Arne Nysveen

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.

Direct electrical heating of subsea pipelines - technology development and operating experience

The formation of hydrates in the subsea production of oil and gas is a well-known problem. As the unprocessed well stream cools down, hydrates start to form around 25 degC, depending on the water cut and pressure in the pipeline. Several solutions are available to solve this problem. Generally, chemicals (i.e., methanol) have been used. Methanol reduces the critical temperature where hydrates are formed. Alternatively, hydrates can be prevented by using thermal insulation in combination with direct electrical heating (DEH). Thus, the well stream is kept above the critical temperature for hydrate formation. DEH heats the pipeline by forcing a large electric current to flow through the pipeline steel. The system model for design and sizing of the system is presented. DEH uses a single-phase system where the heated pipeline is electrically connected to the surrounding sea water. Thus, the system current is divided between sea water and pipeline, requiring additional sacrificial anodes on the pipeline. The anode system for a pipeline with DEH is discussed. There are currently more than 100 km of DEH pipelines on the Norwegian Continental Shelf. The operating experience from these installations is discussed. This paper presents the research and development for application of the system for pipelines with lengths up to 50 km

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.

A Review of Results From Thermal Cycling Tests of Hydrogenerator Stator Windings

Thermal cycling of generators, occurring during intermittent operation of power stations, increases the thermomechanical stress on the stator groundwall insulation and can, therefore, contribute to failure of the generator. Thermal cycling stresses can, to some extent, be replicated and accelerated in laboratory tests. Such tests are performed to ascertain whether a new stator winding insulation system can handle intermittent operation, to evaluate the relative difference between different types of insulations or to investigate degradation processes and failure mechanisms. Results from thermal cycling tests of hydrogenerator stator windings are reviewed. Only stator windings of air-cooled hydrogenerators with thermoset groundwall insulation of mica-epoxy or mica-polyester are considered. The published literature of laboratory experiments performed in order to study effects of thermal cycling on groundwall insulation through different types of diagnostic measurements is described and discussed. Partial discharge (PD) and tanδ measurements gave, in some cases, an indication of a trend in the condition during thermal cycling, while in other cases it was difficult to interpret the test results. Many agree that measurement of breakdown voltage of stator windings can give an indication of the aging and remaining lifetime of the insulation. The ability of other measurements such as tan δ, PD, and voltage endurance to assess the remaining lifetime of the stator winding is less convincing since the results vary greatly.

Charging of dielectric barriers in rod-plane gaps

The electric properties of an air gap between a hemisphere-rod covered with 3 mm thick silicone rubber and plane metal electrode have been examined. Observation of discharge activity and measurement of the discharge current have been made under impulse voltage stress. When applying an impulse voltage close to the inception voltage, discharge activity is observed around the hemispheric tip of the rod. Observed light emission seems to be evenly distributed along the hemispheric tip comprising a number of independent discharges similar to discharges in a homogeneous electrode configuration. Applying a conductive layer at the rod tip reduces the discharge activity and measured discharge current considerably. The discharge activity was concentrated close to the tip end of the rod. Relaxation time constant for the deposited charge was measured to be about 4 h. By comparing conductivity of the insulation material, the dominating relaxation mechanism was found to be conduction through the solid insulation.

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.