K. J. Kang

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.

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.

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

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 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