Yongsu Han

Sensorless displacement estimation of a shape memory alloy coil spring actuator using inductance

To measure the displacement of a shape memory alloy (SMA) coil spring actuator for feedback control, displacement sensors larger than the actuator are normally required. In this study, a novel method for estimating the displacement of an SMA coil spring actuator without a sensor is proposed. Instead of a sensor, coil inductance is used for estimating the displacement. Coil inductance is estimated by measuring the voltage and the transient response of the current. It has a one-to-one relationship with the displacement of the coil and is not affected by load. Previous methods for estimating displacement using resistance measurements are heavily affected by load variations. The experimental results herein show that displacement is estimated with reasonable accuracy under varying loads using coil inductance. This sensorless method of estimating the displacement of an SMA coil spring actuator can be used to build a compact feedback controller because there is no need for a bulky displacement sensor.

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.

Single external source control of doubly-fed induction machine using dual inverter

This paper proposes a drive system and a control method for a double inverter fed wound machine, where the stator and the rotor are fed by two variable frequency inverters. Compared with the conventional induction machine drive system, the proposed system can improve the operating area and the efficiency because the rotor side inverter allows an additional degree of freedom in control. Also, it has no external power source for the rotor side inverter. Due to this feature, the inverter can be integrated on the rotor shaft and connected without any slip ring. In this paper, the possible operating areas in the topology are analyzed, considering the rotor power balance for the stabilization of the rotor side dc link voltage. In addition, vector control method is proposed to regulate the torque, the flux and the rotor side dc link voltage. The experimental results present the feasibility of the proposed system.

Wound Rotor Machine Fed by a Single-Phase Grid and Controlled by an Isolated Inverter

This paper proposes a novel energy conversion system for variable speed drives. It consists of a wound rotor machine and an inverter without any rectifier and input filter. In the proposed system, the stator of the machine is directly connected to a single-phase grid and the rotor is connected to three-phase inverter isolated with any external power source. The inverter can not only be connected through slip rings but also integrated on the rotor due to the structure. In this paper, based on the positive and negative sequence model, the rotor, stator powers, and the torque capability in the rotor energy balance and unity grid power factor are analyzed. From these analyses, the vector control methods of the torque, speed, the dc-link voltage of the isolated inverter, and the grid power factor are proposed. Since the power supplied from the single-phase grid is pulsating and the machine is directly connected to the grid, the controlled torque and power inevitably pulsates at twice the grid frequency. Nevertheless, the machine can start, accelerate, and decelerate in the wide range. The experimental results present the performance and the feasibility of the proposed system.

Design of single switch DC-DC converter with parasitic parameters

This paper proposes an isolated single switch dc-dc converter suited to a high frequency switching operation. The impedance seen at the drain to source node of the switch is mainly designed for achieving zero voltage switching (ZVS) operation and reducing the voltage stress across the switch device. In the proposed converter, leakage inductances of the transformer are integrated to the resonant network. By the proposed design procedure, the transformer is designed to have a proper coupling coefficient in order to satisfy the design requirement solely by the leakage and magnetizing inductances without extra inductors. Thus, it is possible to miniaturize the converter size and achieve high power density. In addition, the junction capacitance of the switch device is used for establishing the resonant network, so it is possible to mitigate the non-linear effect of the junction capacitance of the switch device. As a design example, a 10MHz-18W GaN-based prototype is presented, and the performance of the proposed converter is verified.

Dynamic model and control of wound-rotor machine in single-phase grid system

This paper proposes a novel wound-rotor machine which has a single-phase stator winding and three-phase rotor windings. The machine is called Single-Phase Wound-rotor Machine (SPWM). The machine can be driven in motoring and generating modes in single-phase grid system. The dynamic model and control system of the machine are also designed. Typically, the double revolving field model is used for singlephase machines, but it includes only the steady-state characteristics. So, the double revolving voltage model was proposed for dynamic analysis. However, the model cannot be applied to the proposed machine since the model requires more than two stator windings. In the proposed model, the SPWM is reconstructed from a d-q equivalent circuit of the three-phase wound-rotor machine. It includes the dynamic characteristics of the machine. Besides, the vector control based on the model is considered by applying an isolated inverter to the rotor side. The speed and grid power factor can be simultaneously controlled through the inverter while the rotor power is regulated. Simulation and experimental results present the validity of the proposed model and control.

A Family of High-Frequency Single-Switch DC-DC Converters with Low Switch Voltage Stress Based on Impedance Networks

This paper presents a novel family of single-switch resonant dc–dc converters with low switch voltage stress. The single-switch resonant converter which has a ground-referenced switch is advantageous for implementing the gate drive circuit and operating at several-MHz switching frequency. However, the conventional ones mostly have high voltage stress on the switch, roughly 4–5 times the input voltage. In this paper, we propose the single-switch converter topologies derived from the drain-source impedance networks consisting of two inductors and two capacitors. The switch voltage of the proposed converters is shaped into a near trapezoid by designing the resonant networks to have the desired drain-source impedance. Furthermore, a simple and specific design scheme is presented here so that the peak switch voltage is lowered to 2.2–2.5 times the input voltage while zero voltage switching is achieved. Experimental results from a 20-W GaN-based prototype operating at 10 MHz demonstrate the feasibility of the proposed converter topologies and the design method.

Design and control of 10-MHz classs E DC-DC converter with reduced voltage stress

This paper presents a novel design method for class E dc-dc converter and a control scheme capable of operating in a wide load range. The class E converter is suitable for several-MHz switching frequency; however, the conventional design has two drawbacks: high voltage stress on the switch (theoretically about 3.6 times the input voltage), and a large input inductor. Here we propose the design scheme to reduce the peak switch voltage up to 2.2-2.5 times the input voltage without additional circuit element and to improve transient performance by a small input inductor. In addition, an on-off control is implemented to regulate the output voltage, allowing the fixed-frequency, fixed-duty-cycle operation and zero voltage switching (ZVS) at light load. Experimental results from a 20-W laboratory prototype operating at 10 MHz verify the feasibility and the effectiveness of the proposed method.

Copper loss minimizing control of Doubly Fed Wound Machine

For efficient control of electrical machines, it is necessary to generate a current command set that satisfies a minimum copper loss condition, where the iron loss can be neglected. In Doubly Fed Wound Machine (DFWM), of which the stator and rotor are fed by two voltage sources, more efficient control is possible since the rotor currents are also controlled, but the calculation of the optimal current command set is more difficult than in the cases of single inverter fed systems. In this paper, the stator and rotor current command set for the minimum copper loss of DFWM is mathematically determined, considering constraints of the stator and rotor currents, the rotor flux, and the stator and rotor voltages. In addition, the torque control method based on the result is proposed. The experimental results of a wound machine are presented to show the feasibility of the proposed control method.

A doubly-fed induction machine and the control in a single-phase grid connection

This paper proposes a drive system different from the conventional one using an inverter and rectifier or a back-to-back converter. It consists of a machine and an inverter without any input filter. Even though it has the simple structure, it can control the torque, speed and power quality of the connected grid simultaneously in variable speed. In the proposed system, the stator of the machine is directly connected to a single phase grid and the rotor is to a three phase inverter isolated from any power sources. The inverter can be integrated on the rotor or connected to it through the slip rings. The integrated configuration has merits in footprints and maintenances. In this paper, the possible operation areas in this topology are analyzed. The power balance of the rotor isolated from the grid is considered for the operation. The inverter of the rotor side can control the torque based on the vector control method while the rotor power is regulated. The power factor for the grid and shape of the grid current can be also controlled. The experimental results present the feasibility of the proposed system.