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A novel E-core hybrid-excited switched-flux permanent-magnet (SFPM) brushless machine is proposed based on an E-core SFPM machine, which has significantly less magnet and higher torque density than those of a conventional SFPM machine. The proposed machine has a simple structure. The main flux path of dc excitation does not affect the magnet excitation because it is not through magnets. The combination of stator and rotor pole numbers of the proposed machine is optimized, and the flux-enhancing and flux-weakening capabilities are investigated by 2-D finite-element analyses and experimentally validated. [ABSTRACT FROM PUBLISHER]

A new simplex wave winding permanent-magnet (PM) brushless DC machine is proposed in this paper. Besides PM excitation and electric commutation, the machine has close-connected winding identical to the brushed DC machine. By properly controlling the power electronic switch status, the current direction of coils under N or S pole is unchanged. Thus, the new machine's operation principle is exactly as same as the brushed DC machine. Furthermore, as the adoption of PM and electric commutation, the new machine can be designed with larger pole number and lower slot number per pole, which respectively leads to less iron yoke and shorter end winding. Therefore, the proposed new machine not only keeps the characteristics of a brushed DC machine, but also improves the efficiency and power density. A 200 W, 10-pole, and 11-slot new machine is designed, prototyped, and tested to verify the analysis. [ABSTRACT FROM AUTHOR]

This paper proposes a new direct active and reactive power control (DPC) for the three-phase grid connected dc/ac converters. The proposed DPC strategy employs a nonlinear sliding mode control (SMC) scheme to directly calculate the required converter's control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any rotating coordinate transformations. Meanwhile, there are no extra current control loops involved, which simplifies the system design and enhances the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the design of the ac harmonic filter. Simulation and experimental results are provided and compared with those of the classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC strategies. The proposed SMC-DPC is capable of providing enhanced transient performance similar to that of the LUT-DPC, and keeps the steady-state harmonic spectra at the same level as those of the VC scheme. The robustness of the proposed DPC to line inductance variations is also inspected during active and reactive power changes. [ABSTRACT FROM PUBLISHER]

This paper presents a new direct active and reactive power control (DPC) of grid-connected doubly fed induction generator (DFIG)-based wind turbine systems. The proposed DPC strategy employs a nonlinear sliding-mode control scheme to directly calculate the required rotor control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations. Thus, no extra current control loops are required, thereby simplifying the system design and enhancing the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the designs of the power converter and the ac harmonic filter. Simulation results on a 2-MW grid-connected DFIG system are provided and compared with those of classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC. The proposed DPC provides enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the same level as the VC strategy. [ABSTRACT FROM AUTHOR]

The investigation on flux-switching permanent-magnet (FSPM) machines in existing papers has been restricted to a specific combination of stator and rotor pole numbers, viz., 12/10 stator/rotor poles. In this paper, the influence of stator and rotor pole numbers on the optimal parameters of the FSPM machines are investigated by the finite-element analysis and the lumped-parameter magnetic circuit model, respectively. It shows that, although the optimal rotor pole width for maximum torque varies with the rotor pole number, the optimized ratio of rotor pole width to the rotor pole pitch almost maintains constant, viz., ~1/3. It also shows that the 12/13 and 12/14 stator/rotor pole FSPM machines exhibit ~10% larger electromagnetic torque than the conventional 12/10 stator/rotor pole FSPM machine. The analyses are validated by an experiment made on a 12/14 stator/rotor pole FSPM machine. [ABSTRACT FROM AUTHOR]

A simple analytical method is developed to compare the combinations of stator and rotor pole numbers in flux-switching permanent magnet (PM) machines in terms of back electromotive force (EMF) and electromagnetic torque. The winding connections and winding factors of machines having all poles and alternate poles wound, and different numbers of phases, from two to six, are determined by the coil-EMF vectors. Their differences from analyzing the conventional fractional-slot PM machines with concentrated nonoverlapping windings are highlighted. The general conditions are established for balanced symmetrical back-EMF waveform. It shows that the optimized rotor pole number should be close to the number of stator poles, whereas larger torque can be obtained by the machine with relatively higher rotor pole number. The analysis is validated by finite-element analyses and experiment. [ABSTRACT FROM AUTHOR]

This paper presents the results of an investigation focused on the rotor core eddy-current losses of interior permanent-magnet (IPM) synchronous machines. First, analytical insight into the rotor core eddy-current losses of IPM machines is developed. Next, major design parameters that have the most significant impact on the rotor core eddy-current losses of IPM machines are identified. Finite-element analysis results are then presented to compare the predicted eddy-current losses in the machine core of IPM machines with one- and two-layer rotors coupled with concentrated- and distributed-winding stators. It is shown that the lowest total eddy-current losses in the machine core are achieved using a combination of distributed stator windings and two magnet layers per rotor pole, whereas minimizing only the rotor core eddy-current losses favors replacement of the rotor with a single-layer configuration. [ABSTRACT FROM AUTHOR]

A simple analytical method is developed for determining the optimal combination of stator pole and rotor pole numbers for a multi-tooth flux-switching permanent magnet (FSPM) machine, and the optimal design of the stator teeth for maximum torque. In addition, the rotor pole width and the split ratio are optimized by finite element analysis. The optimized multi-tooth FSPM machine exhibits a significantly higher torque capability and requires a significantly lower volume of magnet material than a conventional FSPM machine. The performance of the proposed multi-tooth FSPM machine is validated experimentally. [ABSTRACT FROM AUTHOR]

The electromagnetic performance of three-phase permanent magnet brushless motors in which the slot and pole numbers are similar and either all the teeth or only alternate teeth carry a concentrated coil is compared. Analytical and finite element techniques are employed to predict the air-gap flux density distribution, the cogging torque, electromotive force (EMF) and static torque waveforms, and the winding inductances, predicted results being validated by measurements. It is shown that there is a significant difference in both the phase and line EMF waveforms, the static torque-rotor position waveforms, and the self and mutual inductances. [ABSTRACT FROM AUTHOR]

Surface mounted permanent magnet Vernier (SPM-V) machines, because of their high torque density and multi-pole structure, are promising candidates for low-speed direct-drive applications. To achieve high torque density, the SPM-V machines are generally designed with high gear ratios. Therefore, their operating frequencies can be much higher than those of the conventional SPM machines. This potentially increases the alternating current (AC) winding losses, especially those with form-wound coils proposed for high-power applications. This article investigates the AC losses (including skin effect, proximity effect, rotor PM induced, and circulating current losses) in form-wound stator coils of a 3 MW direct-drive SPM-V machine with different slot/pole number combinations. The study reveals that the SPM-V machines have significantly higher AC winding losses than their conventional SPM counterparts for similar operating conditions. To reduce the AC winding losses in SPM-V machines, a novel flux shunt concept is proposed along with other conventional techniques such as increasing the number of turns/coil (with terminal voltage being kept constant) and parallel strands/turn, providing extra clearance at slot opening. With the loss reduction techniques implemented, the SPM-V machines can achieve comparable efficiency but much higher torque density than their conventional counterparts. Moreover, the proposed flux shunt was also found to be very effective in reducing the potential risk of permanent magnet (PM) irreversible demagnetization, a key issue for SPM-V machines at high power ratings. [ABSTRACT FROM AUTHOR]

This article investigates the effect of airgap length on the electromagnetic performance of direct-drive surface mounted permanent magnet Vernier (SPM-V) machines with different power ratings. Using 3 kW machine as an example, its performance is comprehensively compared with a conventional SPM machine with the same airgap lengths using two-dimensional finite-element analysis. For each airgap length, the slot/pole number combination for the SPM-V machine is investigated to achieve the optimal performance compared to the conventional SPM machine. In order to make the study more generic, the slot/pole number and the airgap length variations are expressed as normalized pole pitch, i.e., $\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $ (ratio of rotor pole pitch to electromagnetic airgap length). The results show that for 3 kW machines, $\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $ >2.2 is a good design criterion for the SPM-V machines to achieve higher average torque and efficiency than the conventional SPM machines. In addition, a reasonably good power factor (>0.9 in this case) can be achieved. Although the power factor of SPM-V machines drops significantly at multi-MW power level, i.e., 3 and 10 MW, the criterion $\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $ >2.2 still results in achieving a performance closest to their optimal capability. However, when $\overline {{{\boldsymbol{\tau }}_{\boldsymbol{r}}}} $ >2.2, special consideration should be paid to avoid potential irreversible magnet demagnetization at multi-MW power levels. [ABSTRACT FROM AUTHOR]

An improved brushless AC motor model is proposed for use in signal-injection-based sensorless control schemes by accounting for cross-coupling magnetic saturation between the d- and q-axes. The incremental self- and mutual-inductance characteristics are obtained by both finite-element analysis and measurements, and have been successfully used to significantly reduce the error in the rotor position estimation of sensorless control. [ABSTRACT FROM AUTHOR]

In permanent magnet (PM) brushless motors, a magnetic drag torque, which results from the effect of magnetic hysteresis, exists at low speeds but is often neglected. However, in some applications it can have potentially serious consequences. In this paper, a practical method for predicting the magnetic drag torque in a PM brushless motor is presented. It is based on the fact that the average drag torque over one mechanical revolution can be determined from the energy associated with excursions around appropriate hysteresis loops of the steel. Finite element analysis is employed to calculate the variation of flux in each element throughout the stator core, as the rotor rotates, and, hence, the hysteresis loss. The method is applied to two PM brushless motors having 18-slots/12-poles, 12-slots/10-poles, respectively. The predicted magnetic drag torques are compared with the measurements. [ABSTRACT FROM AUTHOR]

A simple analytical technique is proposed for synthesizing the cogging torque waveform of a permanent magnet brushless machine from the cogging torque waveform that is associated with a single stator slot. The machine may have either uniformly or nonuniformly distributed stator slots and/or a skewed rotor, in which the skew is realized by circumferentially displacing the magnets of each pole. The technique is validated by finite element analysis and measurements. [ABSTRACT FROM AUTHOR]

This paper combines the use of multi-phase configurations and current harmonic injection methods to achieve significantly improved performance for doubly salient synchronous reluctance machines (DS-SRMs). In order to understand the torque production mechanism of multi-phase DS-SRMs with current harmonic injection, some general analytical torque models have been developed. The models are based on Fourier Series analysis of self- and mutual-inductances. It has been established that with current harmonic injection, the torque ripple of both 5-phase and 6-phase DS-SRMs can be significantly reduced and the average torque can be enhanced. It contrast, harmonic injection always has a detrimental impact on 4-phase machines. The investigations of losses and dynamic performance with current harmonic injection for multi-phase machine have also been carried out and demonstrated that the proposed method can improve the efficiency in constant torque region and maintain comparable performance to fundamental current supply in the flux weakening region. Experimental validation based on single layer, 6-phase, DS-SRM has been performed. [ABSTRACT FROM AUTHOR]

This article investigates the impact of scaling on the electromagnetic performance of surface-mounted permanent-magnet Vernier (SPM-V) machines with a main focus on open-circuit induced Electro Motive Force (EMF). Three different power ratings, i.e., 3 kW, 500 kW, and 3 MW, have been chosen for this article. For each power rating, the SPM-V machines are analyzed for different slot/pole number combinations to compare their optimal performance with a conventional SPM machine. Step-by-step development of an analytical equation is presented for the prediction of induced EMF taking into account the interpole leakage of rotor permanent-magnets. 2-D finite-element analysis (FEA) has been used to validate the analytical equation across different power ratings. The analytical equation is thereafter utilized to study the influence of different geometric parameters on the performance of the SPM-V machines. It reveals that the back EMF and torque of SPM-V machines, for a given normalized pole pitch (rotor pole pitch to magnetic airgap length), are unaffected by the increase in airgap length due to scaling. However, the power factor of SPM-V, unlike the conventional SPM, reduces significantly with the increase in electrical loading due to the scaling effect. The analytical model for induced EMF and the 2-D FEA predicted results are validated by experiments using conventional SPM and SPM-V machine prototypes. [ABSTRACT FROM AUTHOR]

This paper proposes a novel flux reversal permanent magnet (FRPM) machine with asymmetric-stator-pole (ASP) configuration. Different from the conventional FRPM machine with uniform “NS-NS-NS” PM sequence, the proposed ASP-FRPM machine is characterized by a “NSN-S-NSN” magnet arrangement. Hence, the interpolar flux leakage is significantly reduced with the developed design, which can improve the torque capability. The machine topologies, features, and operating principle are introduced, respectively. A simplified magnetic circuit model is established to reveal the underlying flux leakage reduction mechanism of the ASP design, and the rotor pole number is analytically optimized as well. The design parameters are then globally optimized so as to improve the torque quality. In addition, the electromagnetic characteristics of the ASP- and conventional FRPM machines are compared. Finally, experiments have been carried out to validate the theoretical results. [ABSTRACT FROM AUTHOR]

Variable flux reluctance machines (VFRMs) are developed as magnetless electrical machines. To extend the application of VFRMs, the enhancement of torque density is a key during machine design. In this paper, a novel second-harmonic current injection method is developed for torque density enhancement for 6-stator-slot/4-rotor-tooth (6/4) VFRM. By using the analytical method, the optimal current profile with injected second-harmonic current is obtained. The average torque of 6/4 VFRM is improved by 20% with the proposed method under all load conditions. Moreover, the proposed current profile has fixed harmonic proportions and is easily applicable to machines with different specifications. All the conclusions are confirmed by both finite-element analyses and experimental results. [ABSTRACT FROM AUTHOR]

This paper investigates the contribution of each order inductance harmonic to the torque (both average torque and torque ripple) of multiphase doubly salient synchronous reluctance machines (DS-SRMs). Such machines are similar to switched reluctance machines but supplied with sine wave currents. The investigations in this paper are as follows. First, a general analytical torque model based on Fourier series analysis of inductances has been built for machines with different phase numbers, slot/pole number combinations, and also winding configurations. The instantaneous torque for DS-SRMs with any given phase number can then be accurately predicted. Using such model, contribution of each order inductance harmonic to torque can be investigated separately. It is found that the torque ripple frequency of the DS-SRM only depends on phase number. For example, for an m-phase machine, there will be m $\times $ kth-order torque ripple if mod(mk, 2) = 0, where m is the phase number and k is a natural number. This paper also explains why certain phase numbers inherently produce lower torque ripple than others. The findings in this paper provide a future direction for potential torque ripple reduction methods either from machine design or advanced control. The simulations have been validated by experiments using a six-phase DS-SRMs. [ABSTRACT FROM AUTHOR]

This paper proposes high-torque-density dual-stator permanent magnet machines with biased permanent magnet (PM) excitation in the inner stator. The developed machine can be geometrically considered as an outer-rotor stator PM machine plus a separate outer stator similar to the conventional fractional-slot machines. Consequently, the proposed designs feature two different stator structures. Meanwhile, two sets of armature windings are employed to improve the space utilization ratio and torque density. The machine topologies and operating principles are first described. In addition, the analytical models of the machine are introduced, which are utilized to optimize the stator/rotor pole combination as well as the power splitting ratio between two stators. This design optimization is performed in order to maximize the torque capability with the constraint of copper loss. The electromagnetic characteristics of the proposed machine with different inner stator structures are evaluated and compared by the finite-element (FE) method. Finally, an optimized dual-stator biased PM machines prototype is manufactured and tested to verify the FE analyses. [ABSTRACT FROM AUTHOR]

This paper proposes a novel dual-sided permanent magnet memory machine (DSPM-MM) by combing the distinct advantages of “high torque density” of conventional rotor-PM machine and “convenient online PM flux control” of stator-PM MM. In the proposed design, the consequent-pole NdFeB PMs are placed in the rotor, whereas the low coercive force (LCF) PMs are mounted between the adjacent stator teeth to enable flexible air-gap flux adjustment. Meanwhile, since the LCF PMs can be either remagnetized or demagnetized bidirectionally by a current pulse, the associated flux-weakening copper loss is eliminated, and hence, the proposed DSPM-MM can maintain high efficiency operation over a wide operating range. The machine topology and operating principle are introduced and addressed from the perspective of the nonlinear hysteresis behavior of LCF PMs as well as dual flux modulation effect, respectively. Then, the electromagnetic characteristics of the proposed machines having various available rotor poles are investigated and compared. The feasibility of the proposed design is validated by both finite element and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, the torque production mechanisms of flux-reversal permanent magnet (FRPM) machine and Vernier permanent magnet (PM) machine are analyzed and compared based on air-gap field modulation. Working harmonics of PM magnetomotive force (MMF) and air-gap permeance in two machines are analytically identified and compared, indicating that the fundamental PM MMF together with all permeance harmonics contribute to the torque production of Vernier machine whereas all PM MMF harmonics but only fundamental permeance in FRPM machine produces the torque. Thanks to the utilized large dc component of air-gap permeance, the torque density of Vernier machine is revealed to be better. Influence of critical parameters on machine performance, such as PM thickness and slot width ratio of the modulation pole, is also investigated. It shows that FRPM machine is more sensitive to the design parameters. Both finite-element analysis and experimental validation are conducted to verify the conclusion. [ABSTRACT FROM AUTHOR]

This paper proposes a resonant-based backstepping direct power control (BS-DPC) strategy for doubly-fed induction generator (DFIG) under both balanced and unbalanced grid conditions. Proper formulae for the BS-DPC strategy are obtained based on the elaborated analysis of the mathematical model of DFIG. The influence of the unbalanced grid voltage on the normal BS-DPC is analyzed. Furthermore, a resonant-based improved strategy is proposed to achieve different control targets under unbalanced grid condition without the need of decomposition of positive- and negative-sequence components. Comparative experimental studies of the resonant-based BS-DPC and the normal BS-DPC for DFIG are conducted to validate the effectiveness of the proposed strategy under both balanced and unbalanced grid conditions. In addition, the experimental results also prove that the proposed resonant-based BS-DPC can achieve a satisfying transient performance. [ABSTRACT FROM AUTHOR]

In this paper, electromagnetic performances of 12-phase 24-stator-pole redundant switched flux permanent magnet (SFPM) machines with 20-, 22-, 26-, and 28-rotor-pole rotors designed for wind power generation are comparatively analyzed. The influence of key design parameters on the open-circuit phase electromotive force and cogging torque is comparatively evaluated by a 2-D finite-element analysis (FEA). FEA results show that the 24-/26-pole SFPM machine exhibits the highest electromagnetic torque and efficiency than the other three, while the 24-/22-pole one has the smallest cogging torque and torque ripple as well as relatively good voltage regulation and efficiency, which are essential for a wind generator. Furthermore, the investigation on redundancy performance shows that cross coupling among the four sets of three-phase balanced windings and the saturation effect can be neglected when the generator operates with relative light loads. Finally, the 12-phase 24-/22-pole SFPM machine is built and tested to validate the simulations. [ABSTRACT FROM AUTHOR]

In this paper, in order to enlarge its torque density, an overlapping winding (OW) is applied to the 24/16-stator/rotor-pole flux-switching permanent magnet (FSPM) machine. By adopting an OW-configuration with full-pitch coils, the pitch factor and coil-flux cross-sectional area in the 24/16-pole FSPM machine with nonoverlapping winding (NOW) can be doubled. Thus, the fundamental open-circuit phase flux-linkage and back-EMF can be effectively enhanced by 115.28%, making it comparable to the conventional 12/10-pole NOW-FSPM machine. Therefore, by injecting the same current density, the proposed OW-FSPM machine can produce 65.2% higher torque density than its NOW counterpart, and 17.6% higher torque density than the conventional 12/10-pole NOW machine despite 21.5% lower ampere turns due to a smaller slot area. However, the proposed machine suffers from high torque ripple due to high cogging torque and back-EMF harmonics. Torque ripple reduction by harmonics current injection and rotor skewing in the proposed machine are investigated. Results show that both methods can effectively reduce the torque ripple in the proposed 24/16-pole NOW-FSPM machine, albeit with the level of torque density being compromised. Experiments are performed in order to validate the analysis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article investigates some novel modular consequent pole PM machines (CPMs) with E-core and C-core stators. Different slot-pole number combinations including 12-slot/10-pole (Ns>2p) and 12-slot/14-pole (Ns<2p) have been investigated. Their static and dynamic electromagnetic performances have been investigated, e.g. the phase back-EMF, on-load torque, torque-speed curves, power factor-speed curves and also efficiency maps are compared. It is found that the existence of flux gaps (FGs) can improve the average torque of the 12-slot/14-pole E-core modular CPMs while the C-core structure can be a better candidate where relatively low torque ripple is desirable. Moreover, by selecting proper FG width, the 12-slot/14-pole E-core modular CPMs can achieve better flux-weakening capability and higher efficiency while the 12-slot/10-pole C-core modular CPMs can have higher power factors over the whole speed range. The finite element simulation results have been validated by a series of experiments using 12-slot/14-pole modular CPMs with both C-core and E-core stators. [ABSTRACT FROM AUTHOR]

This paper proposes a coordinated control strategy for the rotor-side converter (RSC) and the grid-side converter (GSC) of an LC-filtered doubly fed induction generator (DFIG) under distorted grid voltage condition, to eliminate the distortion of the total output current (including the stator current, the GSC current, and the filter capacitor current). For an LC-filtered DFIG connected to a distorted grid, the low-order current harmonics can freely flow into the filter capacitor thereby polluting the total output current, even when the stator current and the GSC current are both controlled as sinusoidal. Although this problem can be addressed by measuring the output current or the capacitor current, the extra current sensors are undesired in industrial applications. In this paper, a novel hybrid virtual impedance method is implemented in both the RSC and the GSC, to reshape the parallel impedance of the stator, the GSC and the filter capacitor to nearly infinite for low-order harmonics, thus, the freely flowing output current harmonics can be mitigated. Meanwhile, the low-order current harmonics required by the output current compensation can be flexibly distributed between the stator and the GSC according to their operating conditions, to disperse the side-effects on the drive train and the GSC, thereby better protecting the whole system. The effectiveness of the proposed strategy is theoretically analyzed and verified with both simulation and experiment. [ABSTRACT FROM AUTHOR]

In this paper, two flux-switching permanent-magnet (FSPM) wind power generators with 9- and 12-phase windings are designed and comparatively analyzed. Both two generators are designed under the rated specifications of 10 kW output power and 220 V phase voltage at 500 r/min. The static characteristics and power generating performances including output voltage, power, and efficiency at rated and variable load/speed conditions are predicted by finite-element analysis and validated by experimental tests based on the two FSPM prototypes. It shows that the 12-phase 24-stator-slot/22-rotor-pole FSPM generator exhibits a higher air-gap flux density, a higher torque/power density, and a lower voltage regulation factor. Besides, it has a better overload capability than its 9-phase 36-stator-slot/34-rotor-pole counterpart when the load and wind speed exceed the rated levels. The comparative study reveals the benefits of the lower leakage flux and permeance from the larger stator-slot opening and longer magnetic circuit. [ABSTRACT FROM AUTHOR]

In this paper, the influences of design parameters on the on-load demagnetization behavior of a switched flux hybrid magnet memory machine (SF-HMMM) are evaluated, which aims to provide a useful design guideline to avoid this performance degradation. The machine topology and on-load demagnetization phenomenon are described, respectively. Furthermore, some key design parameters are selected to optimize to elevate the operating point of the low coercive force (LCF) permanent magnets (PMs). Then, some geometric modifications are suggested to alleviate the on-load demagnetization effects. Finally, an SF-HMMM prototype is fabricated and tested to experimentally verify the finite-element analysis. [ABSTRACT FROM AUTHOR]

A memory machine (MM) equipped with hybrid permanent magnets (PMs), i.e., NdFeB and low coercive force PMs, exhibits acceptable torque capability at low speeds and high efficiency at high speeds. Previous literatures have addressed that the constant power speed range (CPSR) of MMs can be further extended by online PM flux control and the requirement of the flux-weakening (FW) inverter current can be reduced as well. Nevertheless, how to coordinate the d-axis vector FW and PM magnetization control over a whole operating range in a reasonable manner remains unreported. Therefore, this paper proposes and implements a stepwise magnetization control strategy on a dc-magnetized MM based on the operating characteristics under various PM magnetization states and speed ranges. The configuration, principle, and mathematical model of the investigated machine are introduced first. Then, the proposed control strategy is established by dividing the operating envelop into several FW regions and an appropriate FW control scheme is utilized at each stage. It demonstrates that the CPSR can be effectively extended by simply applying the demagnetizing current pulses in several steps. This simplifies the control efforts considerably without resorting to continuous PM flux control and frequent actions of the switching devices. The effectiveness of the proposed control strategy is verified by experimental results. [ABSTRACT FROM AUTHOR]

The split ratio, i.e., the ratio of rotor outer diameter to stator outer diameter, is one of the most vital design parameters for permanent-magnet (PM) machines due to its significant impact on the machine torque or power density. However, it has been optimized analytically in the existing papers with due account only for the stator copper loss, which is reasonable for low-to-medium speed PM machines. For high-speed PM machines (HSPMMs), the negligence of stator iron loss and the mechanical stress on the rotor will lead to a deviation of optimal split ratio and actual torque capability. In this paper, the optimal split ratio of HSPMM is investigated analytically with the consideration of stator iron loss as well as the mechanical stress on the rotor. The influence of air-gap length and rotor pole pairs on the optimal split ratio is elaborated. Both the analytical and finite-element analysis reveal that the optimal split ratio for HSPMM will be significantly reduced, when stator iron loss and mechanical constraints are taken into account. [ABSTRACT FROM AUTHOR]

This paper analytically derives the optimal third harmonic current to improve the output torque in five-phase surface-mounted permanent magnet (SPM) machines under the constraints of the same peak/rms value. The relationship between the output torque and the third harmonic current is established and its optimal value for maximizing the output torque is derived. It is found that the optimal third harmonic current is dependent on the ratio of the third harmonic back electromotive force (EMF) to the fundamental one but it is not 1/6 of the fundamental one as pulse width modulation in an inverter. In order to improve the output torque by increasing the fundamental and third harmonic winding factors and the third harmonic airgap flux density, the unequal width tooth and permanent magnet (PM) segmentation are employed in the 10-slot/8-pole five-phase SPM machine. The optimal unequal tooth dimensions and the number of PM segmentations are analyzed for maximizing the output torque. The airgap flux density, back EMF, cogging torque, and output torque of the machine with equal/unequal tooth width and unsegmented/segmented PM are compared. Finally, the experiments on the prototype 10-slot/8-pole PM machine with unequal tooth width and segmented rotor are given to verify the analytical and finite element analyses. [ABSTRACT FROM AUTHOR]

In this paper, four rotor shaping methods, i.e., eccentric circular, inverse cosine, inverse cosine with third harmonic, and multi-step shaping methods, are developed and compared for torque ripple mitigation in variable flux reluctance machines (VFRMs). By using a 6-stator-pole/7-rotor-pole (6/7) VFRM as an example, the design criterions and capabilities of these four methods are illustrated. It is found that all the rotor shaping methods are capable of torque ripple mitigation and applicable to all the VFRMs except those with 6k/(6i ± 2) k (k, i = 1, 2, 3…) stator/rotor pole combinations. Moreover, the inverse cosine with third harmonic and multi-step shaping methods are found to have the best performance. They are able to reduce the torque ripple by 90% at a cost of only 3% torque density reduction. A 6/7 VFRM with both conventional and shaped rotors is prototyped and tested for verification. [ABSTRACT FROM AUTHOR]

Compared to nonmodular machines, modular topologies become increasingly attractive due to their simplified manufacture process, better fault-tolerant capability, and potentially reduced material consumption. In order to maintain or even enhance the machine performance while achieving high fault-tolerant capability, novel modular, single-layer winding switched reluctance machines (SRMs) with different pole numbers are proposed, which are supplied by rectangular wave current with different conduction angles. The influences of the pole number and flux gap width between E-core segmented stators on the electromagnetic performance have been investigated in terms of self- and mutual inductances, electromagnetic torque, copper loss, iron loss, and radial force. It has been found that the modular structures with higher rotor pole numbers than stator slot numbers (12-slot/14-pole and 12-slot/16-pole SRMs) can maintain and even improve the average torque due to the nature of self and mutual inductances. In addition, the torque ripples for modular machines are significantly reduced (below 50%), so do the iron loss and radial force, leading to higher efficiency albeit with potentially lower vibration and acoustic noise. Two prototypes with 12-slot/8-pole and 12-slot/14-pole combinations have been built with both nonmodular and modular structures to validate the predictions in terms of inductances and static torques. [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear hybrid field model to predict the open-circuit magnetic field distribution in surface-mounted permanent-magnet machines. It combines the complex permeance model (CPM) with the magnetic equivalent circuit (MEC). The saturation effect is accounted for in the CPM by considering the magnetic potential distribution on the stator bore, which is calculated by the MEC and can be transformed to the virtual current on the slot. The proposed model significantly improves the calculation accuracy for saturated machines, which is verified by the finite-element analysis and the experimental results. [ABSTRACT FROM PUBLISHER]

This paper proposes a sliding-mode direct power control (SMDPC) strategy for doubly fed induction generator (DFIG) under both balanced and unbalanced grid conditions using extended active power. When the traditional power theory is used under unbalanced grid condition, the control strategies usually need to be modified and become more complicated. Therefore, an extended active power is proposed in this paper, which is effective under both balanced and unbalanced grid conditions with a simple control strategy. Based on the extended active power, elaborated analysis of the mathematical model of DFIG is obtained. Furthermore, an SMDPC strategy using the extended active power is proposed, which can obtain sinusoidal stator currents and restrain electromagnetic torque ripples under unbalanced grid condition without the need of decomposition process and phase-locked loop (PLL). Comparative experimental studies of the SMDPC using the extended and traditional active powers for DFIG are conducted to validate the effectiveness of the proposed strategy under both balanced and unbalanced grid conditions. In addition, the dynamic performance and robustness of the proposed SMDPC are also proved to be satisfying by the experimental results. [ABSTRACT FROM AUTHOR]

In this paper, the torque production mechanism of variable flux reluctance machines (VFRMs) is investigated. Initially, based on an analytical instantaneous torque model developed from the Lorentz force law and airgap field harmonic analysis, the magnetic gearing effect is revealed in the torque production of VFRMs. Then, by using a 6-stator-pole/4-rotor pole (6s/4r) VFRM as an example, all its torque components, including synchronous torque, reluctance torque, and cogging torque, are analyzed separately under linear condition. Their average torque and torque ripple characteristics are comprehensively illustrated. Finally, the influence of saturation effect is also accounted for with the help of finite element and frozen permeability methods. The analysis results are verified experimentally. [ABSTRACT FROM AUTHOR]

This paper investigates the interaction of the zero sequence component into both permanent magnet (PM) shape and phase current to improve the output torque in dual three-phase surface-mounted PM (SPM) machines. The optimal zero sequence components are analytically derived and validated by finite element (FE) analyses. It is found that the optimal zero sequence components into both PM shape and current waveform are dependent on the fundamental and zero sequence harmonic winding factors. For the dual three-phase SPM machines having rotors without shaping, Sine shape, Sine shape with the zero sequence components, the electromagnetic performances including the back electromotive force (EMF) waveform, cogging torque, average torque, and torque ripple are compared. It is demonstrated that the average torque with the optimal zero sequence component in PM shaping and current waveform can be improved by >30% while the torque ripple remains similar to that of the one with Sine shaping. Finally, the experiments are given to verify both the analytical and FE analysis. [ABSTRACT FROM AUTHOR]