The mechanism of torque production in the Vernier machine is difficult to understand since the stator teeth originating flux modulation effects, upon which the torque is produced, are usually minor factors which are ignored in normal synchronous machines. This paper begins with an analytical study of a spoke type Vernier PM Machine (SVPM), which has a generic number of stator slots per pole per phase, upon which the key sizing equations are developed. An equivalent winding factor is defined to model stator teeth originating flux modulation phenomena and resulting multi-harmonic field coupling effects. Performance predictions and design observations concerning SVPMs are then obtained. A new topology for an SVPM using ferrite magnets is proposed where a consequent pole design having an alternating leakage flux blocking ability is developed for the rotor. Overall, this improved SVPM design significantly boosts the back-EMF compared to a conventional SVPM, and the torque production even surpasses that of a benchmark rare earth IPM machine although with a slightly lower power factor.

This paper proposes a novel Bi-Magnet machine design concept in which ferrite and rare earth magnet materials are both utilized for torque production. The proposed design aims to reduce the cost of permanent magnet machines by minimizing the use of expensive high-grade rare earth magnet materials without compromising the high performance associated with them. The decreased performance due to the reduced rare earth magnet content can be compensated by adding inexpensive ferrite magnets. The design was optimized to achieve maximum cost-savings by minimizing the rare earth magnet content and improving the saliency of the rotor structure. A comparative study was conducted to highlight the advantages of the proposed concept using the Camry 2007 model as the baseline design. The results show that the proposed design is capable of achieving the same torque and power density as the baseline design using 30% less rare earth magnet materials, resulting in 20% active material cost reduction. The Bi-Magnet machine concept is shown to maintain the superior performance of conventional rare earth permanent magnet machines at a lower cost.

This paper proposes a novel shaped-magnet surface permanent magnet machine design concept featuring magnets shaped according to a sinusoid plus a third harmonic along the axial direction, and full-pole-pitched distributed stator windings with a unity winding factor to provide high torque density with essentially zero torque ripple for high performance motor applications. The magnet thickness is uniform along the radial direction so as not to impair the magnet's ability to handle demagnetization force. The proposed design was compared against a full-pole-pitched magnet design with maximum magnetic loading and a conventional bread-loaf magnet design. The results demonstrate that the proposed shaped-magnet design uses the magnet materials efficiently to achieve torque density comparable to that of the full-pole-pitched magnet design, and essentially zero torque ripple without compromising its resistivity against a demagnetization force, which conventional bread-loaf magnets cannot offer.

This paper develops an improved dual-stator Vernier machine using ferrite magnet materials, aiming to replace an industrial interior permanent magnet machine with rare earth magnets in a low speed industrial application. The improved Vernier design adapts the single winding dual-stator topology with an empty inner stator core mainly to improve the torque density and eliminate the heat from the inner stator windings of conventional dual-stator Vernier machines. A drum rotor is also proposed to provide mechanical and structural enhancements. The paper compares the performance of the improved Vernier design against an industrial interior permanent magnet machine with rare earth magnets. The comparison shows that the improved design using ferrite magnets has even higher torque density and efficiency than those exhibited in the industrial interior permanent magnet machine with rare earth magnets. The improved Vernier machine design has considerable potential to replace industrial rare earth interior permanent magnet machines in low speed applications.

This paper proposes a new reference-generation-strategy for direct torque control (DTC) of doubly fed induction generator (DFIG), under symmetrical voltage-dips. Since DTC has no current-control loop, it cannot prevent overcurrent during voltage-dips. The proposed method prevents overcurrent in the rotor side converter (RSC), by damping the transient-flux and modifying the references of rotor-flux and torque, during voltage dip and recovery. The analysis of DTC for DFIG is presented by λ-i equivalent circuit which is decomposed into forced and natural circuits. Based on the natural λ-i circuit, the method adds a transient compensation term to the rotor-flux reference, which is obtained by multiplying the stator-natural-flux with a proposed decaying-ramp function. Moreover, this method reduces the forced component of rotor-flux proportional to the grid voltage. As a result, this method adjusts the transient-flux damping time and ensures overcurrent prevention in the rotor and stator windings. The torque reference is modified to maintain the torque-angle (between rotor and stator flux vectors) constant which results stable control of the DFIG during grid faults. The effectiveness of the proposed method is confirmed by experimental results of a 3-kW test set-up and simulation results of a 2-MW DFIG.

The vernier machine can utilize magnetic-gear effect, which conspicuously improves the torque production over other type of synchronous machines. However, a dual-rotor/dual-stator structure enabled high torque density vernier design is often unfavorable for industrial applications, due to manufacturing complexity and thermal management challenges. This paper successfully addresses this problem, by building magnetic-gear effect into a consequent pole enabled alternating flux barrier structure with single-rotor single-stator (SRSS). In this way, the proposed machine can achieve a superior torque density as a vernier machine does, while maintaining an equivalent manufacturing complexity as a conventional SRSS synchronous machine. This paper begins with elaborating generic analytical equations, that can transform into a vernier machine or a SRSS synchronous machine. More specifically, the magnetic-gear effect is manifested via stator teeth originating flux modulation concept and resulting multi-harmonic field coupling effects in the equations. A SRSS vernier topology with consequent pole and V-shape magnet is then proposed. A considerable 52.6% improvement on the torque production with comparable efficiency are validated by experiments, where the comparison is made over that of a benchmark rare earth PM machine. Power factor would fall behind with increasing current load. Overall, this new design achieves outstanding torque density, showing solid potential for industry adoption.