Design of variable magnetization pattern machines for dynamic changes in the back-EMF waveform

One of the requirements for the electric machine designer is to provide an optimal back-EMF waveform in a high-performance application such as electric vehicles (EV) or hybrid EVs (HEV) systems. This paper presents a design technique for the shaping of back-EMF waveforms in brushless permanent magnet (PM) AC and DC machines. At first, back-EMF waveforms are calculated by an analytical model based on the subdomain field model and the complex permeance function. The finite element and experimental results are used for the verification of the analytical model. Secondarily, the heuristic optimization algorithm is applied to the analytical model in order to search for optimal solutions where the objective function is composed of harmonics in target back-EMF. As a result, the optimal back-EMF waveform can be traced by matching the odd harmonics of the design candidate with those of the target waveform. Also, a detailed investigation of the winding factor is carried out to provide the key design techniques regarding the shape of back-EMF.

Variable magnetization pattern permanent magnet synchronous machines (VMP-PMSMs) capable of dynamic changes in the magnetization spatial distribution are discussed in this paper. VMP-PMSMs are able to change the harmonic components in the air gap magnetic flux density due to the magnet by intentional demagnetization; hence, they can effectively manipulate the torque ripples and losses according to the required operating points. The objective of this paper is to provide a procedure to design VMP-PMSMs that can have both the sinusoidal and the trapezoidal back EMF waveforms by switching magnetization pattern. The performance of the designed VMP-PMSM following the reported procedure is confirmed through experiments as well as finite element analysis (FEA) simulations in terms of the back EMF shaping, torque ripple reduction, and loss reduction. Loss analysis of VMP-PMSMs is also given as contrasting with variable flux permanent magnet machines (VF-PMSMs), showing that VMP-PMSMs can reduce more losses than VF-PMSMs with the same torque density.

This paper describes the application of direct torque control (DTC) to a PM brushless AC machine whose back-emf waveform is non-sinusoidal, and compares its performance with that which is achieved when the back-emf waveform is assumed to be sinusoidal. It also demonstrates the utility of employing a sliding mode observer to estimate the back-emf waveform, and, thereby, to calculate the instantaneous electromagnetic torque and to minimise torque ripple. The effectiveness of this approach is validated by both simulations and measurements.

This paper reviews the relative merits of induction, switched reluctance, and permanent-magnet (PM) brushless machines and drives for application in electric, hybrid, and fuel cell vehicles, with particular emphasis on PM brushless machines. The basic operational characteristics and design requirements, viz. a high torque/power density, high efficiency over a wide operating range, and a high maximum speed capability, as well as the latest developments, are described. Permanent-magnet brushless dc and ac machines and drives are compared in terms of their constant torque and constant power capabilities, and various PM machine topologies and their performance are reviewed. Finally, methods for enhancing the PM excitation torque and reluctance torque components and, thereby, improving the torque and power capability, are described

This paper presents a novel, low-cost, high-performance single-phase adjustable-speed motor drive by using a three-phase permanent magnet brush-less DC (BLDC) machine. This system has been designed and implemented by the multidisciplinary student team of Illinois Institute of Technology (IIT) for the 2003 IEEE/DOE/DOD international future energy challenge and won the first prize overall award for the motor competition. Machine configurations and driver topologies have been investigated to develop a low-cost innovative system. To satisfy the requirements of the challenge, a commercially available motor has been selected and redesigned including the stator winding and permanent magnet rotor. The redesigned motor has been fabricated and tested successfully. For the designed power electronic driver, the main goal was to achieve high performance and reliability at low cost. Simulation and experimental results are provided to verify the proposed approach.

Finite element analysis is used extensively for the design and performance prediction of all types and topologies of permanent magnet machine, (e.g. brushed DC motors, hybrid stepper motors, brushless DC and AC motors), and for calculating field distributions, winding inductances, back-EMF waveforms, torque and force, demagnetisation withstand of magnets etc. Usually, it complements analytical techniques, which can provide a rapid and reliable means of design optimisation. The presentation focuses on permanent magnet brushless machines, and illustrates the application of finite element analysis to both linear and rotary machine topologies, demonstrates how it facilitates the prediction of iron loss and eddy current loss, and shows how it can account for the influence of magnet powder alignment and impulse magnetisation systems on machine performance.

This paper investigates the influence of different magnetization patterns on the performances of the surface-mounted permanent magnet synchronous machines (SMPMSMs). Three magnetization patterns are employed, which are radial, parallel, and ideal Halbach magnetizations. These magnetization patterns are applied to 9-slot/10-pole and 15-slot/4-pole permanent magnet (PM) machines. The PM machines are designed and simulated by using Opera 2D finite element. The performances of three PM motors, such as airgap flux density, phase back-EMF, and cogging torque, are evaluated under the influence of different magnetization patterns. The total harmonic distortion of phase back-EMF (THDv) for the motors are investigated. The PM motors with ideal Halbach magnetization provide the lowest cogging torque and the lowest total harmonic distortion of phase back-EMF. Besides that, the optimum setting of the magnet pole-arc can reduce the total harmonic distortion of phase back-EMF and achieve lower cogging torque. The optimum magnet pole-arc produced by radial magnetization in 9-slot/10-pole motor is 24.8° mech., with cogging torque of 0.45 Nm, and THDv of 2.69 %. Meanwhile, the optimum magnet pole-arc produced by parallel magnetization in 9-slot/10-pole motor is 26.0° mech., with cogging torque of 0.41 Nm, and THDv of 2.00 %.

This paper describes a computationally efficient approach for mapping rotor power loss in permanent magnet (PM) machines. The PM loss mapping methodology discussed here utilizes a small number of time-step finite-element analyses (FEAs) to determine the parameters of a functional representation of loss variation with speed (frequency) and stator current and is intended for a rapid evaluation of machine performance over the entire torque-speed envelope. The research focus is placed on field-oriented-controlled brushless AC PM machines with surface-mounted PM rotor construction, although the method could be adapted for other rotor formats. The loss mapping procedure accounts for the axial segmentation of the PM array through the use of an equivalent electrical resistivity of the segmented PM array, which is obtained from three-dimensional (3-D) FEA. The PM loss can be accurately mapped across the full operational envelope, including the field-weakened mode, through a single 3-D and four two-dimensional time-step FEAs. The proposed methodology is validated on an 18-slot 16-pole surface-mounted brushless AC PM machine design. The loss mapping procedure results closely agree with the computationally demanding alternative of direct 3-D finite-element prediction of the PM power loss undertaken at each of the machine's operating points.

Brushless dc (BLDC) machines with a surface mounted permanent magnet (SMPM) rotor meet the high-torque and high-efficiency requirements for automotive applications. However, their constant-power operation region is limited due to the low phase inductance. As an alternative to the electrical field-weakening methods, the speed range of radial-flux BLDC machines can be extended by mechanically reducing the axially overlapping length of the stator and the rotor. In this paper, the no-load performance of an SMPM-rotor BLDC machine with an axially displaceable permanent-magnet rotor is analyzed. The effectiveness of this mechanical field-weakening method is limited through the flux components due to the stator/rotor misalignment and the additional losses. The cause of the flux components due to the stator/rotor misalignment and the dependence of back-electromotive-force waveforms on the axial rotor position are investigated by using 3-D finite-element method (FEM) analysis, where the effects of the end-winding geometry and design are taken into account. Moreover, the additional loss mechanisms due to the stator/rotor misalignment are identified, and the no-load additional losses are determined by using the experimental and 3-D FEM analysis results. Finally, the numerical results are verified by using test-bench measurements.

This paper outlines a method using finite element analysis to model the magnetization process of a small brushless surface-magnet 3-phase motor with isotropic ferrite magnets and concentrated windings. This is aimed at allowing in-situ magnetization using the 3-phase machine winding. Radial magnetization is the nominal manufactured magnetization pattern; however, the in-situ method leads to a variation in the magnetization pattern and the performance is examined using open-circuit simulations. The properties of the magnet material (a ferrite 5 type) are examined in order to simulate the magnetization. Bulk magnetization H s-sat levels for ferrite type 5 indicate a 10 kOe; however, in this paper localized magnetization is examined using finite element analysis. The use of magnetization functions allows the magnetization pattern to be applied to the simulations. This paper uses a standard manufactured machine to carry these out. Open circuit experiments are conducted to validate the presented method.

This work reports a finite element analysis (FEA) model study of the stator current spectra of a commercial permanent magnet (PM) AC machine operating with rotor PM demagnetisation faults. A range of PM uniform and local demagnetisation fault scenarios at different severity conditions are examined. The aim of this study is to investigate the extent to which different modes of PM demagnetisation could be distinguished by inspecting the spectral signature patterns these generate in the stator current signal. To this end, a generalised mathematical characterisation of PM fault spectral signature in the stator current is examined and a 2D FEA software used to establish a model of the studied PM machine design. The FEA model was employed to predict the current signal spectral signature patterns of a range of different PM demagnetisation faults. These are then correlated with those arising from the derived expressions to examine whether the predicted and expected spectral trends are in agreement and whether their observation as the potential to provide diagnostic information.

This paper presents an analytical magnetic field calculation for slotted brushless permanent-magnet (PM) machines equipped with surface inset magnets. The analytical expressions can be used to calculate both open-circuit and armature reaction field distributions. Although the expressions can be used for brushless PM machines with any magnetization pattern, the results of the open-circuit magnetic field are presented for three different magnetization patterns: radial, parallel, and Halbach. Different winding structures, such as overlapping, nonoverlapping with all teeth wound and nonoverlapping with alternate teeth wound, can be considered to compute the armature reaction field distribution. The analytical expressions can be used for the surface inset magnet structure with airspace between the magnets and the iron interpoles but they cannot be used in the case of brushless machines with surface mounted magnets. The calculations account for slotting and slot-opening effects via the subdomain technique. The analytical results for a four-pole, six-slot brushless PM machine with three magnetization patterns and nonoverlapping winding are presented and evaluated by comparing them with those obtained from the finite element method.

Axial flux magnetic-geared double-rotor machines (AMGDRMs) are characterized as a potential core component of the power splitting system in hybrid electric vehicles (HEVs). Conventional AMGDRMs suffer from low mechanical strength due to high axial magnetic force. In this paper, an interior-modulating-rotor axial flux magnetic-geared double-rotor machine (IMR-AMGDRM) is proposed to solve this problem. An analytical model of the IMR-AMGDRM is proposed to predict the magnetic field distribution and electromagnetic performances. Since analytical model is based on 2-D geometry, a 2-D approximation of IMR-AMGDRM which has similar output performance to 3-D finite element analysis (FEA) is presented based on the permanent magnet (PM) magnetic energy equivalent principle. Utilizing subdomain method, the magnetic field distribution and electromagnetic performances are calculated. Finally, the accuracy of the proposed analytical model is validated by 3-D FEA.Index Terms—Analytical model, axial flux, magnetic-geared machine, double-rotor machine.I. IntroductionAxial flux magnetic-geared double-rotor machines (AMGDRMs), which enable high torque density, have been considered a promising candidate for power splitting in hybrid electric vehicles (HEVs) [1]. However, conventional AMGDRMs suffer from low mechanical strength due to high axial magnetic force. To enhance mechanical strength, an interior-modulating-rotor axial flux magnetic-geared double-rotor machine (IMR-AMGDRM) which can reduce axial magnetic force dramatically is proposed in this paper. Fig. 1 (a) and (b) illustrate the configuration of the proposed IMR-AMGDRM.As axial flux type machine, IMR-AMGDRMs have complex 3-D magnetic circuits. 3-D finite element analysis (FEA) can offer accurate results by taking into account the nonlinear factors such as magnetic saturation and flux leakage. But the major drawback of 3-D FEA is serious time cost. Analytical model which is much lower time-consuming provides an alternative for electromagnetic performance prediction in the initial design procedure.In this paper, an analytical model of the IMR-AMGDRM is proposed. The geometry of analytical model is derived based on PM magnetic energy equivalent principle to guarantee similar output performance to 3-D FEA. The solution to analytical model is calculated by subdomain method. To evaluate the accuracy of the proposed analytical model, the results computed by analytical model and that of 3-D FEA are compared.II. Configuration of IMR-AMGDRMIn conventional configuration, the inner and outer rotors are the permanent magnet (PM) rotor and the modulating rotor, respectively. Since the modulating rotor is closer to the PM rotor than stator, the modulating rotor suffers from high axial magnetic force. Furthermore, the modulating rotor is hollow and thus the low mechanical strength of modulating rotor becomes the major drawback of conventional configuration [1]. To solve this problem, the IMR-AMGDRM which switches positions of the PM rotor and the modulating rotor is proposed. In interior-modulating-rotor configuration, as shown in Fig. 1 (a) and (b), the PM rotor is between the modulating rotor and stator, i.e. the inner and outer rotors are the modulating rotor and the PM rotor, respectively. Owing to that the PM rotor suffers from reverse axial magnetic forces from both sides simultaneously, the total axial magnetic force is dramatically reduced. Hence, although the outer rotor is hollow in both configurations, the axial magnetic force of interior-modulating-rotor configuration is significantly decreased, leading to higher mechanical strength and reliability.III.Geometry of 2-D ApproximationGenerally, the unrolled section of axial flux machine at mean radius is used as 2-D approximation. However, few references are found to evaluate the electromagnetic performance equivalence of such approximation. To make the electromagnetic performance of 3-D model and 2-D approximation equal, the amount of magnetic energy of the PM should remain unchanged after approximation, i.e. PM magnetic energy equivalent principle [2]. A 2-D approximation of IMR-AMGDRM which has identical output performance is presented by using PM magnetic energy equivalent principle.IV. Analytical Calculation of IMR-AMGDRMThe proposed analytical model is illustrated in Fig. 1 (c). It can be seen that the model consists of four subdomains. Region i, I, II and III represent the ith slot of the modulating rotor, the inner air gap, the PM rotor and the outer air gap, respectively. Utilizing subdomain method, the magnetic field distributions in the inner and outer air gaps are obtained by the solution of Laplace’s and Poisson’s equations. Then, the electromagnetic performances such as no-load back EMF, torque and axial force can be derived from the calculated magnetic field distributions.Comparisons of computation accuracy between analytical model and 3-D FEA is shown in Fig. 2. As can be observed, the results of analytical model are well consistent with that of 3-D FEA, validating the proposed method. **

The paper describes a relatively new topology of 3-phase permanent magnet (PM) brushless machine, which offers a number of significant advantages over conventional PM brushless machines for automotive applications, such as electrical torque boosting at low engine speeds for vehicles equipped with downsized internal combustion engine (ICEs). The relative merits of feasible slot/pole number combinations for the proposed 3-phase modular PM brushless ac machine are discussed, and an analytical method for establishing the open-circuit and armature reaction magnetic field distributions when such a machine is equipped with a surface-mounted magnet rotor is presented. The results allow the prediction of the torque, the phase emf, and the self- and mutual winding inductances in closed forms, and provide a basis for comparative studies, design optimization and machine dynamic modeling. However, a more robust machine, in terms of improved containment of the magnets, results when the magnets are buried inside the rotor, which, since it introduces a reluctance torque, also serves to reduce the back-emf, the iron loss and the inverter voltage rating. The performance of a modular PM brushless machine equipped with an interior magnet rotor is demonstrated by measurements on a 22-pole/24-slot prototype torque boosting machine.

This paper presents an optimization design for a three-phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$12\mathrm{s}/8\mathrm{p}$</tex> surface-mounted permanent magnet synchronous machine (SMPMSM) with a RM pattern applied in the electric vehicle by using the computing framework of finite element analysis (FEA) and genetic algorithm (GA). The framework is to determine the optimal settings of permanent magnet (PM) motor for higher average electromagnetic torque <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(T_{em_{-}avg})$</tex> and lower its ripple <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(T_{em_{-}rip})$</tex> . Several motor performances are investigated for the initial and the optimal designs of PM machines during open-circuit and on-load conditions, i.e., magnetic field distribution across the motor, magnetic flux density distribution in the mid air gap, phase back-EMF, electromagnetic torque, and its ripple through FEA. The magnet pole-arc and slot opening angle of the PM motor are taken into consideration. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{em_{-}avg}$</tex> and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{em_{-}rip}$</tex> are employed to formulate the computing equations to determine the objective function, which is used to search the optimal settings of PM motor in GA framework, where the fitness value produced by the computing framework is further evaluated. The comparison of parameters and motor performances between the initial and the optimal designs of 12s/8p PM motors with the GA framework are validated in terms of electromagnetic torque under BLAC operation using FEA. Therefore, the framework of FEA and GA is verified in reducing the usage of magnet materials and the electromagnetic torque ripple in the design of SMPMSM, which is highly viable for electric vehicle.