J. Y. Song

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.

Real-Time Detection of the Dynamic Eccentricity in Permanent-Magnet Synchronous Motors by Monitoring Speed and Back EMF Induced in an Additional Winding

We developed a real-time method to detect the dynamic eccentricity of a rotor in a permanent-magnet (PM) motor by monitoring a fault detection signal induced in an additional winding, without performing any further postprocessing, even under a nonstationary rotational speed. After deriving a mathematical equation of the back electromotive force (EMF) induced in a tooth-coil winding, we proposed a fault detection signal, which is the back EMF in an additional winding divided by the rotational speed, when the additional winding is wound around the teeth corresponding to an even number of pole pitches. We used the 2-D finite-element model of a three-phase PM motor with eight poles and 12 slots to verify the proposed method. We also developed an experimental setup which can change the dynamic eccentricity of a PM motor and we performed the experiment for PM motors with dynamic eccentricities of 0%, 25%, and 50% to verify the proposed method. Through the mathematical equation, numerical simulation, and experiment, we confirmed that the fault detection signal proposed in this paper can successfully detect the dynamic eccentricity in a PM motor in real time.

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.

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.

Comparison of one-way and two-way coupled analyses of electromagnetic machines considering magnetic and structural interactions

We compared one-way and two-way coupled analyses of electromagnetic machines considering magnetic and structural interaction to identify the frequency components of magnetic excitation and to determine the structural coupling effects predicted only by the two-way coupled analysis. We developed finite element models of a C-core switch and an electric motor. In the two-way coupled analysis method, the magnetic force calculated by using the Maxwell stress tensor was applied to the structural finite element model to determine the elastic deformation, and the magnetic finite element model was rearranged by means of the moving mesh method to represent the structural elastic deformation. We showed that two-way coupled analysis predicted the excitation frequency of 80 Hz (4 times the input current frequency) of magnetic force in the C-core switch and the excitation frequency of 667 Hz (the first natural frequency of the rotor, corresponding to the translational mode) of magnetic force in the electric motor underg...

Unbalanced magnetic pull due to simultaneous existence of static and dynamic eccentricities and uneven magnetization in permanent magnet motors

We mathematically and numerically investigate the characteristics of unbalanced magnetic pull (UMP) in a permanent magnet (PM) motor due to simultaneous existence of static and dynamic eccentricities and uneven magnetization of PM. We mathematically show that pole number ±1 and pole number±2 harmonic components of UMP are generated due to static eccentricity with uneven magnetization and that pole number±m harmonic components of UMP are generated due to static eccentricity with dynamic eccentricity. Finite element analysis is conducted to verify the mathematical equations.

Effect of air gap between PM segments on cogging torque and acoustic noise of BLDC motor

This research investigates numerically the characteristics of the cogging torque in a brushless DC (BLDC) motor caused by the air gap between permanent magnet (PM) segments, and proposes a design method to reduce the effect of the air gap between PM segments on the cogging torque and acoustic noise of a BLDC motor. A 3D finite element model is used to calculate the cogging torque while a microphone is used to measure the acoustic noise and overall sound pressure level of the BLDC motor.

Effect of Additional Harmonics of Driving Current on Torque Ripple and Unbalanced Magnetic Force of the BLDC Motors With Stator and Rotor Eccentricities

This paper investigates the characteristics of the torque ripple and UMF in the BLDC motor with stator and rotor eccentricities due to additional harmonics of the driving current. Torque ripple can be divided into cogging torque due to the interaction between poles and slots, and commutation torque ripple due to driving current. Additional harmonics of driving current affect the characteristics of torque ripple. UMF is not generated in rotational symmetric motors with respect to pole, slot and winding configurations. However, stator and rotor eccentricities of BLDC motors generate additional harmonics on cogging torque and UMF. This research theoretically and numerically investigates the effects of driving current on the torque ripple and UMF of a BLDC motor with rotor and stator eccentricities. It shows that additional harmonics of the driving current, the stator eccentricity, and the rotor eccentricity independently affect the torque ripple. It also shows that additional harmonics of the driving current generate additional harmonics in UMF if BLDC motors have the stator or rotor eccentricities.Copyright