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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]

Electrical machines with high-temperature superconducting (HTS) armature windings can benefit from high power density and efficiency. Furthermore, by using permanent magnets (PM) on the rotor, the cryogenic system can be significantly simplified compared with the fully superconducting (SC) machines or the machines with SC field windings. Compared with the conventional electrical machines, the performance of machines with HTS armature windings can be more sensitive to the design parameters due to the interaction between the operating current and the flux density. In this article, the influence of stator geometry on the machine with HTS armature windings and PM rotor is investigated. By considering the interaction between the operating current and the flux density, an iteration procedure for calculating the operating current of the HTS armature windings is proposed. A 10-MW surface-mounted PM wind power generator with HTS armature windings is studied by the finite element analysis. It is found that machines with different coil angle have different behaviors when changing the tooth width and the dimensions of tooth tips, which is different from conventional electrical machines. A new design and optimization method is then developed. Finally, the optimal design is identified with global optimization. [ABSTRACT FROM AUTHOR]

In this article, the operating current and torque of surface-mounted permanent magnet (SPM) wind power generators with high-temperature superconducting (HTS) armature windings are analyzed. The influence of coil location and current angle on the electromagnetic performance is investigated systematically. The results show that the location of the HTS coils significantly affects the operating current and torque. Furthermore, the maximum torque current angle of SPM wind power generators with HTS armature windings can be different from 180° due to the interaction between the armature and magnet fields. A design optimization method for wind power generators with HTS armature windings is developed in this article. The optimal design is then identified with consideration of the coupling influence of coil location and current angle. [ABSTRACT FROM AUTHOR]

This paper compares the electromagnetic performance of 10-MW superconducting (SC) generators with three different topologies, i.e., iron-cored stator and rotor topology (ISIRT), iron-cored stator and air-cored rotor topology (ISART), and air-cored stator and rotor topology (ASART). The objective is to provide a powerful insight into the advantages and disadvantages of different topologies, and to establish some design guidelines for selecting an appropriate direct-drive SC generator for offshore wind turbine applications. First, the structures of the three SC generator topologies are introduced. Then, the influence of the SC coil cross-sectional area on torque capability is compared. After that three SC generators with different topologies are optimized, respectively, for further comparison, including the active material cost, weight, harmonics in the electromotive force, torque ripple, field harmonics in the SC coil, and forces on the rotor and stator components, etc. It is found that, with the same SC quantity, the torque capability of the ISIRT is much better than that of the other two topologies. However, the advantage becomes less significant when a larger area of the SC coil is employed. The air-gap flux density waveform of the ASART is much smoother than those of the ISIRT and ISART. The torque ripples of the ISIRT and the ISART are much higher than that of the ASART. The field harmonics (both amplitude and frequency) in the SC coil of the ASART are the lowest. For the ISIRT, most of the force on the rotor is acting on the rotor iron, and thus, the SC coil is more likely to be safe from a mechanical performance point of view and the design of the corresponding supporting structure is simple. However, for the air-cored rotor topologies, nearly all the force is acting on the SC coil. For the air-cored stator, the force mainly acts on the armature winding, whereas for the iron-cored stator, it is mainly on the stator teeth. Due to the excellent mechanical performance of iron, the iron-cored stator is therefore more robust. [ABSTRACT FROM PUBLISHER]

This paper investigates the influence of pole number and stator outer diameter on the performance of superconducting (SC) generators. The SC generator has an iron-cored rotor topology. First, the generator structure is introduced and the optimization procedure is described. Then, the influence of design parameters on performance, in terms of generator volume, weight, SC wire utilization, active material cost, etc., is presented. Some relationships for the optimal combinations for different performance attributes are established. In addition, the influence of SC material price on the determination of optimal stator outer diameter and pole number is discussed. Finally, the influence of SC coil area per pole on performance is also investigated. [ABSTRACT FROM PUBLISHER]

This paper describes the operation principle and analyzes the electromagnetic performance of synchronous reluctance (SynR) machines having the structure of switched reluctance machines but excited by an ac sinusoidal current with dc bias, i.e., salient-pole SynR with nonoverlapping concentrated windings and a dc bias excitation. Under such excitation, these machines become similar to permanent magnet (PM) counterparts. The dc bias, which is equivalent to the PM excitation, produces back electromotive force (EMF) in the ac coils as well as cogging torque. Thus, injecting the ac coils by sinusoidal currents results in output torque on the machine shaft. The produced back EMF and cogging torque, and torque ripple magnitudes and periods, as well as the optimal ac current angle and the ac/dc current density ratio, strongly depend on the winding connection configuration. The maximum average torque can be achieved when the ac/dc current density ratio is 2 and the symmetrical winding connection, which results in a lower torque ripple, is employed. A prototype machine is built and tested to confirm the analyses and conclusions. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study of torque–speed characteristics and cogging torque of the fractional-slot interior permanent magnet brushless ac machines having different slot openings, viz. open slot, closed slot, and hybrid slot (sandwiched open and closed slots), and unskewed and step skewed rotors. The influence of such alternate slot openings on the magnetic cross-coupling and cogging torque is particularly emphasized. Furthermore, the repercussion of the manufacturing limitations and tolerances on the cogging torque of machines having these alternate slot openings is also investigated. The magnetic cross-coupling level and the sensitivity of cogging torque to manufacturing limitations and tolerances strongly depend on the slot opening materials. The finite-element analyses are experimentally validated. [ABSTRACT FROM PUBLISHER]