M. Tadokoro

3-Phase Inverter Using Frequency Divide Function of Ferro-resonant Circuit

Ferro-resonant type power supplies are highly reliable, and are therefore used as power supplies for electronic devices. Recently a three-phase inverter (called a phase-conversion type inverter), a combination of a single-phase square-wave inverter and a phase converter utilizing the phase conversion function of a ferroresonant circuit, has been proposed. However, the use of a phase converter causes difficulties, in that the output voltages are not balanced and the efficiency remains at about 60%. This paper reports experiments, analyses and reviews three-phase inverters (frequency-divide type inverters) which combine a phase converter utilizing a frequency-divide function with a single-phase square-wave inverter. As a result, the device characteristics were determined, and a high output balance and high system efficiency were achieved.

Unstable Phenomena in a Two-Phase Nonlinear Circuit

A ferroresonant-based two-phase circuit functions as a parametric oscillator, and is widely used in various kinds of power devices. When the load conductance of the circuit is very small, the parametric oscillations occurring in the circuit become unstable. It has been reported that the occurrence of unstable oscillations is due to the resistance of the power supply side of the circuit. When the magnetization curve of the circuit reactor has a sharp saturation characteristic, unstable oscillations differing from the above-mentioned mode occur in the first circuit (load side) of the two-phase circuit. This paper clarifies the relationship between the unstable state of the circuit and the nonlinearity of the magnetization curves of the cores used in the circuit, and confirms experimentally the validity of the analysis.

An Analysis of Abnormal Behavior in Parametric Oscillations

Parametric oscillations occurring in nonlinear magnetic circuits have wide applications in such devices as phase converters, parametric transformers and parametric motors. When such devices are operated under a light loadr the parametric oscillations come into a state of fluctuation or quasi-periodic oscillation, and normal device operation becomes impossible. This paper analyzes the outbreak of such abnormal behavior, and the regions in which the phenomena occur.

Quasi-Periodic Oscillations in a Three Phase Ferro-Resonant Circuit

When a three-phase voltage is applied to ferro-resonant circuits connected in a Y-connection, the third harmonic voltage can be obtained in the zero-phase-sequence circuit. Usually, the waveform of this voltage is almost sinusoidal and the oscillation is stable, but under some conditions the voltage changes to a state of quasi-periodic oscillations. This paper deals with the analysis of the mechanism of the quasi-periodic oscillations in a three-phase ferro-resonant circuit.

Frequency-Dividing-Type Three-Phase Inverter with Adjusting Capacitor

Frequency-dividing three-phase inverters, which consist of a single-phase square wave inverter and a three-phase ferroresonant circuit, have excellent efficiency, constant voltage characteristics, and three-phase balance. These advantages make them suitable for use as dc-ac converters and uninterruptible power supplies (UPS). However, when such three-phase inverters are used as UPS devices, a startup circuit with a complicated sequence is needed to reduce the capacitance of the single-phase inverter. If the sequence is simplified a separate problem emerges?the phase sequence of the voltage cannot be determined. In this work, the relationship between circuit imbalance and the phase sequence of three-phase voltages is clarified through both experiments and analyses. The results provide useful information on circuit constants which can be used to reduce costs through simplification of startup circuits for these applications.

Parallel Operation of Magnetic Frequency Triplers

Magnetic frequency triplers provide high reliability and a good output waveform, and therefore have been widely. used This paper reports experiments and analyses of the parallel running of two frequency tripler units to increase the capacity of the device. Parallel running creates a problem involving effective loads, in which the frequency tripler whose self-impedance has a proportionately smaller resistance component is the first to be overloaded. The authors analyzed the parallel running characteristics using the harmonic balance method. The relationship between the circulating power and the device circuit constants was clarified, and parallel running conditions which increase the effective load were determined.

An Analysis of Extraordinary Oscillation of Potential Transformers

A simulation model of extraordinary oscillation in potential transformers (PT) was studied to determine conditions for its occurrence and countermeasures. The extraordinary oscillation is analyzed by solving the simultaneous equations derived from the circuit state equation by the harmonic balance method. Addition of a damping circuit on the PT side, or of a ground compensating capacitance on the system side, are suggested countermeasures, but the analysis results indicate that it is possible to introduce a rational setting method.

Operating Characteristics of Magnetic Frequency Multiplier with Core-Type Zero-Phase-Sequence Circuit

A new type of magnetic frequency multiplier is proposed for improved performance. This device has core-type output windings installed collectively by combining the cores into a T. The length of output winding of this type of device can be reduced to about 56 percent of the conventional type. The core advantages of this type of magnetic frequency multiplier are as follows: (1) high efficiency equivalent to a large-scale device can be obtained easily; (2) fluctuations in the output voltage can be reduced. In this paper, operation of a core-type frequency multiplier is analyzed by the harmonic balance method considering the unbalance of the circuit. The mechanism of the two-step jumps which occur in the nonlinear zero-phase-sequence circuit is clarified by results of the analysis.

Extraordinary Oscillation in Nonlinear Polyphase Circuit

Polyphase circuit phenomena are discussed from the standpoint of improving reliability of conversion detection devices used in measuring equipment. A nonlinear three-phase circuit was analyzed by the harmonic balance method. It was found that the line impedance ZL and line resistance RL have a very large effect on the zero phase voltage and extraordinary oscillation region, and that the line and protective relay load capacitances CL and CO are closely related to the skip phenomenon.

Operating Characteristics of Hybrid Inverter System

As part of research to combine an inverter with a frequency converter, thereby increasing the device frequency limits, the authors investigated the improvement of output waveform distribution of the hybrid system, in an enhanced design to minimize reductions in efficiency due to cascaded connection of the two types of converter. The necessity of system design enabling operation in the oscillation region of the first zero-wave component, but not the second zero-wave component, was demonstrated.