Rotor Eddy Current Loss Research Articles

Rotor Eddy Current Losses Reduction in an Axial Flux Permanent Magnet Machine

A novel rotor structure for a tooth-coil-winding (concentrated winding) open-slot axial-flux permanent-magnet (PM) machine that reduces the eddy currents and increases the main flux linkage is proposed in this paper. A composite-body-based rotor assembly and a steel-laminated layer inserted on top of the magnets are used to reduce eddy-current losses in the PMs. A prototype of an axial flux generator for a heavy-duty vehicle following this structure is optimized, analyzed and measured. The tooth coil windings produce high-amplitude harmonics in the air gap and the large slot openings reduce the rotor flux. Consequently, a rotor structure that decreases the amount of harmonics travelling through the magnets is required. Steel laminations covering the magnets have positive effects on both phenomena. Two-dimensional and three-dimensional finite-element analysis is used to verify and optimize the geometry of the laminations. Computations are verified by experimental measurements of a 100 kW test machine prototype.

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.

Analysis of Iron Loss in Interior PM Machines With Distributed Windings Under Deep Field Weakening

This paper investigates the rotor cavity positioning and the combination of stator slot and rotor effective slot number on the eddy-current loss for interior permanent-magnet/reluctance machines operating under deep field-weakening conditions. A new closed-form expression for the stator and rotor eddy-current loss is developed. The optimal barrier-positioning for the minimum total loss and the effect on the eddy-current loss of varying the stator slot and rotor effective slot number are investigated for one-, two-, three-, and full-layered rotors.

Influence of Rectifiers on High-Speed Permanent Magnet Generator Electromagnetic and Temperature Fields in Distributed Power Generation Systems

A power converter is very necessary in the system of high-speed permanent magnet generators (HSPMG), since the frequency of the output electrical energy is generally hundreds of Hertz or even more than 1000 Hz. However, the converter with switches in fast on–off transitions will have some effects on the generator and, therewith, some current harmonics will be also induced in the device. Taking a 117 kW 60 000 r/min HSPMG as an example, the harmonics in this generator were analyzed. And then, the voltage harmonic content, the current harmonic content, and the current total harmonic distortion could be obtained. In addition, stator core losses and rotor eddy current losses were analyzed comparatively, and the influence of the converter on the generator power factor was further studied. Based on the 3-D coupling field between fluid and temperature, the temperature distributions were obtained when the generator was connected to different loads, and the influence of the rectifier on the generator temperature field could be obtained, which was also compared with the test data.

Rotor eddy loss in high‐speed permanent magnet synchronous generators

Analytical and transient finite element analysis (FEA) methods are used to calculate on-load rotor eddy current power loss in permanent magnet machines taking into account the effect of the reaction of eddy currents. The effect of the phase advance angle between the back electromotive force, and the phase current on rotor eddy current power loss is investigated. Both magnet flux tooth ripple and magnetomotive force harmonics are considered in the analytical calculation of rotor loss; the harmonics of the magnet tooth ripple flux and armature reaction flux are added vectorially. Good agreement between analytical and FEA results is observed when the stator laminations are assumed to have a linear BH curve. However, a significant discrepancy is observed between the analytical and FEA solutions when a non-linear BH curve is used.

Low Space Harmonics Cancelation in Double-Layer Fractional Slot Winding Using Dual Multiphase Winding

One of the main drawbacks of nonoverlapped coils in fractional slot concentrated winding permanent magnet (PM) machines are the high eddy current losses in both rotor core and permanent magnets induced by the asynchronous harmonics of the armature reaction field. It has been shown in the literature that the reduction of low space harmonics can effectively reduce the rotor eddy current losses. This paper shows that employing a combined star-delta winding to a three-phase PM machine with fractional slot windings and with a number of slots equal to 12, or its multiples, yields a complete cancellation to the fundamental magneto-motive force (MMF) component, which significantly reduces the induced rotor eddy current. Besides, it offers a slight increase in machine torque density. A case study on the well-known 12-slot/10-pole PM machine is conducted to explore the proposed approach. With the same concept, the general n-phase PM machine occupying 4n slots and with a dual n-phase winding is then proposed. This configuration offers a complete cancelation of all harmonics below the torque producing MMF component. Hence, the induced eddy currents in both rotor core and magnets are significantly reduced. The winding connection and the required number of turns for both winding groups are also given. The concept is applied to a 20-slot/18-pole stator with a dual five-phase winding, where the stator winding is connected as a combined star/pentagon connection. The proposed concept is assessed through a simulation study based on 2-D finite element analysis.

Electromagnetic and Thermal Analysis of Open-Circuit Air Cooled High-Speed Permanent Magnet Machines With Gramme Ring Windings

This paper investigates the electromagnetic and thermal performances of the open-circuit air cooled high-speed permanent magnet motor (HSPMM) with Gramme ring windings. Take a 36 kr/min, 75 kW HSPMM as an example, the electromagnetic losses, in particular the rotor eddy current loss and the stator iron loss of the motor are calculated. The influences of switching frequency and air gap length on these losses are studied. Then, a computational fluid dynamics (CFD) model is composed to calculate its coolant flow parameters and the temperature distribution. Based on the CFD results, a lumped-parameter thermal network (LPTN) is created and verified. Then, a 24 kr/min, 140 kW prototype with the same structure and cooling configuration is designed using the LPTN. Experiments show that the calculated results are with reasonable accuracy. Finally, a sensitivity analysis is applied to the 140 kW prototype based on the LPTN to disclose the influences of different parameters on the rotor permanent magnet temperature rise.

Reduction of Rotor Eddy-Current Losses for Surface PM Machines With Fractional Slot Concentrated Windings and Retaining Sleeve

High strength nonmagnetic metallic retaining sleeve is widely used in surface permanent magnet (SPM) machines for mechanical reliability, especially to high speed machines. Rotor eddy-current losses would be significantly increased in fractional slot concentrated winding (FSCW) SPM machines using sleeve. In this paper, the novel structure with copper damping rings inserted in rotor lamination is proposed. A current will be induced in the damping rings in reaction to the nonsynchronous harmonics field in the rotor, therefore, the eddy-current losses in sleeve and magnet will be reduced. FSCW SPM machines with common slot and pole combination such as 9-slot/8-pole, 12-slot/10-pole, and 24-slot/22-pole are investigated by the finite element analysis tools, and simulation results indicate that the eddy-current loss in sleeve, particularly on the single layer windings, is significantly reduced using the proposed structure.

A Generic Approach to Reduction of Magnetomotive Force Harmonics in Permanent-Magnet Machines With Concentrated Multiple Three-Phase Windings

This paper proposes a generic approach to reduction of magnetomotive force (MMF) harmonics in permanent-magnet machines with multiple three-phase concentrated windings. Analytical equations for the MMF distributions of the proposed multiple three-phase concentrated windings are derived, and the principle for harmonic cancellation is established and validated by finite-element analysis. A new 18-slot 14-pole nine-phase winding machine in which all sub-MMF harmonics are eliminated is presented based on the proposed generic approach. The proposed approach is also applicable to other slot-pole combinations with concentrated windings except for slot number being equal to 1.5 times the pole number. Retaining all the advantages of the concentrated windings, the new technique improves the fundamental winding factor, enhances the fault tolerance capability, and reduces the detrimental effects of lower and higher order MMF harmonics on the machine, such as high rotor eddy-current and iron losses, less reluctance torque, acoustic noise, and vibration.

Calculation of No-Load Rotor Eddy-Current Power Loss in PM Synchronous Machines

Three analytical methods, and finite-element analysis (FEA), are used to calculate no-load flux harmonics traveling with respect to the rotor of a surface permanent magnet machine. Rotor eddy-current losses caused by each harmonic are calculated analytically using a multilayer eddy-current model of the machine, in which each harmonic is represented by an equivalent current sheet on the bore of a slotless stator. Losses are also calculated using transient time-stepping FEA coupled with the equation of motion of the rotor. Significant discrepancies are observed between the results obtained from the different methods and FEA, especially when the level of saturation in the stator core is significant.

Calculation analysis of thermal loss and temperature field of in-wheel motor in micro-electric vehicle

To improve the efficiency and life of in-wheel motor in micro-electric vehicle, thermal loss and temperature field are calculated and analyzed. The mathematical model of thermal loss and temperature field was established, the equivalent model of stator winding was adequately handled, convection heat transfer coefficients was calculated, and the heat distribution of in-wheel motor was analyzed. Winding copper loss, stator and rotor core loss and eddy current loss of permanent magnet were calculated, which were coupled to the temperature field as the heat sources. This paper effectively simplified and dealt with the inner complex radiating coefficient. Three-dimensional finite element model of temperature field was established, and static and transient state temperatures were simulated and analyzed. Overall temperature of the stator region is higher than that of the rotor region. Temperature of stator iron core is basically accord with the temperature of equivalent insulating film, but both are less than the temperature of equivalent winding. The conformity of the measurement results with the final simulation results shows that three-dimensional finite element method is accurate and feasible to analyze thermal loss and temperature distribution of in-wheel motor, which can afford a theoretical basis to optimize the in-wheel motor.

Six-Phase Fractional-Slot-per-Pole-per-Phase Permanent-Magnet Machines With Low Space Harmonics for Electric Vehicle Application

This paper discusses the development of new winding configuration for six-phase permanent-magnet (PM) machines with 18 slots and 8 poles, which eliminates and/or reduces undesirable space harmonics in the stator magnetomotive force. The proposed configuration improves power/torque density and efficiency with a reduction in eddy-current losses in the rotor permanent magnets and copper losses in end windings. To improve drive train availability for applications in electric vehicles (EVs), this paper proposes the design of a six-phase PM machine as two independent three-phase windings. A number of possible phase shifts between two sets of three-phase windings due to their slot-pole combination and winding configuration are investigated, and the optimum phase shift is selected by analyzing the harmonic distributions and their effect on machine performance, including the rotor eddy-current losses. The machine design is optimized for a given set of specifications for EVs, under electrical, thermal and volumetric constraints, and demonstrated by the experimental measurements on a prototype machine.

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.

Study of Multiphase Superconducting Wind Generators With Fractional-Slot Concentrated Windings

Superconducting (SC) generator offers a promising candidate for offshore direct-drive wind generators with power of 10 MW or higher. Since copper losses commonly dominate in the SC direct-drive wind generators with SC field windings and copper armature windings, an important way to increase the generator efficiency is to improve the armature winding design. Compared to traditional overlapping windings, fractional-slot concentrated windings (FSCW) have shorter end connection and higher slot fill factor, which has a great contribution to copper loss reduction. However, high content of magnetic motive force (MMF) harmonics is a big challenge for applying FSCW in SC generators. The asynchronous MMF harmonics may cause high eddy-current losses in the rotor assembly, resulting in efficiency reduction and more refrigeration burden. A novel multiphase FSCW with fewer MMF harmonics for SC generators is presented in this paper, followed by an MMF harmonic analysis through winding function method. Comparison with three-phase and traditional dual three-phase FSCW is performed. Characteristics of the generators are compared, in terms of back electromotive-force, electromagnetic torque, and rotor eddy-current losses by using finite element analysis. The advantages of the proposed FSCW are verified. Further, it is observed that the alternating components of the flux density at SC coil area is attenuated significantly by the proposed topology, indicating that the AC losses of the SC coils may be substantially reduced. The feasibility of applying FSCW topology in the design of SC wind generator is enhanced through the study.

Comparison Study on Harmonic Loss of MW-Class Wind Generators With HTS Field Winding

This paper investigates the effect of slot-pole combination on harmonic loss such as iron loss and eddy-current loss in wind generators with high-temperature superconductor field winding. In order to study the effect of stator configuration, three wind generators having the same rotor structure but different stators were purposely designed to produce 11 MW. In the analysis, 2-D time-stepped finite element analysis was used to estimate flux density waveforms, and then harmonic iron loss in the stator yoke and rotor eddy-current loss were obtained. From the simulation results, we showed that the harmonic effect on stator iron loss is not significant because magnetic loading by field winding dominates. Meanwhile, we found that the eddy-current loss in the rotor by harmonics of magneto motive force strongly depends on the slot-pole combination.

Rotor Eddy Current Power Loss in Permanent Magnet Synchronous Generators Feeding Uncontrolled Rectifier Loads

Analytical methods and transient finite element analysis (FEA) with rotating mesh are used to calculate rotor eddy current power loss in a permanent magnet synchronous generator connected to an uncontrolled bridge rectifier. Two winding and rectifier topologies are considered: a three-phase winding with a three-phase bridge rectifier and a double three-phase winding with a three-phase rectifier each, connected in series. Both magnet flux tooth ripple and stator magneto-motive force harmonics are considered in the calculation of rotor loss; the harmonics are added vectorially. Good agreement is observed between analytical and FEA for constant dc link current and constant voltage loads. The machine with double three-phase windings was found to have considerably lower rotor losses than the machine with one single three-phase winding.

Analyses on Electromagnetic and Temperature Fields of Superhigh-Speed Permanent-Magnet Generator With Different Sleeve Materials

In this paper, a superhigh-speed permanent-magnet generator (SHSPMG) which has an alloy sleeve on the rotor outer surface is investigated. The purpose of the sleeve is to fix the permanent magnets and protect them from being destroyed by the large centrifugal force. However, the sleeve material characteristics have much influence on the superhigh-speed machine, and therewith, most of rotor eddy-current losses are generated in the alloy rotor sleeve, which could increase the device temperature. Taking a 117-kW 60 000-r/min SHSPMG as an example, the influence of the sleeve on the generator output performance is analyzed when the generator sleeve is made of stainless steel, carbon fiber, copper-iron alloy, and copper. In addition, the eddy-current loss distributions could be gotten, and therewith, the variations of the eddy-current losses in different kinds of sleeves are analyzed. Based on the 3-D coupling field between the fluid and temperature, the temperature distributions were obtained when the sleeve adopts different materials. Moreover, the temperature variations of the permanent magnets are further analyzed. The obtained conclusions may provide some references for the design and analyses of the SHSPMG.

Thermal Optimization of a High-Speed Permanent Magnet Motor

This paper investigates the losses of a high-speed permanent magnet motor. The iron losses are calculated by a model that can consider the skin effect and rotational loss. The rotor eddy current losses are estimated by a fast hybrid method that can consider the end effect. Pulse-width modulation (PWM) harmonics brought by the voltage source inverter (VSI) are considered in the loss calculations. Then the temperature distribution of the motor is evaluated by using the calculated loss results and computational fluid dynamic (CFD) modeling. Finally, based on the CFD results, the motor structure is optimized to achieve better rotor cooling. The outer slots are closed to force the cooling air flow through the rotor surface. Calculated temperature distributions and optimization results are verified by measurements.

Copper Losses and Rotor Losses Calculation for the BLDCM Used in a Flywheel Energy Storage System

This paper presents a finite element investigation into the copper losses and rotor eddy current losses in brushless DC motors used in a flywheel energy storage system. The copper and rotor losses generate heat which needs to be dissipated in order to not exceed a defined temperature level for the magnet or winding insulation to avoid demagnetization or damage, which if it occurs will reduce the motor performance. The principle of operation and losses calculations are presented in the paper. The influences on the copper and eddy current losses are analyzed in detail, including the operation mode, the control strategy and the variation of the speed. Simulation and experimental results are presented and discussed.

Reduction of Rotor Eddy Current Loss in High Speed PM Brushless Machines by Grooving Retaining Sleeve

Eddy currents might be significant in the rotor of a high speed permanent magnet brushless machine, especially when a metal retaining sleeve is applied. Various methods have been used to reduce the eddy currents. In this paper, a simple method, by grooving the retaining sleeve, is proposed, such that the flowing path of the eddy currents is obscured and the energy loss is thus suppressed. Circumferential, axial and comprehensive grooving are comparatively studied, particularly on the aspects of loss reduction, sleeve mechanical strength, rotor dynamic balance, and manufacture process complexity. It is proved that the circumferential grooving is the most effective for overall performance improvement and the simplest for practical manufacture with little extra cost.