Beomseok Lee

Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes.

The nano-imprint lithography method was employed to incorporate wide-area (375 x 330 mum(2)) photonic-crystal (PC) patterns onto the top surface of GaN-based LEDs. When the 280-nm-thick p-GaN was partly etched to ~140 nm, the maximal extraction-efficiency was observed without deteriorating electrical properties. After epoxy encapsulation, the light output of the PC LED was enhanced by 25% in comparison to the standard LED without pattern, at a standard current of 20 mA. By three-dimensional finite-difference time-domain method, we found that the extraction efficiency of the LED tends to be saturated as the etch-depth in the GaN epitaxial-layer becomes larger than the wavelength of the guided modes.

Photonic Crystal Slab Waveguides Fabricated by the Combination of Holography and Photolithography

We have fabricated air-bridge type two-dimensional photonic crystal waveguides (2D-PCWs) using two high-throughput processes only: holography and photolithography. Despite the existence of misalignments of a defect line with respect to the air-hole arrays, waveguiding in both straight and 90°-bent PCWs was clearly observed at the wavelength of 1.55 µm. We also estimated the upper bound in propagation loss of our straight PCWs, which was measured to be about 40 cm-1.

Integrated Field and Armature Current Control Strategy for Variable Flux Reluctance Machine Using Open Winding

This paper proposes an integrated field and armature current control strategy for a variable flux reluctance machine (VFRM) with an open-winding topology. By using an open-winding inverter, the field and armature currents can be injected into a single coil as a sinusoidal current biased by the dc offset rather than the separated field and armature windings. The integrated current control can reduce the copper loss to half and extend the operating speed range owing to the reduction in the winding resistance. In order to utilize the zero-sequence current as a field current, a zero-vector modification technique is proposed, in which the switching-on time of the zero vectors is modified to generate the constant zero-sequence voltage between two inverters. The proposed scheme is implemented in a synchronous

A Mechanical Flux Weakening Method for Switched Flux Permanent Magnet Machines

dq0

Integrated field and armature current control strategy for variable flux reluctance machine using open winding

-axis frame with space vector modulation. For the validation of the proposed method, a machine model of the VFRM is developed and implemented in MATLAB/Simulink. The simulation and experimental results verify that the proposed strategy can effectively reduce the copper loss and extend the operating speed range.

Contribution of Current Harmonics to Average Torque and Torque Ripple in Switched Reluctance Machines

Several switched flux permanent magnet (SFPM) machines with mechanically movable flux adjusting technique are proposed, analyzed, and compared for applications which demand high speed and efficiency. By using ferromagnetic pieces--one per every stator pole or every alternative stator pole, which are named as flux adjusters (FAs) and located on the outside surface of the stator-the airgap flux density can be weakened and therefore the operation speed range and flux weakening capability can be improved. The influence of using all or alternative FAs on the open circuit results, electromagnetic performance, and torque-speed characteristics of the SFPM machine is investigated through three SFPM machines with different stator/rotor pole combinations, i.e., 12/10, 12/13, and 12/14. Moreover, although using FAs in all stator poles can significantly improve the flux weakening capability, alternative FAs can also achieve remarkable improvement although higher torque ripple is observed. Additionally, it has been found that the 12/13 combination is considered to be the most suitable candidate for alternative FAs technique. In addition to 2-D finite-element analysis (FEA) and analytical methods, 3-D FEA results accounting for the end-effect are presented. Furthermore, experiments are performed to validate the results.

Torque ripple reduction for 6-stator/4-rotor-pole variable flux reluctance machines by using harmonic field current injection

This paper proposes an integrated field and armature current control strategy for a variable flux reluctance machine (VFRM) with an open-winding topology. By using an open-winding inverter, the field and armature currents can be injected into a single coil as a sinusoidal current biased by the dc offset rather than the separated field and armature windings. The integrated current control can reduce the copper loss to half and extend the operating speed range owing to the reduction in the winding resistance. In order to utilize the zero-sequence current as a field current, a zero-vector modification technique is proposed, in which the switching-on time of the zero vectors is modified to generate the constant zero-sequence voltage between two inverters. The proposed scheme is implemented in a synchronous dq0-axis frame with space vector modulation. For the validation of the proposed method, a machine model of the VFRM is developed and implemented in MATLAB/Simulink. The simulation and experimental results verify that the proposed strategy can effectively reduce the copper loss and extend the operating speed range.

Integrated Field and Armature Current Control for Dual Three-Phase Variable Flux Reluctance Machine Drives

This paper investigates the contribution of current harmonics to average torque and torque ripple in switched reluctance machines. In order to account for the magnetic saturation under different current amplitudes and waveforms, the self-inductances are obtained by finite element calculation using frozen permeability method and subsequently used in the analytical derivation of an instantaneous torque equation based on the Fourier series analyses. Five different current waveforms are also considered in order to identify the contribution of current harmonics. It is found that the average torque is mainly contributed by the dc, first, and second current harmonics. Meanwhile, the fourth and fifth current harmonics have high influence on the torque ripple but much lower contribution to the average torque. In addition, the optimization of current waveforms in terms of the maximum average torque under a given RMS current is also discussed. All analytical analyses are validated by direct nonlinear finite element and experimental results.

On-line compensation of dead-time and inverter nonlinearity with disturbance voltage observer

The variable flux reluctance machines (VFRMs) are electrically field excited synchronous machines having both field and armature windings on the stator. In order to investigate the torque production of the 6/4 VFRM, an instantaneous torque equation is derived for the first time considering the inductance harmonics by using Fourier series. According to the equation, the undesirable torque ripples having multiples of third harmonic exist due to the inductance harmonics of the field and armature windings. Hence, the harmonic field current injection methods are proposed for the torque ripple reduction. The torque waveforms calculated by 2D-finite element analysis (FEA) are compared with the predicted results from the derived torque equation. Although the methods are proposed at which the magnetic saturation does not occur, the torque ripple can be also reduced in saturation region. Since the proposed method utilizes the machine parameters the influence of the parameter mismatch is investigated. It shows that the torque ripple reduction performance is not affected much unless the fundamental component of nominal inductance is underestimated. The experimental results verify that the proposed approach can effectively reduce the torque ripple in the VFRM.

Simple on-line dead-time compensation scheme based on disturbance voltage observer

An integrated field and armature current control strategy is proposed for dual three-phase variable flux reluctance machines (VFRMs). The proposed method utilizes a sinusoidal current biased by dc offset rather than separated field and armature currents in an external field current control. In order to generate the dc and ac components together, a dual three-phase inverter is adopted, in which one side of each stator winding is connected to a neutral point. The dq-axis currents are controlled by the dual three-phase inverter, whereas zero sequence current is generated as a field current by the zero vector redistribution between two inverters. Since the integrated current control allows an integrated winding, which reduces the winding resistance to half, the copper loss can be also reduced to half. To apply the vector control scheme, the voltage and torque equations are derived for dual three-phase VFRMs in a synchronous dq-axis frame. These equations are also utilized for the dynamic simulation with the aid of MATLAB/Simulink. The experimental results validate that the proposed strategy can effectively increase the efficiency and extend the operating speed range.