investigated in depth by finite-element analysis (FEA). Consequently, the optimal rotor skewing angle is determined. After that, comparisons at the rated speed 1500 r/min and at the rated RMS current density of 5 A/mm2 among the 6-armature-slots/5-rotor-poles (6/5) straight- and skewed-WFFSMs, and two representative SRMs, namely conventional 12/8 SRM and the short-flux 12/10 SRM with the same stator outer diameter and stack length are conducted in terms of phase flux-linkage, inductance, and torque performance. The results indicate that for these two PM-free machines, at the rated speed and the rated RMS current density, the skewed-rotor WFFSM exhibits considerably larger torque capacity, much smaller torque ripple, stronger saturation withstand ability. In addition, the experiments of the skewed-WFFSM and the 12/8 SRM prototypes working at the mechanical character curve are conducted. The experimental results present that the power factor of the WFFSM is much higher and the efficiency is slightly lower than that of the 12/8 SRM.