Wook-Jin Lee

DC-Link Voltage Stabilization for Reduced DC-Link Capacitor Inverter

In conventional motor drive systems using pulsewidth modulation (PWM) inverters, large electrolytic capacitors are used for stabilization of the dc-link voltage. Since the electrolytic capacitors are bulky and reduce reliability of the system due to short lifetime, there have been many efforts to eliminate or reduce the electrolytic capacitors in the motor drive system. However, the PWM inverter with reduced dc-link capacitor has a problem that the dc-link voltage is less stable compared to the conventional inverter because the capability of storing energy is also reduced. In this paper, a dc-link voltage stabilization algorithm using an active damping is proposed so that the dc-link voltage can be stabilized with reduced dc-link capacitor. To achieve load-/source-independent stabilization, a source state estimator which estimates both source voltage and current is also proposed. The fluctuation of the dc-link voltage due to a step load change can be also suppressed under the tolerance range using the estimated source current. The effectiveness of the proposed methods is evaluated by experimental results.

DC-link voltage stabilization for reduced dc-link capacitor inverter

A PMW inverter with reduced dc-link capacitor has a problem that the dc-link voltage is less stable compared to the conventional inverter because of the lack of energy storage capability. The proposed dc-link voltage stabilization algorithm using active damping gives a solution to this problem. To achieve load/source independent stabilization, the source state estimator which estimates both source voltage and current is also proposed. The proposed methods are evaluated by experimental results. While the inverter with a small DC link capacitor tripped due to dc-link overvoltage fault at step load change, the fluctuation of dc-link voltage of the inverter is suppressed under the tolerance range with the proposed method at the same step load change.

New flux weakening control for high saliency interior permanent magnet synchronous machine without any tables

This paper presents a new flux weakening algorithm for high saliency Interior Permanent Magnet Synchronous Machine. The proposed method modifies the current reference not by look-up tables, but by a controller in order to achieve a flux weakening. The current reference modification is determined by the d-q axis current and output voltage using the gradient descent method. The proposed method consists of two part, the determination of the flux weakening region and the current reference modification. The flux weakening region is determined by the angle between the constant torque direction and the voltage decreasing direction, and the current reference is modified by the flux weakening direction and the voltage magnitude according to the flux weakening region. Moreover, this method can be applied to the infinite speed drive system as well as the fininite speed drive system. The feasibility of the proposed method has been verified by the experimental results.

Single-Phase Active Power Filtering Method Using Diode-Rectifier-Fed Motor Drive

This paper presents a single-phase high power factor motor drive system with active power filter function. Since most of electrical equipment connected to the grid must comply with regulations regarding grid current harmonics, motor drive systems are generally equipped with Power Factor Corrector (PFC) which is comprised of power switches and reactive components, e.g., inductor and capacitor. The reactive components are bulky and increase system cost especially in low-cost applications such as electrical home appliances. In this paper, a new motor drive algorithm which is capable of both driving a permanent magnet motor and filtering harmonic currents produced by other non-linear loads belong to the system is proposed. Since the input current of the drive system is directly controlled by manipulating not the motor current reference but the output voltage reference of the inverter, it is possible to achieve exact and immediate control of the grid current. The effectiveness of the proposed algorithms is validated by experiments with a permanent magnet motor drive system.

Single-phase active power filtering method using diode-rectifier-fed motor drive

This paper presents a single-phase high power factor motor drive system with active power filter function. Since most of electrical equipment connected to the grid must comply with regulations regarding grid current harmonics, motor drive systems are generally equipped with Power Factor Corrector (PFC) which is comprised of power switches and reactive components, e.g., inductor and capacitor. The reactive components are bulky and increase system cost especially in low-cost applications such as electrical home appliances. In this paper, a new motor drive algorithm which is capable of both driving a permanent magnet motor and filtering harmonic currents produced by other non-linear loads belong to the system is proposed. Since the input current of the drive system is directly controlled by manipulating not the motor current reference but the output voltage reference of the inverter, it is possible to achieve exact and immediate control of the grid current. The effectiveness of the proposed algorithms is validated by experiments with a permanent magnet motor drive system.

A Doubly Fed Induction Generator Controlled in Single-Sided Grid Connection for Wind Turbines

This paper proposes a configuration of wind turbine using a doubly fed induction generator (DFIG) in single-sided grid connection. The stator of the machine is connected with a direct power grid, and the rotor is controlled by an inverter without any external power source. This is called a single external feeding of DFIG (SEF-DFIG). The conventional DFIG uses an additional grid power converter to regulate the rotor power; but in SEF-DFIG, the external source of a grid power is only connected to the stator windings. Due to this feature, the rotor-side inverter can be integrated on the rotor without any slip ring. In this paper, for applying the proposed scheme to the wind turbine, the performances of the generating operation are analyzed. The experimental results of a 2.4-kW DFIM system are presented to prove the feasibility of the proposed system and method.

A Protection Scheme Against Voltage Sags for Electrolytic Capacitorless AC Drives

This paper presents a protection strategy for the semiconductor switches in an electrolytic capacitorless converter. The proposed protection strategy is capable of protecting the semiconductor switches from overvoltage breakdown caused by the regenerative energy flowing from the load to the grid after the source lines are disconnected by the grid interruption. The energy stored in the machine inductance also may result in excessive DC-bus overvoltage due to the small DC capacitance in the electrolytic capacitorless converter. According to the proposed algorithm, the energy in the q-axis inductance is transferred to the d-axis inductance in the rotor reference frame while the DC-bus voltage is maintained within an allowable range by the proposed DC-bus voltage regulation during the grid (or source) interruption. The proposed protection strategy was verified by the experiments with an elevator system with a 10.8-kW 380-V interior permanent-magnet (IPM) hoist machine.

Input filter analysis and resonance suppression control for electrolytic capacitor-less inverter

This paper presents an analysis on electrolytic capacitor-less inverter and proposes a strategy for resonance suppression control for a DC-link voltage and input current in electrolytic capacitor-less inverter. The proposed electrolytic capacitor-less inverter can operate both in the motoring mode and regenerating mode owing to its inherent bidirectional power flow capability of the active front-end rectifier whose switching frequency is equal to the input line frequency. However, in the electrolytic capacitor-less inverter, the DC-link voltage is fluctuating with six times of the frequency of input three phase source due to its small DC link capacitance. Also, an input filter should be employed to remove the high frequency component generated by PWM inverter. By the input filter, the DC-link voltage can be resonant, and a specific resonance suppression control is required when the input line-to-line voltage is low and the demanded power is large. In this paper, a novel resonance suppression strategy is proposed for a stable operation of the electrolytic capacitor-less inverter. The experimental results show the effectiveness of proposed strategy.

A framework to support qos-aware usage of web services

Web services offer an easier, effective, efficient way to support a service paradigm. However, there is lack of adequate QoS support in using web services. In this paper, we propose a framework to support QoS aspects of web services. Three new elements are added to the original web service framework: QoS Server, QoS UDDI Broker and QoS Agent. In this framework, a requester is able to query more flexibly by using history QoS data of each web service with few load.

Sensorless vector control of super high speed turbo compressor

The goal of this paper is to control 118,000r/min turbo compressor driven by 12 kW, 2 pole surface mounted permanent magnet synchronous motor (SMPMSM) in a position sensorless mode. PLL-type observer is adopted for the angle and the speed estimation because of its simplicity and easy implementation. The analysis of the observer is described, and based on that analysis, the observer gains are set. Also, the experimental results to verify the analysis are shown.