Robert Nilssen

Influence of Pole and Slot Combinations on Magnetic Forces and Vibration in Low-Speed PM Wind Generators

In this paper, radial forces and torque ripple characteristics are investigated in permanent magnet (PM) machines having different pole and slot combinations. Using the PM machines with concentrated windings could be beneficial in direct-drive wind generators since it is possible to reduce the size and weight of the generator. The PM machines with concentrated windings having a large number of poles are compared to investigate the effect of pole and slot combinations on force and vibration characteristics in low-speed generators. Cogging torque waveforms and torque ripple are investigated using time-stepping finite-element analysis. Analysis of radial forces is presented, including investigation on radial force density distribution, total forces on teeth, and time-dependent force waveforms on a tooth. Structural analysis and experimental modal analysis are performed on the prototype generator. The main mode of vibration in the prototype machine is observed experimentally and the results are in good agreement with the simulations.

Performance Comparisons Among Radial-Flux, Multistage Axial-Flux, and Three-Phase Transverse-Flux PM Machines for Downhole Applications

The aim of this paper is to provide performance comparisons among conventional radial-flux, multistage axial-flux, and three-phase transverse-flux permanent-magnet machines for downhole applications where the outer diameters are limited by well sizes, but the axial lengths can be relatively long. The comparison procedure is based on a high ambient temperature of 150°C, a small outer diameter of 100 mm, a current density of 4 A/mm2 , an electrical loading of 20 kA/m, and a constant speed of 1000 r/min, with their output torques being from several newton meters to 105 N·m and power up to 18 kVA. Three machine prototypes are chosen and optimized individually in terms of maximum torque density on the basis of some common constraints. The comparisons are focused on the torque density, machine efficiency, and power factor with respect to different pole numbers and axial lengths. For a specific downhole application without an external cooling system, the obtained results provide an indication of machines best suited with respect to performance and size.

Characteristic investigations of a new three-phase flux-switching permanent magnet machine by FEM simulations and experimental verification

In this paper a new flux-switching permanent magnet (FSPM) machine with 12 stator poles and 14 rotor poles is investigated, and compared to a machine with the same stator but 10 rotor poles. Two prototypes are studied by both finite element method (FEM) analysis and experimental measurements. The results show that the 12/14 pole prototype can provide about 7-12% higher torque, the torque ripple reduces from 8.5% to 5.1% and its synchronous inductance is also 15% higher. After optimization, the FEM simulation results show the 12/14 pole machine could provide 19% higher torque than the 12/10 pole machine and the torque ripple is further reduced to 2.3%.

Slot Harmonic Effect on Magnetic Forces and Vibration in Low-Speed Permanent-Magnet Machine With Concentrated Windings

In this paper, the influence of slot harmonics on magnetic forces and vibration is studied in a 120-slot/116-pole low-speed PM machine at no-load. It is shown how the lowest mode of vibration is produced at no-load due to slotting. Comparing the cases of open slots, semi-closed slots and magnetic wedges, the effect of slot closure on radial forces and torque production capability is discussed. Magnetic flux distribution in the airgap is computed using finite element analysis. Spatial harmonics due to slotting are investigated in different cases. Maxwells stress tensor is employed to calculate radial and tangential components of the force density in the airgap. Spatial distribution of the total forces on the teeth and also time-dependent force waveform on one tooth are analyzed and discussed for different cases. It is shown how the magnitude of the lowest mode of vibration is reduced in the case of using semi-closed slots and magnetic wedges. Tangential force density distribution and torque production capability are also discussed. Structural analysis is presented to compute the maximum amplitude of the stator deformations due to the radial forces. Experimental results of the prototype generator are presented verifying the existence of the lowest mode of vibration at no-load because of the slot harmonics.

Ironless Permanent-Magnet Generators for Offshore Wind Turbines

Because nonmagnetic material is used in the stator, ironless permanent-magnet generators (iPMGs) have negligible normal force between the rotor and the stator, and there is low requirement for the strength of the supporting structures. Therefore, the generator can be light. This feature is attractive in offshore direct-drive energy conversion systems where lightweight design is preferred. The objective of this paper is to investigate systematically different concepts of iPMGs. A design strategy is developed, and codes for finite-element analysis are embedded in this design strategy to ensure the calculation accuracy. A genetic algorithm (GA) is used to find the optimal designs. The influences of machine types and diameter to the machine performances are presented and discussed. Furthermore, the laboratory test of an existing ironless axial-flux permanent-magnet generator confirms the high accuracy of the field and inductance calculations of this design strategy, and the comparison with the parametric study is conducted to demonstrate the excellent performance of the GA used.

Magnetic forces and vibration in permanent magnet machines with non-overlapping concentrated windings: A review

Permanent magnet machines with non-overlapping concentrated windings have been gaining importance in the last few years. Significant advantages such as short end-windings, high efficiency and low cogging torque make them an attractive option in several applications. However, due to a large harmonic content in the MMF and also particular pole and slot combinations, the vibration level of these machines is considerably higher than the machines with distributed windings. This paper discusses this problem and reviews the works presented in the literature concerning magnetic forces and vibration in PM machines with concentrated windings.

Analytical design of a high-torque flux-switching permanent magnet machine by a simplified lumped parameter magnetic circuit model

This paper presents how to analytically design a high-torque three-phase flux-switching permanent magnet machine with 12 stator poles and 14 rotor poles. Firstly, the machine design parameters are studied addressing on high output torque and its flux distribution is also investigated by finite-element method (FEM) analysis. Then a simplified lumped parameter magnetic circuit model is built up for analyzing design parameters. And a design procedure is also presented. The analytically designed machine is verified by FEM simulations.

Investigation of a three-phase flux-switching permanent magnet machine for downhole applications

This paper investigates a newly designed flux-switching permanent magnet machine with 12 stator poles and 14 rotor poles for downhole application where the ambient temperature is around 150°C. This machine having an outer diameter of 100 mm and an active axial length of 200 mm can provide ∼ 2.7 kW with an output torque up to 25 Nm, an efficiency of ∼88% and power factor of 0.87. The maximum temperature in the machine is around 200°C without external cooling. Additionally, the machine losses, inductance and magnet demagnetization field are also studied.

Force Analysis in Design of High Power Linear Permanent Magnet Actuator with Gas Springs in Drilling Applications

This paper mainly describes the force analysis in the design optimization of a reciprocating linear permanent magnet actuator with interior magnet and slotless stator. This paper predicts the force characteristics according to design parameters such as pole pitch, magnet width and stator inner diameter on the basis of numerical calculations. The design is unique because of the combination of a heavy piston and gas springs. The gas springs allow the piston to oscillate at high frequency and with a long stroke length, in spite of pistons large weight. A large electromagnetic force, due to the use of permanent magnets makes a high power linear electric actuator available. The heavy piston causes a considerable vibration in the housing which is utilized in the drilling application. The load can then be attached directly to the housing and the machine can be made hermitically sealed, which eliminate leakage problems. These features make the actuator suitable as a hammer in oil drilling applications.

Magnetic Equivalent Circuit Modeling of the AC Homopolar Machine for Flywheel Energy Storage

This paper develops a magnetic equivalent circuit model suitable to the design and optimization of the synchronous ac homopolar machine. The ac homopolar machine is of particular interest in the application of grid-based flywheel energy storage, where it has the potential to significantly reduce self-discharge associated with magnetic losses. The ac homopolar machine features both axial and radial magnetizing flux paths, which requires finite element analysis to be conducted in 3-D. The computation time associated with 3-D finite element modeling is highly prohibitive in the design process. The magnetic equivalent circuit model developed in this paper is shown to be a viable alternative for calculating several design performance parameters and has a computation time which is orders of magnitude less than that of 3-D finite element analysis. Results obtained from the developed model are shown to be in good agreement with finite element and experimental results for varying levels of saturation.