Power electronics developement in some important fields, such as utility systems, electric railways and industry applications, are outlined and future trends are discussed. Many of line commutation converters will be replaced by forced commutation converters using GTOs or IGBTs. Power electronics will be much applied to utility systems. Higher harmonics problems may increase their significance and suppression technologies using power electronics should be widely applied in various applicaton fields.

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Official notice of guideline on harmonics-control schemes dated 30, September, 1994 is expected to solve future harmonic interference problems by promoting the installation of active filters (AFs) in addition to LC filters on consumers' side.
We describe the concept and performance of our AF with simple estimation method of AF rating necessary for the harmonic compensation.

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The resonant type DC-DC converter with clamped tank capacitor voltage is very effective to reduce the resonant tank voltage and to avoid inverter arm short-through due to diode recovery times. However, the clamping diodes make the operation complex, which is difficult to apply the conventional analytical method. In this paper, a numerical method is applied to investigate the operation of the parallel resonant type. The circuit for given parameters is found to have 108 topological modes and 35 operation modes. The computed results are validated through experimental results of a proto-type IGBT circuit.

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Recently, the methods for controlling chaos, in which OGY method is famous, are proposed. Many of them can acheive the linear control in the region near the unstable periodic points locally. In this paper, the delayed feedback control method, which is proposed by Pyragas for the continuous system, are applied to control the magneto-elastic chaos and stabilize the unstable periodic orbital in them experimentally. In addition the analogy between the system and the metal cutting dynamics are discussed. The discussion will show the system in consideration has the wide physical meaning.

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Three-phase induction motor vector control and such like are carried out on the basis of equation of states. In the case of the squirrel-cage induction machine, the measurement of the constants in the equation of states are very difficult, because the equivalent rotor winding cannot be opened. Therefore, the authors define 2 new rotor winding constants K, M for the induction machine and clarify the measurement method of them. Next, the authors transform the usual equation of states into the one having measurable constants K and M. The simulation results obtained by the use of the K, M measured values and the above equation of states agree approximately with experimental results. It becomes clear from the above facts that the proposed measurement method is valid.

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Permanent magnet linear synchronous motors (PM-LSM) have been developed for factory automation, transportation applications and so on. The direct control of air gap flux is not available in the PM-LSM because the field flux is made by the permanent magnet. The air gap flux, however, can be weakened by the demagnetizing effect due to the d-axis armature reaction. This technique is called the flux-weakening. In the rotary type permanent magnet synchronous machines, the wide speed and constant power operation are realized by using the reluctance torque due to the saliency and the flux-weakening control.
In this paper, we examine the effect of interior permanent magnet arrangement for wide speed operation and the end effect in the linear structure.

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A ZVS Multi-Resonant DC-DC Converter (MRC) can be operated at high switching frequency. The MRC circuit can reduce the switing-loss and the effect of EMI, becouse it can absorb parasitic reactance into the resonant circuit and can prevent parasitic oscillations. However the conventional MRC circuit has some disadvantages such as control characteristics are non-liner.
In this paper, we describe the new MRC circuit which has two switcing-devices and two capacitors of the same capacitance. The output voltage of the new MRC can be controled from zero to the DC source voltage at both of posssitive and negative load-current.

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A mechanical system composed of some masses connected with flexible shafts is called multi-mass resonant system. When a response frequency of the system is close to a mechanical resonant frequency, a resonant phenomenon occurs. It is an important problem to realize an effective vibration suppression control in multi-mass resonant system. Recently, the researches on two-mass resonant system have been done vigorously, in which a disturbance observer, H_∞ control theory, etc, are used for reduction of vibration.
In this paper, we applied the disturbance observer or the H_∞ control theory to a symmetrical type three-mass resonant system like an elevator system. Effectiveness of vibration reduction method is confirmed by simulation results.

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Solar energy is more attractive and acceptable as one of available power sources in future from a view point of a clean and inexhaustible power supply. However, the electric power generated from a solar photovoltaic array largely depends on weather conditions. Therefore a constant mechanical power can not be obtained from one motor which is driven on the basis of the electric power from a solar photovoltaic array. In order to produce a stable mechanical power, the authors propose the parallel operation system using two induction machines.
This paper presents on the stability of the proposed system in which the machine A is related to a solar phtovoltaic array coupled with a general purpose inverter and another machine B is interconnected directly to the utility-grid AC line. It is shown that the system can generate a stable mechanical power with cooperation of the machine B even if the output power of the machine A becomes zero. Furthermore the operating frequency of the inverter is determined by utilizing the relation of the maximum power and the optimal operating voltage of a solar photovoltaic array. As a result, the output power of the machine A becomes always a maximum value under variable weather conditions.

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With enormously growing implementation of power electronic apparatus, harmonic current problems in the feeding power lines also have emerged up to our big task.
This paper deals with newly developed switching type ac/dc converter with three phase sinusoidal input. The main circuit contains only three active devices (IGBTs. The system only needs simple switching control with fixed duty factor to obtain sinusoidal three phase input currents in almost unit power factor; while it needs to vary duty factor control to adjust the output dc voltage. In this paper, experimental results are added in addition to the simulating work, and the characteristics of the system are investigated.

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It is well known that non-linear semiconductor circuit applications such as rectifier circuits have been widely used, so that various harmonics are generated in the power system. As a result the large capacity of the conventional thyristor rectifier circuit will be controlled to use. Consequently, the PWM converter using self-turned off switching devices will be put into wide applications.
In this paper, we propose the novel strategy of PWM wave control to minimize the surge voltage at the turned off of switching devices. For the first method, the saw-tooth carrier wave is compared with the input power phase voltage wave for sine reference, in which the polarity of the saw-tooth carrier waveform is reversed alternately at every 60° interval of the fundamental frequency. For the second method, the line voltage waveform is used for sine reference, in which the saw-tooth carrier wave is reversed at every 30° interval of the fundamental frequency. The input current PWM waveforms using the novel strategy are analyzed by means of the proposed switching function.

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The Large Helical Device is a fusion test device which has six pairs of superconducting coils that generate 4T of magnetic field at a plasma center. These coils are driven by six power supply units. Each unit has a two-paralleled double-star thyristor rectifier unit, a dc filter and a quench protection system. The quench protection system includes a new type of dc circuit breaker which consists of a conventional ac vacuum circuit breaker, a power fuse and a diode. Current control error of the power supply unit is limited by 0.04%+6A of target current value. Voltage ripple is also limited less than 1% (rms) of rated output voltage. Current range of these power supply units is 31.3kA to 17.3kA.

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