Sreeju S. Nair

Demagnetization Assessment of Fractional-Slot and Distributed Wound 6-Phase Permanent Magnet Machines

A 6-phase fractional-slot-per-pole-per-phase interior permanent magnet (IPM) machine having a novel topology of 18-slot, 8-pole and a 6-phase, 48-slot, 8-pole IPM with distributed windings, both designed for a segment-A electric vehicle, are assessed for the risk of partial irreversible demagnetization under various fault conditions. This paper describes a more accurate approach of demagnetization assessment based on 2-D transient finite-element analysis. It is shown that due to the presence of low-order space harmonics in the fractional-slot IPM machine, the demagnetization risks across all pole pairs are different. Compared with the distributed wound machine, the fraction-slot machine is less vulnerable to demagnetization due to relatively high winding inductance, although its demagnetized regions are not uniform in each pole. It is also shown that although the demagnetizing current of one 3-phase short-circuit (SC) is greater than that of 6-phase SC, the resultant demagnetization risk is lower than that of 6-phase SC in the fractional-slotmachine.

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.

Post-Demagnetization Performance Assessment for Interior Permanent Magnet AC Machines

This paper assesses the post-demagnetization performance of interior permanent magnet (IPM) ac machines by employing the more accurate recoil line approach based on a 2-D transient finite-element analysis (FEA). The method predicts continuous demagnetization of each magnet element undergoing partial demagnetization and evaluates the machine behavior after an event of short-circuit faults across its terminals. Along with the short-circuit faults, a failure in a drive controller or a position sensor, which may lead to a reverse voltage across the machine terminals that can eventually be more fatal and can cause significant reduction in the performance due to high levels of demagnetization, is analyzed as the worst case scenario. The FE predicted post-demagnetization performance is validated by experimental measurements in which a six-phase IPM machine designed for electric vehicle traction is allowed to lose its synchronization with the inverter when forced to operate on a torque-speed envelope, which is way beyond the drive voltage setting.

Design aspects of high torque density-low speed permanent magnet motor for electric two wheeler applications

This paper focuses on practical aspects of designing a direct drive electric motor having extremely high torque density for a two wheeler electric/hybrid vehicle. The design starts with arrival of motor specification based on the vehicle requirements, progress with finalizing the sizing parameters and examining the aspects which affect the performance of the motor with high torque density. Skin effect, magnet demagnetization, eddy current loss, thermal stability and cycle efficiency are the main concerns which are examined. The use of strip conductors to reduce the resistance and enhance the thermal stability is also discussed. Finally, the prototype is made and the experimental results are presented.

Prediction of 3-D High-Frequency Eddy Current Loss in Rotor Magnets of SPM Machines

This paper proposes a computationally efficient method for accurate prediction of 3-D high-frequency eddy current loss in the rotor magnets of surface mounted permanent magnet machines, employing the imaging method. 2-D finite element analysis (FEA) is used to generate the information on radial and tangential 2-D magnetic field variations (eddy current sources) within the magnet. The diffusion of eddy current sources along the axial plane of the magnet computed analytically is incorporated in the imaging method to establish the 3-D eddy current source variations within the magnet. The modified method is validated with results from 3-D time-stepped FEA for an 8-pole, 18-slot permanent magnet machine, evaluating its magnet loss considering axial and circumferential segmentation.

High-Torque-Density Single Tooth-Wound Bar Conductor Permanent-Magnet Motor for Electric Two Wheeler Application

The design of an in-wheel direct-drive motor for electric and hybrid electric two wheelers is studied, with particular focus on the use of bar-wound conductors in stator. In serving the space-constrained design problem of simultaneously achieving conflicting objectives of weight, cost, and short-term torque rating, this paper aims at comparing machines equipped with bar-wound stators with multistrand wound machines. This paper proposes different conductor shapes and arrangements that combine the advantages of fractional slot concentrated windings with bar conductors and describes the special fabrication method adopted for semiclosed slot geometry. The implications of using bar conductors on eddy effects and thermal equivalent circuit of the motor are studied and characterized experimentally. Finally, to investigate the impact of bar windings over multistranded windings on the ultimate performance metrics, a design study is conducted using a multiobjective optimizer for the two cases, and the resulting Pareto optimal sets are compared. The concept of the bar conductor tooth-wound motor is tested on a laboratory prototype, and the test results for skin effect, thermal contact resistance, and load tests are presented.

Computationally Efficient 3-D Eddy Current Loss Prediction in Magnets of Interior Permanent Magnet Machines

This paper proposes a computationally efficient method based on imaging technique, for accurate prediction of 3-D eddy current loss in the rotor magnets of interior permanent magnet (IPM) machines. The 2-D time-stepped finite-element analysis (FEA) is employed to generate the radial and the tangential 2-D magnetic field information within the magnet for the application of the 3-D imaging technique. The method is validated with 3-D time-stepped FEA for an 8 pole, 18 slot IPM machine evaluating its resistance limited magnet loss with the increase in axial and tangential segmentation. Magnet loss considering eddy current reaction at high frequencies is evaluated from the proposed method by employing the diffusion of the 2-D magnetic field variation along the axial plane. The loss associated with all the frequencies together in the armature currents is evaluated by considering each of the harmonics separately in the proposed method employing the frozen permeability to account for magnetic saturation. The results obtained are verified with 3-D FEA evaluating the magnet loss at fundamental, 10 and 20 kHz time harmonics in armature currents.

Analytical Prediction of 3-D Magnet Eddy Current Losses in Surface Mounted PM Machines Accounting Slotting Effect

This paper presents a novel analytical technique for predicting three-dimensional (3-D) magnet eddy current losses accounting the slotting effect of any pole–slot combinations for a surface mounted permanent magnet machine under any conditions of load. The slotting effect is incorporated from a subdomain model and the 3-D boundary conditions are imposed with the current vector potential to represent the 3-D eddy currents circulating in the magnets. The proposed model in polar coordinate system is demonstrated on a fractional slot rare-earth permanent magnet machine by analyzing its magnet losses as functions of axial and circumferential segmentations. The results have shown an excellent match with 3-D numerical calculations. The analytical prediction has also been validated by experimental tests. The interaction of the armature reaction field with the slotting harmonics is analyzed and their effect on eddy current loss in rotor magnets is established. The proposed technique is employed to evaluate the effect of slotting on magnet loss with increase in field weakening angle.

Computationally efficient 3D rotor eddy current loss prediction in permanent magnet machines

This paper proposes a computationally efficient method, for accurate prediction of 3-dimensional (3D) rotor eddy current loss in permanent magnet (PM) machines. 2D time stepped finite element analysis (FEA) is used to generate the information on radial and tangential time-derivatives of 2D magnetic field (eddy current sources) in the magnets and the retaining sleeve for application of the 3D imaging technique. The proposed method is employed to evaluate the 3D magnet loss associated with surface mounted PM (SPM) machine, interior PM (IPM) machine and also the eddy current loss in the retaining sleeve. The method is validated by 3D time-stepped finite element analysis (FEA) for an 8-pole, 18-slot SPM and IPM machines.

Experimental validation of 3D magnet eddy current loss prediction in Surface Mounted Permanent Magnet Machines

This paper presents the experimental validation of three-dimensional (3-D) Fourier method employed for predicting magnet eddy current loss in surface-mounted permanent magnet (SPM) machines. The magnet loss is measured for a 12-slot 14-pole SPM machine from experimental tests when the machine is operated with inverter under locked rotor conditions by repeating tests with two rotors, one with magnets and one without. The eddy current loss associated with each significant harmonic in the captured armature currents is predicted separately employing the developed method and the total magnet loss is evaluated by applying the principle of superposition. The magnet loss at real operating conditions of the machine is predicted from the method using the phase current captured when the SPM is operating at its maximum speed conditions. The result is used as an example to devise an effective means of further reduction in the total magnet loss.