High-precision current source using low-loss, single-switch,three-phase AC/DC converter

A three-phase AC/DC converter based on isolated Cuk topology feeding an inductive load is presented. The main goal is to get a compact, highly stable current source to feed an electromagnet. A high power factor is achieved, at constant duty-cycle and switching frequency, by discontinuous input current mode operation. The converter presents a linear relationship between the duty-cycle and the output current, making it easier to design the control system. Additionally the voltage stress on the power transistor is constant and does not depend on the duty-cycle. An auxiliary circuit allows zero voltage turn-off while limiting the over-voltage on the switch produced by the transformer leakage inductance. Pulse-width modulation (PWM) control is used to reduce sensitivity to line disturbances and to eliminate the 300-Hz ripple on the output current. Experimental measurements taken on a 400-W prototype confirm theoretical forecasts     

Self-excited induction generator controlled by a VS-PWMbidirectional converter for rural applications

This paper concerns an application of a three-phase cage induction machine (IM) as a self-excited generator connected to the AC side of a voltage-source PWM bidirectional inverter. The generator is supposed to be driven by a low-head unregulated shaft hydraulic turbine. The proposed system is intended to be applied in rural plants as a low-cost source of high-quality AC sinusoidal regulated voltage with constant frequency. Simulation results are obtained based on the αβγ stationary reference frame model of the IM. The experimental results demonstrated that the system presents satisfactory behavior when feeding AC loads and during the startup of induction motors     

Improved three-phase high-quality rectifier with line-commutated switches

The paper presents a three-phase diode rectifier with an add-on simple cell with line-frequency commutated ac switches that is able to greatly improve both power factor and output voltage regulation of rectifiers with passive L-C filters. The boost action introduced by the commutation cell allows for a complete compensation of the voltage drop across the input inductors, so as output voltage regulation can be maintained from no-load to full-load. Moreover, as compared to other line-frequency commutated rectifiers, the proposed circuit allows compliance with the low-frequency harmonic limits defined in the technical report IEC 61000-3-4 for any power range. High-efficiency and minimum EMI are obtained due to the low-frequency commutations. A converter prototype was built and tested. The results confirm the theoretical analysis.     

Linear, steroidal, and triterpene esters, and steryl glycosides from Festuca argentina

Ester waxes and steryl glycosides of the grass Festuca argentina were studied. Saponification of the waxes from the petroleum ether extract led to n-hexacosanol as the major single linear alcohol, along with pentacyclic triterpenols, such as β-amyrin, germanicol, isobaurenol, lupeol, hopenol-a and hopeol, and low amounts of sterols, such as cholesterol, campesterol, stigmasterol, sitosterol and dihydrositosterol, identified by gas chromatography/mass spectrometry (GC/MS). Fatty acids were identified as methyl esters as C_12∶0, C_14∶0, C_16∶0, C_18∶0, C_18∶2, and C_20∶0. The occurrence of a wide chainlength range of fatty acids and a single linear alcohol closely matched for other reports on the tribe Festuceae. On the contrary, pentacyclic triterpenols with a variety of skeletons, especially isobauerenol, are not usual as esters of fatty acids in the Gramineae. Low amounts of steryl glycosides were also obtained from the methylene chloride percolate of the methanol extract. Upon acetylation followed by hydrolysis, aglycones were identified by capillary gas-liquid chromatography (GLC) and GC/MS. As Δ⁷-cholesterol, campesterol, stigmasterol, sitosterol, dihydrositosterol, and the sugars as glucose, xylose, and arabinose by GLC of the respective alditol acetates. This is the first report on the linear, steryl, and triterpenyl esters of F. argentina. It is noteworthy that Δ⁷-steryl glycosides are rare, and steryl monoarabinosides have not been proviously reported on the family Gramineae.     

Interaction between EMI filter and power factor preregulators withaverage current control: analysis and design considerations

The effects of a nonnegligible source impedance, due to the presence of an input EMI filter, on the stability of power factor preregulators (PFPs) with average current control are analyzed by using a state-space averaged model. Different from previous approaches, it allows us to derive a simple expression for the loop gain in terms of the converter current loop gain. The overall system stability was studied for boost, Cuk, and SEPIC PPP topologies. Based on this model, a simple modification of the standard current control loop is proposed which increases the converter robustness against instabilities. Comparison between model forecasts and experimental measurements was carried out using two prototypes, one based on the boost topology and the other based on the SEPIC topology, both rated at 600 W. Finally, the model accuracy was investigated with measurements at different current loop bandwidths     

Induction-generator-based system providing regulated voltage withconstant frequency

The electrical characteristics of an isolated induction-generator-based system are improved through the association with a voltage-source pulsewidth modulation (PWM) inverter. The electronic converter allows the achievement of a better system behavior in many aspects: voltage regulation, frequency stabilization, and reactive power compensation. The system operation strategy consists of maintaining constant synchronous frequency at the induction generator via an association with a PWM inverter. The system power balance and the generator voltage regulation may be accomplished by two different means: through the rotor speed regulation, or by sending part of the energy stored in the inverter DC side to the grid through a single-phase line, in case the rotor speed is not regulated and a single-phase grid connection is available. The obtained results demonstrated the system is stable, robust, and an effective source of regulated three-phase voltages     

A Line-Interactive Single-Phase to Three-Phase Converter System

This paper describes a line-interactive single-phase to three-phase converter. A typical application is in rural areas supplied by a single-wire with earth return system. The traditional objective of feeding a three-phase induction motor is not anymore the main concern for such conversion. Due to the evolution of the farm technology, some of the local loads (as electronic power converters, computers, communication equipments, etc) require high power quality that is intended as sinusoidal, symmetrical, and balanced three-phase voltage. Additionally, to maximize the power from the feeder, the system provides a unity power factor to the grid. A three-phase voltage source inverter-pulsewidth modulation converter is used for this purpose. The power converter processes a fraction of the load power and the energy necessary to regulate the dc link voltage. As it does not need to supply active power, it is not necessary to have a source at the dc side. However, if island mode operation is needed, a dc source must be available at the dc link to supply the load. The control strategy, design criteria, and experimental results are presented     

Performance improvement of soft-switched PWM rectifiers withinductive load

A new soft-switched AC/DC pulse width modulation (PWM) converter structure is presented. It is useful for current-fed inverters and PWM rectifiers. Typical applications are magnet power supplies, high-power AC motor drives, and active power filters with magnetic energy storage. Soft switching is provided at the expense of a limited increase of the circuit complexity as compared to usual hard-switching solutions. A discussion of the soft commutation process is presented. By a proper switching sequence, overvoltages across the converter switches can be fully eliminated. Moreover, light-load operation can be achieved. Both typical limitations of soft-switched current-fed topologies are therefore overcome. Simulation and experimental results demonstrate the actual converter capabilities