W. Q. Chu

Average Torque Separation in Permanent Magnet Synchronous Machines Using Frozen Permeability

This paper investigates the average torque separation in permanent magnet (PM) synchronous machines. In order to accurately separate the PM and armature fields, and, hence, the torque components accounting for the magnetic saturation and crosscoupling, the frozen permeability (FP) method is often employed, while the torque can be calculated by different methods, such as Maxwell stress tensor and virtual work principle. Although these two methods result in identical torques in normal finite element (FE) analyses when appropriate FE meshes are used, the average torques calculated by these two methods are found to be different when the FP method is employed due to the influence of equivalent rotational magnetic saliency in the stator, which causes a part of PM torque being improperly attributed to the reluctance torque when Maxwell stress tensor method is employed. However, by using the virtual work principle, this is eliminated, and, hence, the average torque components can still be appropriately separated and analyzed.

Investigation of Torque Ripples in Permanent Magnet Synchronous Machines With Skewing

This paper investigates the influence of skewing on torque ripples, including the electromagnetic (EM) torque ripple, and the cogging torque, in permanent magnet (PM) machines. It is found that the effectiveness of skewing largely depends on the axial variation of a torque ripple phase but less on its magnitude under skewing. It is further found that, in both linear and nonlinear cases, the EM and the on-load torque ripple cannot be fully eliminated by skewing one on-load torque ripple period or any other angles, except 360° electrical, which is impractical. This is due to the fact that the EM torque ripple, especially its phase, of each slice is different due to the axial variation of the equivalent current phase advance angle introduced by skewing. In nonlinear cases, the EM torque ripple variation is further aggravated by the axial variation of magnetic saturation. The EM torque ripple as well as the on-load torque ripple may be even increased by skewing, especially when the cogging torque is low and electric loading is high.

Simplified Analytical Optimization and Comparison of Torque Densities Between Electrically Excited and Permanent-Magnet Machines

This paper presents the simplified analytical optimization and comparison between electrically excited (EE) and permanent-magnet (PM) machines in terms of torque per volume T/V and torque per weight T/G for low-speed applications when their copper loss and overall size are the same. Analytical torque models for both machines are individually developed and optimized to obtain the optimal flux density ratio, split ratio, and maximum torque densities. Furthermore, the variation of optima with the number of poles and machine size is also investigated. The analytical analyses are validated by both finite-element analyses and experiments. It is concluded that torque densities of PM machines can be more than √2 times higher than those of EE machines. For EE machines, there is an optimal pole number to maximize torque densities, and large volume applications are preferred. In actual applications, EE machines are more likely to compromise the torque density to meet the thermal constraints. It also shows that the optimal T/G designs have significantly higher split ratio and are more cost- and weight-effective than the optimal T/V designs.

On-Load Cogging Torque Calculation in Permanent Magnet Machines

This paper investigates the on-load cogging torque calculation in permanent magnet (PM) machines. The vast majority of existing methods for calculating the cogging torque, no matter whether analytical or numerical methods, neglect the influence of load, or are inappropriate in considering the influence of load. Without using the frozen permeability method, the torque calculated by the virtual work principle includes the magnetic energy due to the armature field. When using the frozen permeability method, the resultant torque based on the Maxwell stress tensor with the on-load PM field only has nonzero average torque and, hence, is not the on-load cogging torque. A new on-load cogging torque calculation method is proposed in this paper based on the combination of the virtual work principle and frozen permeability method. For its implementation, an improved frozen permeability method, which makes the magnetic energy with on-load PM field that can only be calculated according to the B–H curve, is also developed. By using the new method, all the shortcomings of existing methods can be avoided and, hence, the on-load cogging torque can be calculated appropriately in both linear and nonlinear cases.

A Wound Field Switched Flux Machine With Field and Armature Windings Separately Wound in Double Stators

In this paper, a double stator (DS) wound field (WF) switched flux (SF) (DS-WFSF) machine is proposed. In the DS-WFSF machine, field and armature windings are separately placed in two different stators. Compared with the conventional WFSF machine with single stator, in which both field and armature windings are located, nonoverlapping concentrated windings and large slot areas can be obtained in the DS-WFSF machine. The proposed DS-WFSF machine exhibits >19% higher torque than the conventional WFSF machine, with both machines having the same space envelope and being globally optimized. The influence of leading design parameters, such as copper loss ratio between the field and armature windings, air-gap diameter, and rotor iron piece thickness and widths, on the average output torque is investigated for the DS-WFSF machines having 12/10, 12/11, 12/13, and 12/14 stator slots/rotor iron pieces. All the analyses are confirmed by both finite element and experimental results.

Influence of Flux Gaps on Electromagnetic Performance of Novel Modular PM Machines

In order to simplify manufacture processes and improve fault-tolerant capabilities, modular electrical machines, especially the ones with segmented stators, are increasingly employed. However, flux gaps between segments are often inevitable. In this paper, to take advantage of these flux gaps to enhance the machine performance, novel modular permanent magnet machines with different slot/pole combinations have been proposed. The influence of these flux gaps on the electromagnetic performance of modular PM machines, such as winding factor, open-circuit air-gap flux densities, back-EMFs, cogging torque, on-load torque, inductances, magnetic saturation and copper losses, are comprehensively investigated and general rules have been established. It is found that for modular machines having slot number higher than pole number, the flux gaps between stator segments degrade the electromagnetic performance due to the lower winding factor and the flux defocusing effect. However, for modular machines having slot number lower than pole number, the electromagnetic performances can be significantly improved using proper flux gap width due to the higher winding factor and the flux focusing effect. The finite element results are validated by experiments using two prototype modular machines.

Reduction of On-Load Torque Ripples in Permanent Magnet Synchronous Machines by Improved Skewing

Skewing is one of the most widely used methods to reduce the torque ripples. However, it was reported that the on-load torque ripple was not always reduced by the conventional skewing, in which the skewing angle is one period of torque ripple. This paper proposes the way of improving the skewing to reduce the torque ripple when the conventional skewing fails. It is found that by optimizing the skewing angle and current phase advance angle together, the improved skewing is able to meet the stringent torque ripple requirement over the whole load range.

Investigation on Operational Envelops and Efficiency Maps of Electrically Excited Machines for Electrical Vehicle Applications

In this paper, the operational envelops and efficiency maps of an electrically excited (EE) machine with/without employing flux weakening control via armature current and/or with/without the field excitation regulating are obtained and comprehensively compared for electric vehicle applications. It shows that even in EE machines, only using the field excitation regulating but without the flux weakening armature current control, the maximum power at high speed is not constant but inversely proportional to the machine speed. Only by employing the flux weakening armature current control, the maximum constant power operation at high-speed region can be achieved while the operational high-speed region can be greatly extended. The maximum efficiency in this extended high-speed region can be achieved when both the field excitation and flux weakening d-axis armature current change proportionally. The main benefit of the field excitation regulating is that the efficiency in low-torque region can be significantly improved. All the analyses are validated analytically.

Analytical modeling and investigation of transient response of PM machines with 3-phase short-circuit fault

This paper aims to find effective methods to avoid irreversible demagnetisation in permanent magnet machines by suppressing the short-circuit current. An analytical model is developed to predict the transient currents, overcoming the problems of previous steady state models which may significantly underestimate the short-circuit current. Although the developed analytical model is simple, it can accurately reveal the relationship between the short-circuit response and design parameters, as its high accuracy is validated by transient finite element analysis and experiment. The characteristics of short-circuit behavior are investigated and several guidelines for improving the short-circuit performance are obtained to aid the machine design.

Novel Dual Phase Shift Control with Bi-directional Inner phase shifts for Dual Active Bridge Converter Having Low Surge Current and Stable Power Control

To transmit constant powers particularly at light load, the isolated dual active bridge (DAB) with conventional dual-phase-shift (DPS) control exhibits large variations of currents, losses, and efficiencies if there is a small change of outer phase shift ratio, which is caused by the change of command to adjust power transmission. This is mainly because of the narrow operation region of outer phase shift ratio if the conventional DPS modulation is used. To solve this problem, a novel DPS control with bidirectional inner phase shifts for the DAB is developed to reduce the current surge in the high-frequency transformer, which can stabilize the output power with high efficiency. From the analysis of operation modes and power characteristics of the proposed DPS control, it is shown that due to wider operation region of outer phase shift ratios compared with the conventional DPS control in which the inner phase shifts of both the primary and secondary H-bridges are in the same direction, lower surge current can be obtained in the change of phase shift command, which results in stable power transmission particularly at light load without scarifying the efficiency. Moreover, the DAB of using the proposed DPS control can transmit bidirectional energy with the inner and outer shift ratios of 0–1. For experimental verifications, the test results are also presented in this paper.