Janne Nerg

Thermal Analysis of Radial-Flux Electrical Machines With a High Power Density

A lumped-parameter-based thermal analysis applicable to radial-flux electrical machines with a high power density is presented. The modeling strategies using T-equivalent lumped-parameter blocks as well as conventionally defined thermal resistances are discussed. Special attention is paid to the modeling of the convective heat transfer in the air gap of radial-flux electrical machines at different rotational speeds. A brief overview of the evaluation of different loss components is given. The performance of the developed thermal model was verified by comparing the calculated temperature values with the measurements in three different applications.

Direct-Driven Interior Magnet Permanent-Magnet Synchronous Motors for a Full Electric Sports Car

The design process of direct-driven permanent-magnet (PM) synchronous machines (PMSMs) for a full electric 4 × 4 sports car is presented. The rotor structure of the machine consists of two PM layers embedded inside the rotor laminations, thus resulting in some inverse saliency, where the q-axis inductance is larger than the d-axis one. An integer slot stator winding was selected to fully take advantage of the additional reluctance torque. The performance characteristics of the designed PMSMs were calculated by applying a 2-D finite-element method. Cross saturation between the d- and q-axes was taken into account in the calculation of the synchronous inductances. The calculation results are validated by measurements.

Dynamic Torque Analysis of a Wind Turbine Drive Train Including a Direct-Driven Permanent-Magnet Generator

Mechanical interactions between a wind turbine and a direct-driven permanent-magnet synchronous generator (PMSG) are studied, and a model to analyze the system behavior is suggested. The proposed model can be applied to analyze the mechanical vibrations of direct-driven wind turbine installations both in steady state and in dynamic cases. The cogging torque and torque ripple of the PMSG are used as excitation sources in the mechanical model. Four different permanent-magnet rotor constructions are analyzed. It is shown that the maximum allowable value of the cogging torque of the direct-driven permanent-magnet wind generator in this case is 1.5%-2% of the rated torque even when the corresponding resonance frequency does not occur in the operational speed range of the wind turbine. Furthermore, it was noticed that the resonance caused by the excitation torque should occur at the lowest possible speed.

High-Speed High-Output Solid-Rotor Induction-Motor Technology for Gas Compression

This paper investigates the suitability of solid-rotor induction-motor technology for a natural-gas-compression application with a high power output. To this end, a new solid-rotor design for an 8-MW 6.6-kV 12 000-min-1 motor without any copper parts in the rotor was developed, and the motor performance was tested. In this paper, solid-rotor material selection, rotor slitting, and the end effects of the purely solid rotor are discussed. A frequency-dependent end-effect correction factor is introduced, and a method for the rotor-end-leakage correction is presented. The performance of the proposed end-effect correction factor is verified by comparing the calculated torque and power factor with the measured values.

Harmonic Loss Calculation in Rotor Surface Permanent Magnets—New Analytic Approach

The paper introduces an analytic permanent-magnet eddy-current loss calculation method for rotor surface permanent magnets. The problem has been studied widely in the literature, yet many of the suggested methods are difficult to use. A straightforward analytic solution is needed to speed up the motor preliminary design as present day computational capabilities are not fast enough to be used for 3-D time-stepped finite element analysis (FEA) of permanent-magnet eddy-current losses in every day design problems. An analytic approach is suggested to calculate different-harmonics-caused eddy-current losses. The results are compared both with 3-D FEA and measurement results. A comparison between the theoretical and experimental results is reported for an axial-flux two-stator single-rotor machine with no rotor yoke. The method has, however, been used in evaluating permanent-magnet losses also in machines with narrow slot openings where the flux density dips do not penetrate through the whole magnet or in machines having a laminated rotor yoke.

Permanent-Magnet Length Effects in AC Machines

Permanent-magnet synchronous machines (PMSMs) are gaining popularity in the industry and, especially, in distributed generation, for instance in windmill applications. Some of the traditional analytical design principles seem not to be valid for permanent magnets in machine excitation. We report on the effects of the permanent-magnet length in machine design.

Numerical solution of 2D and 3D induction heating problems with non-linear materml properties taken into account

A numerical calculation model for the solution of 2D and 3D induction heating problems, which takes the nonlinearities of both the electromagnetic and thermal material properties into account, is described. The solution of a 2D-coupled field problem is found by traditional FEM. In a 3D analysis nonlinear surface impedances are utilized in the magnetic field problem and the power transfer to the workpiece is modeled using heat fluxes. The performance of the model was verified by comparing the calculated temperature profiles with the measurements.

FEM calculation of rotor losses in a medium speed direct torque controlled PM synchronous motor at different load conditions

Summary form only given. In direct torque control (DTC), the switching frequency of the converter is not constant but it varies according to optimal switching condition. For example, when driving a permanent magnet synchronous motor at no load operation, the bandwidth of the switching frequency can be several kHz which cause high distortion of stator currents and, therefore, increase in rotor losses. In this work rotor losses of a medium speed permanent magnet synchronous motor driven by a direct torque controlled electrical drive were calculated using 2D time-stepping finite element method coupled with the motion equation. Circuit equations were used in the model in order to take the effects of the distorted phase currents into account. Calculations were performed at different load conditions.

Hysteresis Losses in Sintered NdFeB Permanent Magnets in Rotating Electrical Machines

Permanent magnet (PM) materials are nowadays widely used in the electrical machine manufacturing industry. The eddy-current loss models of PMs used in electrical machines are frequently discussed in research papers. In magnetic steel materials we have, in addition to eddy-current losses, there are hysteresis losses when alternating current or a rotating flux travels through the material. Should a similar phenomenon be also taken into account in calculating the losses of PMs? Every now and then, authors seem to assume that some significant hysteresis losses are present in rotating machine PMs. This paper studies the mechanisms of possible hysteresis losses in PMs and their role in PMs when used in rotating electrical machines.

AC Resistance Factor of Litz-Wire Windings Used in Low-Voltage High-Power Generators

New types of heavy litz wires with noninsulated or insulated strands are offered for high-power low-voltage electrical machines, where skin and proximity effects can cause serious problems if traditional windings are used. This paper evaluates the ac resistance of a litz wire and its usability in megawatt-range low-voltage electrical machines. The evaluation is made by finite-element analysis and by resistance measurements with an experimental test setup. A simple amplifier configuration was used to minimize current and voltage phase shift errors, which are critical in this kind of low-impedance and low-frequency measurement. A simple measurement device based on the aforementioned configuration was prepared to ensure a precise measurement result. Tests were done in a frequency range of 50-200 Hz, to cover a wide range of practical high-power low-voltage electrical machines. It is found that the measured litz wire with originally noninsulated strands can be used in large electrical machines up to about 120 Hz if about 50% increase in ac resistance compared to the dc resistance is allowed. Impregnation of the originally noninsulated litz wire with vacuum pressure impregnation improved the ac resistance in one test case, and in the other case, it seemed to have no effect on the ac resistance.