D. Wu

Electromagnetic Performance of Novel Synchronous Machines With Permanent Magnets in Stator Yoke

Novel synchronous machines with doubly salient structure and permanent magnets (PMs) in stator yoke have been developed in this paper. The stator is constituted by T-shaped lamination segments sandwiched with circumferentially magnetized PMs with alternate polarity, while the rotor is identical to that of switched reluctance machines (SRMs). The stator pole number is multiples of six, which is the number of stator poles in a unit machine. Similar to variable flux reluctance machines (VFRMs), the rotor pole numbers in the novel machines are not restricted to those in SRMs. When the stator and rotor pole numbers differ by one (or the number of multiples), the novel synchronous machines show sinusoidal bipolar phase flux linkage and back electromotive force (EMF), which make the machines suitable for brushless ac operation. Moreover, two prototype machines with six-pole stator and five-pole, seven-pole rotors are designed and optimized by 2-D finite element analysis. It shows that, compared with VFRMs, the novel machines can produce ~70 % higher torque density with the same copper loss and machine size. Meanwhile, the proposed machines have negligible reluctance torque due to very low saliency ratio. Experimental results of back EFM, cogging torque, and average torque on the prototypes are provided to validate the analysis.

Influence of Stator and Rotor Pole Arcs on Electromagnetic Torque of Variable Flux Reluctance Machines

The influence of stator and rotor pole arcs on electromagnetic torque of variable flux reluctance machines (VFRMs) having different stator and rotor pole combinations is investigated. Different from 6/4 stator/rotor pole switched reluctance machines, which usually employ equal stator pole arc and stator slot opening, unequal stator pole arc and slot opening can boost the torque density of the VFRM. Moreover, the optimal rotor pole arc to rotor pole pitch ratio in VFRMs is (sim ~1/3) and the optimal stator pole arc is equal to or slightly smaller than the optimum rotor pole arc. For a 6-pole stator, the 4-, 5-, 7-, and 8-pole rotor VFRMs with the optimal stator and rotor pole arcs can increase the average torque by 1%, 4%, 30.7%, and 72.9% compared with those having equal stator pole arc and stator slot opening. In addition, the 6/7 stator/rotor pole VFRM exhibits the largest torque density. Prototype machines with optimal stator and rotor pole arcs are manufactured and measured to validate the analysis.

Comparative Study of Partitioned Stator Machines With Different PM Excitation Stators

The partitioned stator (PS) machine adopts two stators to allocate windings and permanent magnets (PMs) separately, increasing the space for PMs as well as armature coils to boost the electromagnetic torque and improving the thermal condition of PMs. In this paper, the PS switched-flux PM (PS-SFPM) machine and the PS flux reversal PM (PS-FRPM) machine are proved to inherently share the same operating principle and similar machine topology but with interior PM (IPM) and surface-mounted PM stators, respectively. Furthermore, four globally optimized PS machines with different inner stator topologies are compared in terms of back EMF, cogging torque, electromagnetic torque, torque per PM volume, and flux-weakening capability. The results reveal that the spoke-shaped IPM (IPM-spoke) inner stator exhibits the highest back EMF and hence the highest average torque, while the I-shaped IPM (IPM-I) stator has the best flux-weakening capability, and the V-shaped IPM (IPM-V) produces the highest torque per PM volume. Furthermore, four machines are redesigned with the same PM usage volume, and the results show that the PS-SFPM machine still exhibits the highest back EMF as well as torque although with sacrificed advantages. The finite-element analyses and experiments are used to confirm the predictions.

On-Load Voltage Distortion in Fractional Slot Surface-Mounted Permanent Magnet Machines Considering Local Magnetic Saturation

The on-load voltage distortion in fractional slot surface-mounted permanent magnet machines refers to the phase terminal voltage distorted by armature reaction. With the aid of frozen permeability method, the mechanism of this phenomenon has been investigated based on a 12-slot/8-pole model as an example, showing that the local magnetic saturation in tooth-tips is the main cause, especially when small or closed slot openings are adopted. Then, its impacts on machine performance under vector control have been investigated. If the dc-link voltage is sufficient to satisfy all distorted voltage, i.e., operating in constant torque region, the phenomenon mainly contributes to torque ripple. However, it becomes more problematic in the flux weakening region if the dc-link voltage is limited. Both base-speed and torque-speed curve will significantly differ from the expected results calculated by fundamental voltages. Meanwhile, if the machine operates beyond the base speed, or considering the limited bandwidth in real systems, the phase currents may be distorted, which consequently increase the current harmonics and reduce the average torque. Finally, a prototype machine is manufactured and tested to validate the analyses.

Comparative study of novel biased flux permanent magnet machine with doubly salient permanent magnet machine

A comparative study between the novel biased flux permanent magnet machine (BFPMM) and the doubly salient permanent magnet machine (DSPMM) with the same machine size is presented in this paper. The choice of rotor pole number in BFPMM can be any integers except the phase number and its multiples, which is more flexible than that in DSPMM. Different from 6/4 stator/rotor pole DSPMM which has the unipolar phase flux-linkage and trapezoidal phase back-EMF, they become bipolar and essentially sinusoidal in 6/7 stator/rotor pole BFPMM. Hence, compared with DSPMM under the optimal current angle and rated copper loss, the torque ripple in BFPMM can be reduced significantly by ~70%-80% in both the original design (ORI) and the design with flux focusing technique (FC). Moreover, BFPMM exhibits larger average torque than that of DSPMM in both ORI- and FC-designs under the same copper loss and optimal current angle.

On-Load Voltage Distortion in Fractional-Slot Interior Permanent Magnet Machines

The mechanism and the influence of the on-load terminal phase voltage distortion by the armature reaction in interior permanent magnet (IPM) machines with fractional-slot concentrated windings (FSCWs) are investigated in this paper, by taking a 12-slot/8-pole machine as an example. The phenomenon is presented first, which shows that the peak voltage will be much higher than the fundamental value, especially under the flux-weakening operation. Then, the mechanism is investigated with the aid of the frozen permeability method. Due to the geometric feature of the FSCW IPM rotor, the armature reaction flux will asymmetrically enhance the local magnetic saturation in the tooth-tips and the rotor lamination region near the rotor ribs, which disrupt the smooth variation of phase flux linkage and lead to a voltage distortion. With the increasing of current advancing angle (β), this effect will become more obvious and result in the higher distortion level. Furthermore, the influences of voltage distortion on the machine torque performance are investigated, which show that it will only contribute to the torque ripple in the constant torque region, but will largely reduce the flux-weakening operation region compared with the ideal one calculated by the fundamental voltages only. Finally, a prototype is manufactured and tested to validate the analyses.

Analytical Synthesis of Air-Gap Field Distribution in Permanent Magnet Machines With Rotor Eccentricity by Superposition Method

This paper proposes an analytical method that combines the superposition and the subdomain methods to predict the air-gap field distribution in permanent magnet (PM) machines with rotor eccentricity. The original machine with rotor eccentricity is divided into a number of air-gap sections along the circumferential direction. For each air-gap section, a concentric model is employed by adopting an equivalent air-gap length to predict the air-gap field using the subdomain method. The air-gap field distribution of the original model can then be synthesized from these concentric models. Consequently, the electromagnetic performance can be predicted accordingly, such as flux-linkage, back EMF, and so on. Finally, the direct finite element analysis is used to validate the efficacy of the proposed method. The proposed analytical method can be extended to other machines with different slot/pole number combinations and is particularly useful for analyzing the machines with large slot and pole numbers.

Evaluation of efficiency optimized variable flux reluctance machine for EVs/HEVs by comparing with interior PM machine

As one of crucial components for electric vehicles (EVs) and hybrid electric vehicles (HEVs), the electrical machine has been widely evaluated from the prospective of machine topology, torque density, permanent magnet usage, and efficiency. In this paper, novel variable flux reluctance machines (VFRMs), which do not have either brush/slip-ring or rare-earth permanent magnet, is evaluated for EV/HEV application with emphasis on driving efficiency. Two VFRMs are designed to compare with the interior permanent magnet machine (IPMM) used in Toyota /Prius 2010. Both machines have the same outer diameter as that of IPMM. However, one is with the same core length as the IPMM; the other one is with the same machine axial length as IPMM accounting for the end-windings. Based on the quantity evaluations of average torque, torque/power-speed characteristic, efficiency map and energy consumption during the city and highway drive cycles, the VFRM shows it can be a prospective low cost candidate for the EV/HEV application by providing the comparative torque density, efficiency and small torque ripple similar to the IPMM.

Superposition Method for Cogging Torque Prediction in Permanent Magnet Machines With Rotor Eccentricity

This paper proposes an analytical method that combines the superposition concept and the subdomain method to predict the cogging torque of permanent magnet (PM) machines with a rotor eccentricity. The original eccentric machine is first divided into several sections along the air-gap circumferential direction. Then, the equivalent air-gap lengths of all sections will be determined and individually used to build up a series of concentric models representing each section. By the superposition method, the air-gap flux density of the original eccentric machine can be synthesized from that of every concentric model predicted through the subdomain method. Consequently, the cogging torque can be predicted by the integral of Maxwell stress tensor based on the synthesized air-gap flux density. To prove the efficacy of the proposed method, two fractional-slot PM machines with different slot/pole number combinations are taken as examples, together with the validation of a finite-element analysis.

Influence of Slot and Pole Number Combinations on Voltage Distortion in Surface-Mounted Permanent Magnet Machines With Local Magnetic Saturation

The local magnetic saturation in tooth-tips of surface-mounted permanent magnet (SPM) machines generates on-load terminal voltage distortion, especially when small or closed slot openings are adopted. This paper focuses on investigating the influence of slot and pole number (Ns/2p) combinations on this phenomenon in both fractional slot and integer slot SPM machines. The mechanism of this phenomenon and how it is influenced by current advance angle and tooth-tip geometric parameters are introduced first. Then, voltage distortion pattern is proposed to analyze the occurring time or rotor positions of voltage ripples and their relative amplitudes according to the winding arrangements of different Ns/2p combinations, with the influence of PM leakage flux considered. By 2-D finite-element analysis, the voltage distortion of various machines is calculated and compared, such as Ns ± 1 = 2p, Ns ± 2 = 2p, Ns/2p = 3/2 (3/4), and integer slot machines with slot number per pole per phase q = 1 or 2. The analysis results reveal that under the same comparison conditions, the integer slot machines have less voltage distortion than the fractional slot machines, especially when q ≥ 2. Meanwhile, for the same slot number, the fractional slot machines with N8 > 2p suffer from on-load voltage distortion less than their counterparts with Ns <; 2p. Finally, three prototype machines with 12/10, 12/14, and 12/8 slot/pole numbers are manufactured and tested to validate the analyses.