G. H. Jang

Reduction of Magnetically Induced Vibration of a Spoke-Type IPM Motor Using Magnetomechanical Coupled Analysis and Optimization

We present an optimization methodology to reduce magnetically induced vibrations of a spoke-type interior permanent magnet (IPM) motor that we developed by performing magnetic and structural finite element analyses and optimization. The magnetic forces acting on the teeth of the stator were calculated by magnetic finite element analysis and the Maxwell stress tensor method. The natural frequencies and mode shapes of the stator were calculated by structural finite element analysis and verified by modal testing. The vibration of the motor due to the rotating magnetic force was calculated by the mode superposition method, and it was compared with the measured vibration. Finally, two optimization problems were formulated and solved to reduce magnetically induced vibration: minimization of magnetic force and minimization of acceleration. We showed that minimization of acceleration was more effective than minimization of magnetic force at reducing magnetically induced vibrations, because the former method effectively decreased the amplitudes of the excitation frequencies of magnetic force by considering the transfer function of the motor.

Forced Vibration Analysis of an IPM Motor for Electrical Vehicles due to Magnetic Force

This paper discusses a method to analyze the forced vibration of an interior permanent magnet (IPM) motor due to magnetic force. A structural finite element (FE) model of the IPM motor that includes stiffness of ball bearings and laminated effect of core is developed and verified by comparing the simulated natural frequencies and mode shapes with experimental data. A magnetic FE model is also developed and both the radial and tangential magnetic forces are calculated in the air-gap. These forces are subsequently transformed into equivalent nodal forces and applied to the structural FE model to investigate the forced vibration characteristics of the IPM motor. It was found that the magnetically induced vibration of a stator mainly results from the contribution of the dominant harmonics of the magnetic force and structural resonance. In addition, the magnetically induced vibration of the rotor mainly appeared as rigid body modes due to the flexibility of ball bearings.

Effect of solder pads on the fatigue life of FBGA memory modules under harmonic excitation by using a global–local modeling technique

This paper investigates the effect of solder pad size on the fatigue life of fine-pitch ball grid array (FBGA) solder joints in memory modules due to harmonic excitation by using a global–local modeling technique. Finite element analysis of a memory module requires enormous computer memory and computational time because some components such as solder balls are very small relative to the overall size of the memory module. The global–local modeling technique has been suggested as an alternative approach with reasonable accuracy. A finite element model of the memory module with simplified solder joints was developed as a global model, and the natural frequencies and modes were calculated and verified by experimental modal testing. Displacement variations were calculated from the global model due to vibration excitation using the mode superposition method. A finite element model of a part of the memory module, which is composed of a package, PCB, and detailed solder joints, was developed as a local model. Calculated displacements from the global model were then substituted along the boundary of the local model in order to detect vulnerable parts of solder joints under vibration. Utilizing the global–local modeling technique, the interface between the solder ball and pad near the PCB was found to be the most vulnerable part, and the effect of solder pad size on the fatigue life of the memory module was determined by using the Basquin equation and Miner’s rule. It was experimentally verified that the solder pad size in solder joints affects fatigue life as well as the reliability of solder joints under harmonic excitation.

Magnetically Induced Vibrations in an IPM Motor Due to Distorted Magnetic Forces Arising From Flux Weakening Control

This research investigates the characteristics of magnetic forces and magnetically induced vibrations due to changes in the phase angle of applied current arising from flux weakening control of an interior permanent magnet (IPM) motor. The magnetic force is analyzed using the Maxwell stress tensor method, and the vibration induced by the application of a rotating magnetic force is analyzed using the mode superposition method. The experiments are conducted to validate the simulated vibrations due to the distorted magnetic forces. This research shows that flux weakening control may increase the magnetically induced vibration due to increases in tangential magnetic forces.

Failure mechanism of FBGA solder joints in memory module subjected to harmonic excitation

This paper investigates the failure mechanism of Fine-pitch Ball Grid Array (FBGA) solder joints of memory modules due to harmonic excitation by the experiments and the finite element method. A finite element model of the memory module was developed, and the natural frequencies and modes were calculated and verified by experimental modal testing. Modal damping ratios are also obtained and used in the forced vibration analysis. The experimental setup was developed to monitor resistance variation of FBGA solder joints due to the harmonic excitation under Joint Electron Devices Engineering Council (JEDEC) standard service conditions. Experiments showed that the failure of the solder joints of the memory module under vibration mainly occurs due to resonance. Forced vibration analysis was performed to determine the solder joints having high stress concentration under harmonic excitation. It showed that failure occurs due to the relative displacement between PCB and package and solder joints are the most vulnerable part of the memory module under vibration. It also showed that cracked solder joints in the experiments match those in the simulations with the highest stress concentration.

Vibration and Noise in a HDD Spindle Motor Arising from the Axial UMF Ripple

We investigated numerically the characteristics of axial unbalanced magnetic force (UMF) due to axial magnetic design in the spindle motor of a hard disk drive (HDD). The HDD spindle motor has a magnet-overhang and a pulling plate to generate the axial magnetic force, which is applied to the fluid dynamic bearings (FDBs) as a preload in order to increase the axial stiffness of the HDD spindle system. However, the axial UMF ripple with the least common multiple (LCM) harmonics of pole and slot is generated by the pulling plate and magnet-overhang in the HDD spindle motor. We also investigated the characteristics of the axial magnetic forces generated in the pulling plate and the stator core, separately. We found that the axial magnetic forces generated in the pulling plate and the stator core have opposite phases. Furthermore, we proposed an optimal position of permanent magnet with respect to the stator core. We experimentally verified that the LCM harmonics of pole and slot in the vibration and acoustic noise mostly originate from axial UMF ripple and that the proposed design can effectively minimize the LCM harmonics in the vibration and acoustic noise of HDD systems.

Frequency Characteristics of BEMF, Cogging Torque and UMF in a HDD Spindle Motor due to Unevenly Magnetized PM

This research mathematically derives the frequency equations of back electromotive force (BEMF), cogging torque and unbalanced magnetic force (UMF) of a brushless DC (BLDC) motor due to unevenly magnetized permanent magnet (PM). The proposed mathematical equations are experimentally validated by comparing with the measured BEMF, cogging torque and UMF of a hard disk drive (HDD) spindle motor with 12 poles and 9 slots (12P9S). Only 3-multiple harmonics originated from uneven magnetization of the PM in the spindle motor with 12P9S introduce same frequency components to BEMF and slot-multiple harmonics to cogging torque, respectively. However, other harmonics of the uneven magnetization of PM except the 3-multiple harmonics introduce slot-multiple ±1 harmonics to UMF.

Cogging Torque and Unbalanced Magnetic Pull Due to Simultaneous Existence of Dynamic and Static Eccentricities and Uneven Magnetization in Permanent Magnet Motors

We mathematically investigate the cogging torque and the unbalanced magnetic pull (UMP) of permanent magnet motors caused by the simultaneous existence of dynamic and static eccentricities and uneven magnetization. We show that the harmonics of cogging torque and UMP are generated due to the dynamic and static eccentricities and the uneven magnetization, respectively. We also show that the simultaneous existence of dynamic and static eccentricities and uneven magnetization induces additional harmonics of cogging torque and UMP. We investigate the relative amplitudes of the harmonics of cogging toque and UMP generated by the eccentricities. The mathematical investigations are verified via numerical simulation.

Monte Carlo Simulation of the Manufacturing Tolerance of FDBs to Identify the Sensitive Design Variables Affecting the Performance of a Disk-Spindle System

This research investigates the Monte Carlo simulation of manufacturing tolerance of FDBs to identify the sensitive design variables for the friction torque of fluid dynamic bearings (FDBs) and the critical mass of disk-spindle system supported by FDBs. We analyze the characteristics according to design variables of FDBs and it shows that the clearance of journal bearing is most sensitive design variable of both friction torque and critical mass. Also the groove to groove and ridge ratio and groove depth of grooved journal bearing which are manufactured by ECM are also sensitive to determine the friction torque and the critical mass of the FDBs, respectively. This research can be utilized to manage manufacturing tolerance to maintain the consistent performance of FDBs and a disk-spindle system in a HDD.Copyright

Detection of static and dynamic eccentricities in a permanent magnet motor by monitoring BEMF

One of the major faults of electric motors is a breakdown of bearing because it is the most flexible component under cyclic loading between stator and rotor. Most of bearing faults in electric motors result from localized defect of bearing and generate vibration and noise, which eventually degrades the performance of motor-driven systems. Many researches have been proposed various methods to detect bearing faults by measuring vibration and noise. In electrical point of view, bearing fault changes static and dynamic air gap, which change magnetic field and input current of electric motors. Several researchers studied the diagnostic technique of motor faults by monitoring vibration and current. However, another possible signal of electric motors due to static and dynamic eccentricities is back electromotive force (BEMF) and this research proposes a method to detect static and dynamic eccentricities of a permanent magnet (PM) motor by measuring BEMF. It derives mathematical equations of BEMF due to static and dynamic eccentricities of a PM motor, and performed experiment to validate the derived equations.