High-speed Permanent Magnet Machines Research Articles

Advanced Cooling Methods for High-Speed Electrical Machines

High-speed electrical machines are gaining increasing attention, as they enable higher power densities in several applications such as micromachining spindles and turbo compressors. This brings along an important challenge in thermal management due to the higher loss densities in the machine. Therefore, a careful thermal analysis is required along with the electromagnetic and mechanical considerations during the design phase of the machines. In this paper, different forced cooling options are compared for a slotless-type high-speed permanent-magnet machine. Fast, yet sufficiently accurate thermal models are derived for analyzing these cooling concepts. This enables their coupling with electromagnetic models and incorporation into the machine optimization procedure, which would not be feasible when using computationally very intensive methods such as three-dimensional finite element method or computational fluid dynamics. The developed thermal models are first verified on mechanically simplified stator designs (in which no rotor coupling is possible), and later on fully functional high-speed electrical machine prototypes. Using an integrated cooling method instead of a standard cooling jacket, the power density can be nearly doubled while keeping the maximum winding temperature below 80 °C, without altering the rotor or the stator core geometries.

Multi-objective optimization design of a high-speed PM machine supported by magnetic bearings

This paper proposes an optimal design method of permanent magnet machine (PMM) with cylindrical permanent magnet supported by magnetic bearings. The objectives of optimization design are minimizing the rotor loss while maximizing the power density of the PMM as well as the 1st order nature frequency of the rotor, and the constraints are size, the strength safety factor and the phase current. A 30kW, 48,000r/min PMM designed by the multi-objective optimization method is proposed and the results indicate: the rotor loss is decreased from 393W to 290W (is reduced by 26.2%); the power density of the PMM is increased from 1.86kW/kg to 2.19kW/kg (is increased by 17.7%); the 1st order nature frequency of the rotor is increased from 1579Hz to 1812Hz (is increased by 14.7%). The performances of the PMM are improved after optimization, which are verified by experiment.

Evaluation of the electrodynamic forces in high-speed permanent magnet machines with rotor eccentricity

In permanent magnet motors, the presence of rotor eccentricities can alter the airgap field distribution. This results in parasitic radial detent forces that can be reduced by connecting the stator phases in parallel. As a consequence, currents are passively induced in the windings when the rotor spins in an off-centered position, yielding balancing electrodynamic forces. Specific models were developed to predict these forces, but their complexity can be prohibitive. Therefore, this paper proposes to study the effect of the rotor off-centering in permanent magnet motors using a simpler model developed for electrodynamic bearings. This model consists in a linear differential equation with only four parameters that depend neither on the spin speed nor on the rotor position. As an illustration, the paper applies this model to the study of a high-speed, slotted permanent magnet motor. To support this, the main hypotheses of the model are validated in this particular case. Then, the centering electrodynamic forces in a staticeccentricity configuration are predicted using the model and compared to finite element simulation results. Finally, a preliminary study showing the impact of the width and permeability of the stator teeth on the centering force is performed.

Magnet loss analysis for a high-speed PM machine with segmented PM and modified tooth-tips shape

Abstract This paper presents the loss-oriented performance analysis of a radial highspeed permanent magnet (PM) machine with concentrated windings for automotive application. The PM synchronous machine was designed for an operating frequency up to 800 Hz. The main aim of this paper is to analyse the selected methods for magnet eddycurrent loss reduction. The first approach to rotor modification regards magnet segmentation in circumferential and axial directions. The second approach is based on changes in tooth-tips shape of the stator. The best variants of tooth-tip shapes are determined for further investigation, and adopted with a rotor having magnet segmentation. It is found that the machine with a segmented magnet leads to magnet loss reduction by 81%. Further loss reduction by 45% can be realized with the proposed tooth-tip shape. Additionally, owing to the stator and rotor modifications, the main machine parameters are investigated, such as back-EMF, electromagnetic torque, torque ripple and cogging torque. The 2-D and 3-D finite element analysis (FEA) is used for electromagnetic analysis. An experimental approach based on a partially wound stator is employed to verify the 3-D FEA.

A 32 000 r/min Axial Flux Permanent Magnet Machine for Energy Storage With Mechanical Stress Analysis

This paper focuses on the design and analysis of a high-speed axial flux permanent magnet (PM) machine for an aerospace flywheel energy storage system. The design target is to experimentally verify the sinusoidal back electromotive force (EMF) considering the mechanical stress limitation of the machine at a speed of 32 000 r/min. Two machine models based on ferrite and SmCo PMs have been proposed. First, the back EMF analysis using a 3-D finite-element method is performed to determine the performance of the two models. Second, the mechanical stress analysis is performed to check the durability of the rotor strength. Finally, the selected model utilizing the ferrite PMs has been manufactured and tested at 32 000 r/min to validate its effectiveness at high-speed operation.

Analysis of end-winding proximity losses in a high-speed PM machine

Abstract This paper presents a finite element investigation into the proximity losses in a high-speed permanent magnet (PM) machine for traction applications. A three-dimensional (3D) finite element analysis (FEA) is employed to evaluate and identify the endwinding contribution into the overall winding power loss generated. The study is focused on the end-winding effects that have not been widely reported in the literature. The calculated results confirm that the end-winding copper loss can significantly affect the eddycurrent loss within copper and it should be taken into account to provide reasonable prediction of total losses. Several structures of the end-winding are analyzed and compared in respect to the loss and AC resistance. The results clearly demonstrate that the size of the end-winding has a significant impact on the power loss. The calculated results are validated experimentally on the high-speed permanent magnet synchronous machine (PMSM) prototype for selected various winding arrangements.

3-D Analytical Model of Helical Winding PM Machines Including Rotor Eddy Currents

Helical windings (or zigzag windings) are used in a number of applications, however, in electrical machines, mainly employed in low-power, high-speed permanent magnet (PM) brushless dc machines due to the cost effectiveness of the winding type while maintaining reasonable performance. Typically, helical windings are used for low-voltage applications due to their spiral form, which makes them most suitable for a small number of turns. In high-speed electrical machines, such a low number of turns are applicable. It is apparent that high-speed PM machines suffer from rotor eddy-current losses, which in some cases may lead to PM demagnetization due to overheating. The performance of the machine is compromised by these losses; hence, they have to be considered during the design procedure. There are many papers analyzing the magnetic field of these machines employing helical windings; however, none of them present a simple and precise electromagnetic model of a machine with the helical winding. This paper presents an analytical approach to model the resulting 3-D magnetic field of the helical winding, considering eddy currents in the conducting media of the rotor. The model is verified with the 3-D finite-element method by means of comparing magnetic field and rotor eddy-current losses.

The Effect of PWM on Rotor Eddy-Current Losses in High-Speed Permanent Magnet Machines

High-speed permanent magnet machines, supplied by pulsewidth modulation (PWM) voltage source inverters, operate with distorted stator currents. Harmonics present in these stator currents deteriorate the machine performance by generating losses. Mostly, these losses are following the machine design using a transient finite-element model. The precise measurement of these rotor eddy-current losses is extremely difficult, hence, only a few papers provide convincing comparisons between predictions and measurements. This paper presents a fast and precise analytical approach, verified with measurements, to consider rotor losses of machines, supplied by PWM voltages, already during the design procedure.

Rotor Retaining Sleeve Design for a 1.12-MW High-Speed PM Machine

Permanent-magnet (PM) synchronous machines (PMSMs) can provide excellent performance in terms of torque density, energy efficiency, and controllability. However, PMs on the rotor are prone to centrifugal force, which may break their physical integrity, particularly at high-speed operation. Typically, PMs are bound with carbon fiber or retained by alloy sleeves on the rotor surface. This paper is concerned with the design of a rotor retaining sleeve for a 1.12-MW 18-kr/min PM machine; its electromagnetic performance is investigated by the 2-D finite-element method (FEM). Theoretical and numerical analyses of the rotor stress are carried out. For the carbon fiber protective measure, the stresses of three PM configurations and three pole filler materials are compared in terms of operating temperature, rotor speed, retaining sleeve thickness, and interference fit. Then, a new hybrid protective measure is proposed and analyzed by the 2-D FEM for operational speeds up to 22 kr/min (1.2 times the rated speed). The rotor losses and machine temperatures with the carbon fiber retaining sleeve and the hybrid retaining sleeve are compared, and the sleeve design is refined. Two rotors using both designs are prototyped and experimentally tested to validate the effectiveness of the developed techniques for PM machines. The developed retaining sleeve makes it possible to operate megawatt PM machines at high speeds of 22 kr/min. This opens doors for many high-power high-speed applications such as turbo-generator, aerospace, and submarine motor drives.

Performance Assessment of High-speed Flux-switching Permanent Magnet Machine Using Ferrite and Rare Earth Permanent Magnet Materials

This article presents the investigation and comparison of the capabilities and characteristics of high-speed flux-switching permanent magnet machines using ferrite and rare earth permanent magnet materials. Special design considerations for ferrite flux-switching permanent magnet machines under high-speed operation are proposed. Two ferrite and one NdFeB flux-switching permanent magnet machines, rated at 10 kW and 15 krpm, were designed for performance assessment. Various machine performances, including torque characteristics and loss distribution, are compared. In addition, the risk of partial demagnetization and the performance degradation due to partial demagnetization are also investigated. This research reveals several promising characteristics of the ferrite flux-switching permanent magnet machine, such as lower total loss, negligible magnet eddy current loss, and better cooling potential. The results also show the possibility of designing a high torque density ferrite flux-switching permanent magnet machine, which is comparable to the NdFeB flux-switching permanent magnet machine, taking advantage of the flux concentrating effect.

Inductance calculation of high-speed slotless permanent magnet machines

Purpose – The purpose of this paper is to give a simple, fast and universal inductance calculation approach of slotless-winding machines and comparison of inductances of toroidal, concentrated and helical-winding machines, since these winding types are widely used among low-power PM machines. Design/methodology/approach – Harmonic modeling approach is applied to model the magnetic field of the windings in order to calculate the synchronous inductances. The method is based on distinction between electromagnetic properties of different regions in the machine where each region is represented by its own governing equation describing the magnetic field. The governing equations are obtained from Maxwell’s equations by introducing vector potential in order to simplify the calculations. Findings – Results of the inductances of toroidal, concentrated and helical-winding slotless PM machines, which have the same torque and dimensions, obtained by the proposed analytical method are in good agreement with 3D FEM, where the relative difference is smaller than 15 percent. However, the calculation time of the analytical method is significantly less than in 3D FEM: seconds vs hours. Additionally, from the results it is concluded that the toroidal-winding machine has the highest inductance and DC resistance values among considered machines. Helical-winding machine has lowest inductance and DC resistance values. Inductance of concentrated-winding machine is between inductance of helical and toroidal windings; however, DC resistance of the concentrated windings is comparable with resistance toroidal windings. Originality/value – In this paper the inductance calculation based on harmonic modeling approach is extended for toroidal and helical-winding machines which makes the method applicable for most of the slotless machine types.

Loss Calculation and Software Design Based on the Combination of Electric Circuit and Electromagnetic Field Solution in High Speed Permanent Magnet Machine

Mathematics model of loss for high speed machine is discussed. Calculation method of core loss and high-frequency additional copper loss of winding based on finite element analysis is introduced. Experiment results of losses are presented. In order to realize calculation and analysis, a method of PM machine by calling ANSYS and MATLAB based on VB of the combination of electric circuit and electromagnetic field solution is presented in this paper. The design and feature analysis of high speed PM machine takes an example to validate the accuracy and advantage.

Analysis and neutral voltage based detection of inter-turn faults in high-speed permanent magnet machines with parallel strands

This paper analyzes short circuits in high-speed permanent-magnet (PM) machines. Analytical models are used to estimate the consequences of single-strand, parallel-strand, and phase-to-phase turn-to-turn short circuits, as well as open-circuit faults. The parameters for the analytical models are obtained from 3-D finite-element simulations and consider the actual position of each strand inside the slot. It is shown that parallel-strand short circuits are less severe than single-strand short circuits, both before and after fault mitigation actions. The potential for large fault current densities in high-speed PM machines is also demonstrated, with measured rms current densities up to 450 $\hbox{A/mm}^{2} $ . A simple fault detector based on the synchronous detection of the machine and inverter neutral voltage difference is proposed, which is suitable for hardware implementation and can function as an independent observer of a drive system. The dependence of the detectability on the short circuit location and the effects of natural imbalances on the detector output are investigated. Experiments are performed to demonstrate the concept, showing that faults of at least 0.4% can be detected.

Study of the Copper Losses in a High-Speed Permanent-Magnet Machine With Form-Wound Windings

This paper investigates the profile of the copper losses in a 5.0-MW high-speed permanent-magnet machine with form-wound winding. The machine was tested, and the test data were used in conjunction with the results obtained from finite-element simulations to calculate the copper losses over the complete speed range. A retardation test in open-circuit conditions was performed to calculate the eddy current loss induced inside the stator coils by the rotor magnetic flux. The results demonstrate that this component of the copper losses is significantly high in form-wound machines at high-speed operation. The impact of the slot configuration on the proximity effect was also investigated. Open and semiclosed slots were compared, and the study results show that the high flux concentration in the slot opening region associated with semiclosed slots increases the proximity copper loss compared with the open-slot topology. At the end, this paper proposes the stator-alone test as a valid method to calculate resistance per phase at high frequency and estimates its increment over the direct current resistance.

High speed permanent magnet machine design with minimized stack-length under electromagnetic and mechanical constraints

Certain aircraft and military applications require high speed machines with low stack-length and the lowest possible weight. Hence, the accommodation of the highest bore diameter may seem the natural option. However, a rotor design with high diameter results in significant increase in mechanical stresses in the employed magnet retention. In a sleeved magnet retention mechanism, the sleeve thickness can be increased in order to accommodate the stresses. However, this will result is significant drop in air-gap flux-density and will not yield the high power density expected by the machine. This paper presents an analytical technique that combines the sleeve stress model and the air-gap flux density model to calculate the optimal rotor diameter to achieve the maximum power of the machine design for a minimum stack-length. The technique is applied to both a Carbon Fibre sleeve version and a metallic sleeve version. The analytical calculation of the stresses is validated with mechanical finite-element simulations. The machine design with the analytical calculations is validated with electromagnetic finite element simulations. The results confirm the rotor design strategy and the design technique.

Influence of Rotor-Sleeve Electromagnetic Characteristics on High-Speed Permanent-Magnet Generator

Alloy rotor sleeves are used extensively in high-speed permanent-magnet machines since they could fasten the permanent magnets availably. However, it is inevitable that eddy-current losses will be generated in the sleeve, which may make the permanent magnets overheated. In order to reduce the rotor eddy-current losses, this paper focuses on the influence of the rotor-sleeve electromagnetic characteristics. Taking a high-speed permanent-magnet generator (HSPMG) as an example, the variations of output performance and rotor eddy-current losses were analyzed when the generator adopts the steel sleeve and the copper-iron alloy sleeve, respectively, and therewith, the principles of the variations were exposed. Then, the rotor eddy-current losses were further analyzed when the generator sleeve conductivity was changed. The worst range of the sleeve conductivity was also given, in which the rotor eddy-current losses were the largest. Additionally, the increase of the sleeve permeability could reduce the main magnetic circuit reluctance and improve the operating point of permanent magnets. On the other hand, it can increase the pole-to-pole flux leakage dramatically. So, the generator performance should be analyzed by considering the two factors when the rotor-sleeve permeability was different.

An Active Magnetic Damper Concept for Stabilization of Gas Bearings in High-Speed Permanent-Magnet Machines

The successful application of ultrahigh-speed electrical-drive systems in industrial products is currently limited by lacking high-speed bearing technologies permitting high reliability and long lifetime. Promising bearing technologies for high rotational speeds are contactless bearing concepts such as active magnetic bearings or gas bearings. While magnetic bearings usually are major electromechanical systems with substantial complexity, gas bearings allow compact realizations with high load capacity and stiffness; however, poor dynamic stability has been limiting their use at high rotational speeds. Following a hybrid bearing approach with an aerodynamic gas bearing for load support, a small-sized active magnetic damper concept is proposed to enable the stable high-speed operation of the gas bearing with a minimum of additional complexity and costs. As for the effective stabilization of the gas bearing, a high-quality displacement measurement is essential, and a new eddy-current-based rotor-displacement self-sensing concept employing an auxiliary signal injection and rotor displacement measurement circuit is presented. A hardware implementation of the proposed concept is shown providing high-resolution measurement signals.

Application of Soft Magnetic Composite Material in the Field of Electrical Machines

The property of the soft magnetic composite (SMC) material is introduced. Take the traditional material used for electric machine, i.e. the silicon steel lamination, as the reference, this paper summarizes the application advantages of SMC in the typical electric machine, such as tubular permanent-magnet liner machine, high-speed permanent-magnet machine, claw-pole machine, axial flux permanent-magnet machine and radial flux permanent-magnet machine. SMC material has much applied potential in the field of electric machine.

Rotor Combined Design for High-Speed PM Machines

It is quite different to design a high-speed and high frequency machine than designing a common machine. Strength and dynamic characteristic analysis of rotor are key techniques for high-speed permanent magnet (PM) machine. Rotor combined design should be investigated based on mechanical with electromagnetic and dynamic properties analysis. The finite element method is used to analyze stress and electromagnetism field and dynamic characteristic. The multi-physics field iteration design is carried out by optimization analysis of the finite element software. A high-speed PM machine with rated speed of 60000 rpm, 75kW is presented with this method. The experimental measurements validate the correctness of this method.

Design and Analysis of Fan-Cooling for High Speed Permanent Magnet Machine Rotor

Compared with common permanent magnet (PM) machine, the high speed PM machine has smaller size and larger power density. However, owing to high-speed high frequency and small volume, the PM rotor is more easily to become overheated so that irreversible demagnetization of the PM is induced. Traditional cooling methods of machine directly cool the machine stator, the temperature rise of rotor is reduced through heat exchange among stator, air gap and rotor. Owing to rapid temperature rise of high speed PM machine rotor, the method of indirect cooling rotor does not effectively protect PM rotor from overheating. To directly reduce the temperature rise of PM rotor and the volume of machine, design method of fan-cooling for high speed PM machine rotor is proposed in this paper, that is, high speed axial fan rotating synchronously with the high speed PM machine rotor is designed. Finally, the temperature rise of high speed PM machine rotor with the structure of high speed axial fan is analyzed based on the coupling method of fluid-solid, the validity is verified.