Z.J. Liu

Analytical Solution of Air-Gap Field in Permanent-Magnet Motors Taking Into Account the Effect of Pole Transition Over Slots

We present an analytical method to study magnetic fields in permanent-magnet brushless motors, taking into consideration the effect of stator slotting. Our attention concentrates particularly on the instantaneous field distribution in the slot regions where the magnet pole transition passes over the slot opening. The accuracy in the flux density vector distribution in such regions plays a critical role in the prediction of the magnetic forces, i.e., the cogging torque and unbalanced magnetic pull. However, the currently available analytical solutions for calculating air-gap fields in permanent magnet motors can estimate only the distribution of the flux density component in the radial direction. Magnetic field and forces computed by the new analytical method agree well with those obtained by the finite-element method. The analytical method provides a useful tool for design and optimization of permanent-magnet motors.

Accurate Prediction of Magnetic Field and Magnetic Forces in Permanent Magnet Motors Using an Analytical Solution

This paper presents an analytical model suitable for analyzing permanent magnet motors with slotted stator core. By including the effect of the interaction between the pole transitions and slot openings, the model is able to predict the airgap field and magnetic forces with high accuracy, which cannot be achieved using the previously available analytical methods. The results of electromagnetic forces, i.e., the cogging torque and unbalanced magnetic pull, computed analytically agree well with numerical simulations using the finite-element method. The model is used to analyze the magnetic forces developed in permanent magnet brushless motors when the design parameters vary in wide ranges. The model is useful in design and optimization of permanent magnet motors.

Effect of Radial Magnetic Forces in Permanent Magnet Motors With Rotor Eccentricity

This paper studies radial magnetic forces developed in permanent magnet motors having rotor eccentricity. Such forces represent the source of magnetically induced vibration in a disk-spindle assembly, and are of concern in design stage. A new analytical model suitable for studying the relationship between eccentricity effect and design parameters of slotted permanent magnet motors is introduced. The effectiveness of the model to study the effects of the rotor eccentricity and other leading design parameters is demonstrated through comparisons with numerical simulations

Design optimization for cogging torque minimization using response surface methodology

This paper discusses the application of response surface methodology (RSM) to design optimization for brushless dc permanent magnet motors for a hard disk driver to reduce cogging torque. The feasibility of using RSM with the finite-element method in practical engineering problems is investigated with computational examples and comparison between the fitted response and the results obtained from an analytical solution.

Analysis of eddy current and thermal problems in permanent magnet machines with radial-field topologies

This paper describes the analysis of thermoelectromagnetic problems in which eddy current loss induced by rotating magnet poles is present. Analytical and numerical techniques involved in analysis of this type of coupled problem are discussed. Computational predictions are compared with experimental results. >

An improved analytical solution for predicting magnetic forces in permanent magnet motors

This paper describes an analytical model for analyzing magnetic forces developed in permanent magnet motors of radial field topology. The slotting effect is taken into consideration by solving the boundary value problem of the air gap field. The solution is expressed in the form of Fourier series with which the analysis of the harmonic contents in the magnetic forces can be readily performed. In particular, the curvature effect or the flux focusing effect in the slot opening regions is taken into consideration. The results of magnetic forces obtained from this model are verified with numerical simulations using the finite element method and experimental results.

Analysis of Unbalanced-Magnetic-Pulls in hard disk drive spindle motors using a hybrid method

In spindle motor design and analysis, calculation of the torque and force is an essential step for the performance prediction. A method for the calculation of the Unbalanced-Magnetic-Pull (UMP) is proposed in this paper. As the method is based on the combination of the strength of analytical and numerical methods, it can yield more accurate results than traditional methods, and can be conveniently implemented in the computer aided design/analysis softwares for spindle motors.

Electromagnetic design for hard disk drive spindle motors with fluid film lubricated bearings

This paper discusses the problems associated with the electromagnetic design of hard disk drive (HDD) spindle motor having fluid film bearings. It will highlight the effect of the unbalanced electromagnetic pull in the radial direction of FFB spindle which uses permanent magnet brushless DC motors with slotted stator core. An analytical approach is employed for predicting the magnetic field and motor performance. Results obtained from this approach are compared with finite element analysis.

Design of a hybrid fluid bearing system for HDD spindles

This paper introduces a novel design of hybrid hydrodynamic and aerodynamic fluid bearing system. The fluid bearing system consists of an oil lubricated journal bearing unit with herringbone grooves and an air lubricated thrust bearing unit having a conical thrust plate engraved with spiral grooves. This bearing system has lower power consumption and enables a reliable seal design only for its journal bearings. The spindle motor with this bearing system has been prototyped. In this paper some simulation and testing results are reported.

Design trends of spindle motors for high performance hard disk drives

Reducing the unbalanced radial force and reduction of cogging torque are important for the future high performance spindle motors. In this paper, the effects of the pole and slot number combinations on the cogging torque and unbalanced radial force are discussed and a detail investigation on the unbalanced radial force is made using FEM. It is shown that the unbalanced radial force is substantial in the starting condition and in the delta connection winding. The unbalanced radial force can be largely reduced by optimization of the pole arc to pitch ratio.