S. J. Sung

Characterization and Experimental Verification of the Axial Unbalanced Magnetic Force in Brushless DC Motors

The characteristics of the axial unbalanced magnetic force (UMF) in brushless dc (BLDC) motors were numerically and experimentally investigated. Three-dimensional finite element analysis was conducted to characterize the axial UMF. An experimental device was also developed to measure the axial UMF and verify the simulated results. The axial asymmetry of the permanent magnet (PM) with respect to the stator was found to be a major source of the axial UMF, whose driving frequencies are the least common multiples of the pole and slot. Additional uneven magnetization of the PM and rotor eccentricity introduce slot harmonics to the axial UMF, while additional stator eccentricity generates fundamental and pole harmonics.

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.

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.

Torque Ripple and Unbalanced Magnetic Force of a BLDC Motor due to the Connecting Wire Between Slot Windings

In this research, the effect of the connecting wire between slot windings on the torque ripple and unbalanced magnetic force (UMF) of a brushless dc (BLDC) motor was investigated. A 2-dimensional finite element model of a BLDC motor that includes the effect of the connecting wires was also developed. It was found that the connecting wires generate pole harmonics in the torque ripple and UMF. Thus, a novel pattern of the connecting wire where all coils are located at rotationally symmetric positions is proposed in order to eliminate the pole harmonics of the torque ripple and UMF induced by the connecting wire. The experimental findings showed that the proposed pattern can effectively reduce the magnitude of the pole harmonics in the noise and vibration of a BLDC motor.

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.

Noise and Vibration Originated From UMF due to Rotor Eccentricity of the HDD Spindle System

Cogging torque and UMF (unbalanced magnetic force) are major excitation sources of acoustic noise and vibration originated from HDD spindle motors. They are generally outer rotor type motors with fluid dynamic bearings (FDBs). The FDBs support and constrain the rotating disk-spindle system in five degrees of freedom except axial rotating direction. Unbalanced mass of the disk-spindle system generates whirling motion and changes the characteristics of UMF. Several researchers have investigated the harmonic contents of cogging torque and UMF by numerical and analytical methods [1]–[3]. Lee and Jang [4] experimentally and numerically investigated the characteristics of the UMF of a HDD spindle motor due to manufacturing errors such as the uneven magnetization of permanent magnet (PM) and the eccentricity of rotor and stator. However, they discussed only the cogging torque and UMF, and did not investigate the effect of the cogging torque and UMF on acoustic noise and vibration of a HDD spindle system.Copyright

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