Rotor Magnetomotive Force Research Articles

Research on power density improvement for interior permanent magnet synchronous machine based on permanent magnet minimization

In order to improve the power density of the permanent-magnet synchronous machine and reduce the cost of effective materials, this paper investigated approaches of power-density improvement and discussed the influence of flux-weakening level and salient ratio parameters on the characteristics of torque and power for the interior permanent magnet synchronous machine (IPMSM) based on the permanent magnet (PM) minimisation. Firstly, the model of PM minimisation can be established based on the principle of equal area of stator and rotor magnetomotive force. According to the minimisation model, a formula is derived for calculation of structural parameters of the permanent magnets in the IPMSM. Besides, the relation among the interior multilayer permanent magnets is established. Then, comparing and analysing the extremum characteristics of the maximum torque output and maximum power output of the motor under different parameters, the regular influence on the IPMSM can be summarised between the parameter and the internal power factor angle. So, it can be proposed that the corresponding internal power factor angle with respect to the golden section ratio of the maximum torque output is around −35°. Finally, combined with the results of the finite element and analytical calculation, a 45 kW PM motor for electric vehicles was manufactured and tested in no-load, load and thermal experiment, respectively. The peak torque of the prototype lasted for 94 s and reached 2.3 times the rated torque. Compared with the conventional PM motor used for the electric vehicle, the power density was increased by more than 0.5 kW/kg which can verify the correctness of the theoretical analysis.

Magnetic field analysis and performance optimization of hybrid excitation generators for vehicles

Due to the increase of magnetic potential sources and complex magnetic circuit, the hybrid excitation generator (HEG) used in the energy conversion system of the vehicle has the problems of various structure parameters, complicated magnetic field analysis, and complex performance optimization, which can lead to the heavy workload and low accuracy of the traditional analysis method. For these problems, the paper proposes an analytical method of the HEG main magnetic field. The method considers the stator slotting and rotor magnetic circuit distribution and takes Carter coefficient and rotor magnetomotive force as the main calculation parameters. Compared with traditional methods, this method can analyze the various topology of HEG in detail and improve the accuracy of magnetic field analysis. Using this method, the main magnetic field of a new parallel HEG with a combined-pole permanent magnet (PM) rotor and salient-pole electric excitation rotor (EER) is analyzed, the main influencing parameters are determined, and the electromagnetic characteristics of HEG are analyzed to complete the performance optimization. The prototype is trial manufactured and tested. The results show that the optimized HEG can significantly improve the sinusoidal waveform of induced electromotive force (IEMF) and optimize the overall electromagnetic performance.

Analysis of the Rotor Magnetomotive Force of Built-In Radial Permanent Magnet Generator for Vehicle

Permanent magnet generator (PMG) for vehicles has attracted more and more attention because of its high efficiency, high power density, and high reliability. However, the weak main air-gap magnetic field can affect the output performance and the normal use of electrical equipment. Aiming at the problem, this paper took the rotor magnetomotive force (MMF), the direct influencing parameter of the main air-gap magnetic field, as the research object, deduced the analytical expression of rotor MMF of the built-in radial PMG in detail, and analyzed its main influencing factors in analytical expression, including the permanent magnet steel (PMS) material, the thickness of PMS in magnetizing direction, the vertical length of the inner side of PMS, and the effective calculation length of PMS. Based on this, the rotor parameters were optimized to obtain the best values. After that, the finite element simulation and prototype test of the optimized generator were carried out. The comparative analysis results showed that the optimized rotor parameters could effectively improve the rotor MMF and optimize the output performance of the generator.

Experimental verification of power factor adjustor with rotor winding circuit in passive variable flux PM motor utilizing space harmonics

AbstractThis paper proposes a technique for applying LC resonance to a rectifier circuit of a self‐excited wound‐field rotor, which utilizes space harmonics for improving the variable flux function. The rotor magnetomotive force can be improved by setting the resonance frequency of the LC resonance circuit to a low speed rotation range. The adjustable speed drive characteristics and efficiency maps are demonstrated via a prototype.

Analytical analysis and performance characterization of brushless doubly fed induction machines based on general air-gap field modulation theory

Air-gap magnetic field modulation has been widely observed in electric machines. In this study, we present an analytical analysis and performance characterization of brushless doubly fed induction machines (BDFIMs) fed by two independent converters from the perspective of air-gap field modulation. The spiral-loop winding is studied in detail as an example to show the generalized workflow that can also be used to analyze other short-circuited rotor winding types, such as nested-loop and multiphase double-layer windings. Magnetic field conversion factors are introduced to characterize the modulation behavior of special rotor windings and facilitate their comparison in terms of cross-coupling capability, average torque, and harmonic content of the air-gap flux density waveforms. The stator magnetomotive force (MMF), rotor MMF, and resultant air-gap MMF are considered, based on which the closed-form inductance formulas are derived, and the torque equation is obtained along with the optimal current angle for maximum torque operation by using the virtual work principle. The design equations are then developed for the initial sizing and geometry scaling of the BDFIMs. Transient finite element analysis and experimental measurements are performed to validate the analysis.

Flux-Based Detection and Classification of Induction Motor Eccentricity, Rotor Cage, and Load Defects

Motor current signature analysis (MCSA) has been accepted in the field as a reliable means of detecting faults in the rotor cage of induction motors. However, there are many limitations to MCSA-based detection for other types of faults including rotor eccentricity and load defects. Recently, there has been increasing interest in airgap or leakage flux monitoring as an alternative to replace or complement MCSA. Flux monitoring can provide a reliable means of detecting rotor faults since anomalies in the rotor magnetomotive force or airgap can be directly observed. In this article, a new method based on monitoring the attenuation of rotor rotational frequency sidebands in the flux spectra is proposed for reliable detection and classification of rotor cage and eccentricity faults. It is also shown that flux monitoring can provide detection of rotor faults that is insensitive to the load defects. An experimental study under controlled broken bar, eccentricity, misalignment, and load unbalance conditions are given to support the claims. The results show that rotor cage, eccentricity, and load defects can be detected and distinguished for cases where MCSA alone is ineffective. A comparative evaluation between the proposed flux monitoring method and existing methods (MCSA, vibration analysis) is also given.

Torque Calculation of Stator Modular PMa-SynRM With Asymmetric Design for Electric Vehicles

This article proposes a modular asymmetrical permanent magnet-assisted synchronous reluctance machine (PMa-SynRM) combining segmented stator and distributed winding. Meanwhile, an analytical model is investigated for the modular PMa-SynRM, considering the effects of stator slots and flux gaps (FGs). First, the rotor magnetomotive force (MMF) generated by permanent magnets and stator MMF are analyzed by combining the magnetic equivalent circuit (MEC) and conformal mapping. Moreover, an armature reaction magnetic field model is presented considering the influence of the shifted end slots on the winding factor. Besides, the torque of the modular PMa-SynRM is calculated under asymmetrical rotor poles. Finally, a modular PMa-SynRM prototype is manufactured, and the comparison conclusion is validated by the finite-element method (FEM) and experimental results.

Torque calculation of five-phase synchronous reluctance motors with shifted-asymmetrical-salient-poles under saturation condition

In traditional analytical method (AM), the magnetic saturation is always ignored to simplify the calculation process. However, synchronous reluctance motors (SynRMs) often operate around saturation point to achieve higher torque density. Therefore, a new AM is proposed, in which the saturation of stator iron has been considered. The key of the proposed method includes a saturation factor, and an iterative method is adopted to compute the saturation factor in the SynRM by increasing the air-gap length. Especially, the proposed AM can be applied to a SynRM even with shifted-asymmetrical-salient-poles. In the process of AM, the expression of stator magnetomotive force (MMF) is built firstly. Additionally, the air-gap density including slotting effect and salient-poles is calculated. Then, the rotor MMF under saturation of the stator iron is obtained. Therefore, the precision of the instantaneous torque can be improved significantly. Eventually, by the verification of finite elements method (FEM) and experiments, the torque performance of SynRMs with shifted asymmetrical rotor can be predicted accurately by the proposed AM.

Complex vector modeling of a doubly fed cascaded cage rotor induction machine

In this work, a cascaded doubly fed induction machine with a cage rotor is studied. To investigate the machine performance, a model is provided. In this model, the spatial harmonics of the rotor magnetomotive force are exactly considered. For this purpose, each rotor loop is selected as an individual circuit. The winding function theory is applied to compute the self- and mutual inductances of the rotor and stator circuits. To reduce the required simulation time, complex vector representation is adopted. Using the obtained voltage equations, the vector form of the machine state-space equations is yielded. Experimental tests are carried out to show the accuracy of the provided model. In addition, the adequacy of the proposed machine is verified by the appropriate tests.

Open-Circuit Air-Gap Field Calculation of a New PM Machine Having a Combined SPM and Spoke-Type Magnets

This article presents the open-circuit analytical magnetic field distribution model for a new permanent magnets (PMs) rotor configuration designed to improve the utilization of rare earth magnets for flux density production. The air-gap flux density is determined with the superposition method according to the magnetomotive force (MMF) and the air-gap permeance model. Since the new rotor consists of two different rotor topologies, e.g., lateral and spoke magnets, their MMF functions are determined separately. The rotor flux barrier (FB) effect on the rotor MMF harmonics is included and studied closely. The investigations carried out on different rotor topologies show high positive impact of FBs on the third or the operating wave. Compared to the conventional rotor, the utilization of PMs for air-gap flux density production significantly is improved. To verify the analytic model, finite elements (FEs) are used and several simulation results are presented.

Analysis of Magnetic Field and Electromagnetic Performance of a New Hybrid Excitation Synchronous Motor with dual-V type Magnets

Due to the increasing energy crisis and environmental pollution, the development of drive motors for new energy vehicles (NEVs) has become the focus of popular attention. To improve the sine of the air-gap flux density and flux regulation capacity of drive motors, a new hybrid excitation synchronous motor (HESM) has been proposed. The HESM adopts a salient pole rotor with built-in dual-V permanent magnets (PMs), non-arc pole shoes and excitation windings. The fundamental topology, operating principle and analytical model for a magnetic field are presented. In the analytical model, the rotor magnetomotive force (MMF) is derived based on the minimum reluctance principle, and the permeance function considering a non-uniform air-gap is calculated using the magnetic equivalent circuit (MEC) method. Besides, the electromagnetic performance including the air-gap magnetic field and flux regulation capacity is analyzed by the finite element method (FEM). The simulation results of the air-gap magnetic field are consistent with the analytical results. The experiment and simulation results of the performance show that the flux waveform is sinusoidal-shaped and the air-gap flux can be adjusted effectively by changing the excitation current. This study provides design methods and theoretical analysis references for this type of HESM.

Influence of Introducing the Rotor MMF Harmonic on Torque Performance of Spoke Type Permanent Magnet Motor With FSCW

This article proposes a torque optimization method that introduces the rotor magnetomotive force (MMF) harmonics, aiming to increase the average torque without increasing the cogging torque and torque ripple. A spoke type permanent magnet direct drive motor with fractional slot concentrated winding (FSCW) is designed. The FSCW can improve the utilization rate of the winding, but at the same time, it brings a wealth of MMF harmonics. First, the electromagnetic torque expression is derived using the principles of magnetic energy and virtual power. It means that the constant electromagnetic torque is generated when the stator and rotor MMF harmonics with the same space and time order are the same as the main wave speed. The analytical method is then used to analyze the influence of the stator and rotor MMF harmonics on the electromagnetic torque of the three-phase FSCW permanent magnet motor. On this basis, a rotor shape that introduces the third rotor MMF harmonics is proposed. Finally, the finite element software is used to compare and analyze the torque performance of motor before and after optimization. The finite element simulation and experiment results prove the correctness and effectiveness of the method.

Broken Rotor Bar Detection in Induction Motors Based on Air-Gap Rotational Magnetic Field Measurement

This paper presents a novel technique for the online detection of broken rotor bar(s) (BRBs) in induction motors, based on the analysis of the air-gap rotational magnetic field (RMF), being it indirectly sensed using the voltages induced in dual search coils installed in several stator regions. The induced voltages allow obtaining three differential voltages which reveal the symmetricity level of the mentioned field along the stator circumference. Under the presence of the broken bar(s), the RMF is disturbed by the rotor magneto-motive force, being these disturbances detected through the fundamental components of the differential voltages using a new time-domain analysis technique. The proposed diagnostic approach is verified by finite-element analysis and also by several experimental tests. A high sensitivity, robustness in the face of unbalanced voltage sources and motor load level variations are the main advantages of the proposed technique. Moreover, it is a secure method for BRB detection, easily implementable in low-cost digital signal processing units.

Torque Calculation of Five-Phase Interior Permanent Magnet Machine Using Improved Analytical Method

This paper proposes a new torque analytical method (AM) for five-phase interior permanent magnet synchronous machine (IPMSM) with shifted asymmetrical rotor pole. The key of this method is to consider the permanent magnet (PM) torque and the reluctance torque simultaneously, by using the Lorenz force law. The analytical expressions, including stator magnetomotive force (MMF), rotor MMF generated by PMs and rotor MMF generated by stator MMF, are first derived. Then, Lorentz force law is used to calculate PM torque and reluctance torque, respectively. In order to improve the precision of electromagnetic torque calculation, stator slotting effect under shifted asymmetrical rotor pole is taken into consideration. Finally, the AM is verified by finite element method and experiment. The results show that proposed AM is good enough to predict the torque performance of IPMSM with shifted asymmetrical rotor pole.

Novel Rotor Winding Investigation for Reduction of Torque Pulsation and Electromagnetic Noise in Squirrel Cage Induction Motor

In the design of squirrel-cage induction machines, torque pulsation and electromagnetic noise always exist. However, they cannot be minimized simultaneously with no matter which slot combination. Electromagnetic noise is aroused by the spatial harmonics in the air gap which are generated by stator magneto-motive force and rotor magneto-motive force. Generally, stator slot number cannot be changed while rotor slot number and slot size could be modified optionally. One set of uniform-bar rotor winding cannot solve this problem. To investigate this issue, this paper proposes a novel rotor winding connection topology for the simultaneous reduction of torque pulsation and electromagnetic noise. 2D finite element models with conventional rotor winding and novel rotor winding are simulated and compared.

Analysis and Reduction of the Cogging Torque of Flux-Modulated Generator for Wave Energy Conversion

Abstract As the cogging torque of the flux-modulated generator is large, the vibration and noise of the torque reduce the stability of the generator. According to the principle of the flux-modulated generator for wave energy conversion, a novel method is proposed to reduce the cogging torque. The method uses the rotor magnetomotive force produced by permanent magnets to substitute the effect of the rotor. Based on the energy method, the formula of the cogging torque in the gap is derived. The method of unequal tooth width is used to reduce the cogging torque of the generator. By using the finite element method, the simulation model of the proposed generator is built to prove the accuracy. Cogging torque and no-load induced electromotive force prove that the method of unequal tooth width can reduce the effective value of cogging torque effectively. The method is suitable for the promotions of the generator performance.

Reduction of Stator Core Loss in Interior PM Machines for Electric Vehicle Applications

Stator core loss of four interior permanent magnet (IPM) machines is studied for electric vehicle applications in this paper. The no-load air-gap density, core loss density, core loss harmonics, and the segregation of hysteresis and eddy current losses are investigated with finite-element analysis. The results show that the rotor surface shaping of a single-layered IPM machine and increasing the rotor magnet layers can reduce the stator core loss due to the harmonics of the rotor magnetomotive force. The stator core loss reduction effectiveness of rotor surface shaping is similar to a two-layered magnet configuration, but the reduction effectiveness is less than that the magnet layers increased to three numbers. An IPM machine with three-layered magnets rotor is prototyped to verify the analysis.

Unbalanced distributed and balanced concentrated winding comparison in servo motor applications

A comparison between unbalanced distributed winding and balanced concentrated winding is presented in this paper. The application is a torque motor where low speed and high torque are main requirements. Concentrated winding slot/pole combination is selected so that winding factor is maximum and as well as it is equal to the winding factor of the unbalanced winding motor. Stator and rotor Magneto-Motive Force (MMF) functions are obtained for both motor types to model torque ripple components in the frequency spectrum. Finite element analysis (FEA) and experimental results are presented to compare back EMF waveforms, magnetic loading, torque constant and thermal performance of two motors.Two designs have identical stator outer diameter (OD) and active axial length. The detailed comparison shows that two motors have similar torque constant. The behavior of two motors under heavy load differs and concentrated winding design performs better. Thermal performance of the designs is also compared and it is observed that concentrated winding design has also less winding temperature rise.

Analytical Investigation Into Magnet Eddy Current Losses in Interior Permanent Magnet Motor Using Modified Winding Function Theory Accounting for Pulsewidth Modulation Harmonics

The study of the interaction between an interior permanent magnet (IPM) motor and a pulsewidth modulated (PWM) converter is increasingly becoming important, especially in the case of high-speed drives owing to the losses caused by modulation. Considering the significance of machine–converter interaction, this paper proposes a novel method to calculate magnet eddy current losses in an IPM used in traction applications by incorporating both PWM harmonics as well as rotor geometry and design parameters. The voltage harmonics from the PWM are incorporated into a series of steps in order to calculate the stator magnetomotive force (MMF) using the modified winding function theory. The rotor MMF and, subsequently, the magnet losses are computed using a magnetic circuit model incorporating the harmonics from the stator MMF as well as the flux barrier and the rotor geometry. The results have been demonstrated analytically for various cases of modulation and PWM input parameters to study the dependence of magnet losses on converter properties. The analytical results have been validated by finite-element analysis and experimental investigations.

The Amplitude Feature of Current Eigenfrequency for Motor Bearing Banded Failure

The amplitude feature of those current eigenfrequencies resulting from bearing banded failure is merely focused even though it is very important for distinguishing bearing failure. This theme described the torque oscillation resulting from bearing banded failure based on periodic rectangular pulse model, deduced the modulation principle between rotor magnetomotive force and torque oscillation, presented the relation expression between various banded failure and the amplitude of different eigenfrequency. Finally, a series of simulation studies were conducted and results show that the proposed theory is valid.