Juha Pyrhönen

Modeling of axial flux permanent-magnet machines

In modeling axial field machines, three-dimensional (3-D) finite-element method (FEM) models are required in accurate computations. However, 3-D FEM analysis is generally too time consuming in industrial use. In order to evaluate the performance of the axial flux machine rapidly, an analytical design program that uses quasi-3-D computation is developed. In this paper the main features of the developed program are illustrated. Results given by the program are verified with two-dimensional and 3-D finite element computations and measurements. According to the results, it is possible to evaluate the performance of the surface-mounted axial flux PM machine with reasonable accuracy via an analytical model using quasi-3-D computation.

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.

Performance of Low-Cost Permanent Magnet Material in PM Synchronous Machines

Permanent magnet synchronous machines (PMSM) are considered a viable option in various types of applications. However, particularly in consumer and low-power industrial applications, the price may be a factor that limits the use of PMSMs. In addition to a different technology, the main reason for the high price of PMSMs is the use of expensive neodymium or samarium-cobalt magnets. Their use is necessary only if a high motor torque T to linear current density A ratio (T/A) is required. Ferrite permanent magnets are low cost, abundant, and have negligible eddy current losses in low-frequency applications such as motor drives. They have a much lower energy product (BHmax) than the most modern magnets. Because of the high prices of rare earth magnets, many parties are seeking for opportunities to use ferrites instead. In the case of rotor surface ferrite magnets, the air gap flux density remains low. The air gap torque producing tangential Maxwell stress is proportional to the product of the air gap flux density Bδ[ Vs/m2] and the linear current density A [A/m]. If the flux density is low and A cannot be increased, the rotor has to be made larger than in machines having a high air gap flux density. In the case of multiple pole machines, the outer rotor approach, with its low rotor yoke height, offers an interesting alternative. The air gap diameter of these machines can be made larger than in conventional inner rotor type motors without increasing the machine outer dimensions. In this paper, an outer rotor PMSM with ferrite magnets is analyzed and tested. The machine characteristics in a fan drive are compared with an induction machine of the same power.

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.

Effect of Slot-and-Pole Combination on the Leakage Inductance and the Performance of Tooth-Coil Permanent-Magnet Synchronous Machines

The influence of slot-and-pole-number combinations on the leakage inductance of double-layer fractional-slot concentrated nonoverlapping winding (i.e., tooth-coil winding) permanent-magnet synchronous machines is studied. A Fourier-analysis-based method for calculating the harmonic air-gap leakage inductance applied to the current-linkage waveform (winding function) in the air gap is suggested. Different slot-and-pole combinations are considered, and their influence, in particular, on the air-gap leakage inductance is indicated. In tooth-coil machines, the air-gap harmonic leakage can have a surprisingly large impact on the machine performance.

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.

Performance comparison between low-speed axial-flux and radial-flux permanent-magnet machines including mechanical constraints

Performance comparison between different machine topologies is not a straightforward task since many variables exist if electromagnetic, thermal and mechanical aspects are taken into account. In this paper some methods to take into account relevant mechanical constraints in the performance comparison are proposed. A comparison study between low-speed axial-flux permanent-magnet machines and radial-flux permanent-magnet machines is provided with introduced mechanical constraints, respectively

Lumped-Parameter Thermal Model for Axial Flux Permanent Magnet Machines

A lumped-parameter thermal model is presented for axial flux permanent magnet (AFPM) machines. The model provides the steady-state thermal solution to derive the temperatures at different parts of the machine, including the temperatures in the stator windings and the temperature of the magnets. Temperature-dependent thermal properties of the materials used in the machine construction as well as the stator winding resistance and consequently copper losses require temperature update in accurate design of the machine. Therefore, an iterative coupled electromagnetic and thermal design program is proposed in this study. A 5-kW AFPM generator is designed using the proposed program with regarding the thermal behavior of the machine. Tests performed on the designed machine verify that the defined thermal resistance network has a high ability to predict the nodal temperatures accurately.