Losses In Permanent Magnet Machines Research Articles

Analytical Model and Experimental Verification of Permanent Magnet Eddy Current Loss in Permanent Magnet Machines With Nonconcentric Magnetic Poles

Permanent magnet synchronous motors (PMSMs) with nonconcentric magnetic poles (NCMP) structure are widely used as high-precision servo motors. The current subdomain method mainly equates the NCMP to a rectangular magnet to make each subdomain boundaries of magnet parallel when calculating the magnet eddy current loss (ECL), which results in a lower accuracy of the calculation. This article divides the NCMP into nonconcentric region and concentric region along the radial direction. The nonconcentric region is further divided along the circumferential direction, and each divided region has a single thickness. Based on the subdomain method, a novel analytical method of magnet ECL in PMSMs with NCMPs is proposed. Moreover, the three-dimensional distribution coefficient of ECL in the NCMP is derived by the equivalent impedance method. The effects of eddy current reaction and stator slotting are taken into account to improve the calculation accuracy. Then, through theoretical derivation, the relationship between the minimum magnet loss and the pole parameters is obtained, which provides a reference for the design of NCMPs. Finally, the loss separation experiment is performed on four prototypes with NCMPs. The experimental results, finite element method results and analytical results are compared to verify the correctness of the model.

Minimization of AC Copper Loss in Permanent Magnet Machines by Transposed Coil Connection

With the increasing demand for high power density and high-speed electrical machines, ac copper loss issue becomes more severe and poses a significant challenge for machine design. This article proposes a suppression technique by transposing the coil connection in the same branch to balance the inductances of parallel strands, thus reducing the circulating current ac loss. Compared to the commonly used transposition method implemented within one coil (e.g., Litz wire), the proposed method can be beneficial to reduce the transposition points and have no influence on the slot filling factor. The proposed idea is investigated by the field-circuit coupled finite element analysis (FEA), taking the bundle-level circulating current into account. It shows that the copper loss can be reduced by 17% compared to the nontransposed one at 400 Hz for a studied 12-slot 10-pole permanent magnet machine. The influence of winding layout and slot/pole combination on the effectiveness of the proposed transposition method is also investigated. Finally, experiments are implemented on a designed test-rig to validate the FEA method and the effectiveness of the new transposition.

Characterization of rotor losses in permanent magnet machines

PurposeThe purpose of this paper is to present a synthesis of the analysis and modeling of the rotor losses in high speed permanent magnets motors.Design/methodology/approachThree types of losses are as a result of eddy currents in the conductive parts of the rotor. The analysis includes their characterization and the setup of a numerical model using finite element method. The adopted methodology is based on the separation of the losses which allows a better understanding of the physical phenomena. Each type of losses will be modeled and computed separately.FindingsIt is possible to make a precise estimate of the different losses in the rotor while keeping a relatively short computing time.Research limitations/implicationsThe analysis is applied on a high-speed permanent magnet motor for avionic application. The model is validated with the commercial finite element model (FEM) software Flux2D.Originality/valueThe developed model allows an important save in terms of CPU-time compared to commercial FEM software while staying accurate. The separation of each losses and their sources is important for motor engineers and was requested for them to improve the designs more easily.

Novel Empirical Model for Calculation of Core Losses in Permanent Magnet Machines

This article presents a simple model for the calculation of core losses in permanent magnet machines based on the total flux linkage of the stator winding. In reality, the total core losses will vary as a function of permanent magnet field and the combination of $d-q$ current components even in the case when different $d-q$ currents produce the same amount of stator winding flux linkage at a given speed. The classical model with constant equivalent core loss resistance predicts constant core losses for constant flux linkage which leads to incorrect results. The model presented in this paper expands the classical core loss resistance based model with a novel additional term which corrects the classical model and calculates an additional core loss component as a function of $d$ axis current, total flux linkage of the stator winding, and two constant parameters. The accuracy of the model is confirmed by finite element simulations and core loss measurements for the cases of an interior permanent magnet traction motor and an industrial surface permanent magnet motor (only simulations).

Loss analysis of permanent magnet synchronous machines by considering circulating currents of armature windings

AbstractIn this paper, we investigate the losses caused by circulating currents generated in armature windings of permanent magnet synchronous machines by using electromagnetic field analysis. In the analysis, each parallel connected wire in the windings is modeled by finite elements in order to consider the difference in the electromotive forces and the generation of the circulating currents. The calculated losses are compared with experimental results. It is clarified that the armature copper loss in permanent magnet machines with parallel connected wires considerably increases by the circulating currents.

Calculation of rotor losses in PM machines with retaining sleeves using transfer matrices

Accurate calculation of rotor losses in permanent magnet (PM) synchronous machines can be critical because these losses tend to be very small relative to others in the machine. Numerical methods, such as finite element analysis (FEA), can now provide accurate estimates of these losses, but they, especially 3D-FEA, can be time consuming. Analytical methods therefore remain very useful as quick tools for estimating losses at the early design stages. This study presents a new improved analytical method for the calculations of rotor eddy current losses in PM machines using a reformulation of the current sheet model into transfer matrices to solve Helmholtz's diffusion equation. Such methodology reduces the complexity of the problem significantly, particularly in machines with retaining sleeves, and simplifies the numerical evaluation of the resulting equations. A high-speed PM machine with a retaining sleeve is presented as a case study. The analytical results are verified using FEA.

An Analytical Method for Predicting 3-D Eddy Current Loss in Permanent Magnet Machines Based on Generalized Image Theory

This paper proposes an analytical method, based on the generalized image theory, for accurate prediction of 3-D eddy current distributions in the rotor magnets of permanent magnet (PM) machines and the resultant eddy current loss. The analytical framework is established in a 3-D rectangular coordinate system, and the boundary conditions that govern the eddy current flows on the surfaces of magnets are represented by equivalent image sources in a homogenous 3-D space extending into infinity. By introducing a current vector potential, the 3-D eddy current distributions in magnets are derived analytically by employing the method of variable separation, and the total eddy current loss in the magnets is subsequently established. The proposed method has been validated by a 3-D time-stepped transient finite-element analysis (FEA). It is shown that the proposed method is extremely computationally efficient. When combined with the 2-D FEA of magnetic field distributions, the proposed method provides an accurate and computationally efficient means for predicting 3-D eddy current loss in a variety of PM machines with due account of complex machine geometry, various winding configurations, and magnetic saturation.

Modeling and analysis of eddy current losses in permanent magnet machines with multi-stranded bundle conductors

This paper investigates the influence of eddy current losses in multi-stranded bundle conductors employed in out-runner permanent magnet machines, by adopting an analytical model. The analytical model is based on a sub-domain field model that solves the two-dimensional magnetostatic problem using the separation of variables technique for each of the non-magnetically permeable machine sub-domains: PM, airgap and slots. The validity and accuracy of the proposed model is verified using finite element analysis and then used to investigate the eddy current losses. The machine considered for the analysis has 36 slots and 42-poles previously designed for aircraft taxiing. The influence of the number of turns and the conductor cross-sectional area are investigated. It is shown that efficiency can be improved considerably by the choice of multi-stranded bundle conductors.

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.

Impact of the Rotor Yoke Geometry on Rotor Losses in Permanent-Magnet Machines

This paper presents a study on the rotor losses of surface-mounted permanent-magnet (PM) machine with fractional-slot windings. Two rotor losses sources are considered: the losses due to the slot opening (without currents in the stator) and due to the magneto motive force harmonics (mounting a rotor with demagnetized PMs). The rotor losses are measured at no load and under load. It is shown that the rotor losses under load are lower than considering the sum of the two contributions. This phenomenon is investigated using both analytical models and finite-element-based simulations. Then, the impact of the rotor geometry on the rotor losses is considered, and some design advices are reported.

Minimization of Iron Losses of Permanent Magnet Synchronous Machines

In permanent magnet (PM) synchronous machines, iron losses form a larger portion of the total losses than in induction machines. This is partly due to the elimination of significant rotor loss in PM machines and partly due to the nonsinusoidal flux density waveforms in the stator core of PM machines. Therefore, minimization of iron losses is of particular importance in PM motor design. This paper considers the minimizing of iron losses of PM synchronous machines through the proper design of magnets and slots, and through the choice of the number of poles. Both time-stepped finite element method (FEM) and the iron loss model from a previous study are used in this paper to draw the conclusions.

Irreversible demagnetization loss for electric machines with arc-shaped permanent magnets stabilized in air

For electric machines with their permanent magnet assemblies stabilized in air, a certain amount of irreversible loss of magnetization will occur in permanent magnets when magnet material with knee points in the demagnetization curve is used. To estimate quantitatively the irreversible demagnetization loss in permanent magnet machines, the maximum loss of magnetization, the remanent flux loss and the demagnetization area are defined as their three figures of merits. A series of design curves for predicting the irreversible demagnetization loss for arc-shaped permanent magnets stabilized in air is presented in the paper. The effects of material characteristics, magnet geometry and its magnetic orientation on these merits are also discussed. Design curves are obtained from field computation results by using the multi-hysteresis-loop PM model recently proposed by the first author. These design curves are useful for permanent magnet machine designers.