Publications

This paper proposes a drive system for a three-phase hybrid stepping motor, combining sensorless closed-loop control with conventional open-loop control. It is characterized by sophisticated control providing both prevention of pulling out from synchronism and suppression of natural rotor oscillation, without any position sensor attached to the motor shaft. A switching technique in chopper control which can enlarge the speed range controllable in the sensorless closed-loop control is described. Starting and stopping sequences are developed to reduce mechanical natural oscillation produced in the transient state. Finally, the proposed drive system is compared experimentally with a conventional constant-current open-loop drive system. It is shown that the proposed drive system can perform the switchover from starting to sensorless closed-loop operation within 20 ms, and can reduce the natural oscillation caused just after positioning. © 2000 Scripta Technica, Electr Eng Jpn, 131(3): 80–90, 2000

This paper deals with a series active filter for harmonic-current compensation of a large-capacity diode rectifier with a capacitive load. Flux satulation may occur in a series matching transformer for connecting the series active filter to the ac side of the diode rectifier. This paper develops an analog controller of a dc magnetizing current flowing in the matching transformer, along with a model for the controller, to avoid the flux satulation. Operating characteristics of the controller are theoretically and exprimentally discussed, taking into account both an offset voltage in the controller and an error in a current sensor. Finally, this paper discusses a relation between magnetizing characteristics of the matching transformer and the amplitude of the magnetizing current.

An optimum control scheme for current-source inverters using static induction thyristors in various high-frequency induction heating applications is described. The optimum control scheme implies an optimal control of the firing angle and extinction angle relative to the output voltage in order to minimize the surge voltage and the switching loss during the commutation interval. The tailing current problems of the SI thyristors which significantly influence the optimum firing angle and extinction angle are discussed theoretically. The validity of the optimum control scheme is verified by experiments with a 78 kHz, 17.4 kW (290 V, 60 A) inverter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper presents a fully-digital-controlled shunt hybrid filter for damping of harmonic propagation in power distribution systems. The harmonic propagation is caused by resonance between line inductances and power capacitors installed for power factor correction. A possible solution to damping out harmonic propagation is based on installation of a shunt pure active filter at the end of a feeder. This paper proposes a shunt hybrid active filter characterized by series connection of a seventh-tuned LC filter per phase and a small-rated three-phase active filter. Like the pure filter, the hybrid filter is connected to the end bus of a feeder. The capacitor of the LC filter imposes a high impedance to the fundamental frequency, so that the fundamental voltage appears across the capacitor. This unique feature allows us to directly connect the hybrid filter to the 6.6-kV power line without step-down transformers. Furthermore, the capacitor used in this hybrid filter is lighter, cheaper and smaller than the transformer used in the pure filter. Theoretical analysis, along with experimental results obtained from a 200-V, 20-kW laboratory system, verifies the viability and effectiveness of the proposed hybrid filter.

The dramatic growth of power electronics applications has led to serious harmonic pollutions in the power system. The phenomenon of harmonic resonance resulting in severe voltage distortion has been reported. Various suppression techniques have been proposed to resolve this problem. The distributed active-filter system (DAFS) is a more effective solution by deploying multiple shunt active filter units (AFUs) in the system to cooperatively damp the harmonic resonance. This paper proposes a dynamic tuning scheme for the DAFS to enhance its filtering performance. Based on the measured voltage harmonic distortion at the installation point, each AFU can dynamically adjust its own filtering volt-ampere (VA) capacity to maintain the total harmonic distortion of the line voltage at an allowable level. The proposed strategy can be integrated with the droop control between the conductance command and VA consumption so that the harmonic-filtering workload is evenly shared among various AFUs. The operation principles and dynamic analyses are presented in this paper, and test results based on simulations and experiments validate the proposed approach.

This paper provides an experiment-based discussion on a modular multilevel cascade converter based on triple-star bridge cells (MMCC-TSBC) for a low-speed high-torque motor drive. The TSBC is a direct ac-to-ac power converter capable of achieving bidirectional power flow as well as drawing and feeding three-phase sinusoidal input (supply-side) and output (motor-side) currents with any power factor at both sides. This paper discusses active dc-capacitor-voltage control applied to the low-speed, high-torque motor drive. A specially-designed downscaled system combining a 320-V, 38-Hz, 6-pole, 15-kW induction motor with a 400-V, 15-kW TSBC is constructed and tested to confirm the validity of the motor drive. Experimental waveforms obtained from the downscaled system confirm stable operation with the rated load torque across a range from a standstill to the rated speed, including satisfactory start-up performance.

This paper provides a comprehensive discussion on the topology and terminology of the family of modular multilevel cascade converters, including their chronological review. It is followed by answering the following question: "What motivated the author to apply phase-shifted-carrier PWM to the multilevel converters?" The PWM is achievable from integrating inter-cluster balancing or inter-arm balancing control into the middle layer of a hierarchical control system consisting of three layers. This integration makes it easy to expand the phase-shifted-carrier PWM to any bridge-cell or chopper-cell count per cluster or arm.

This paper deals with a curious phenomenon referred to as the "whack-a-mole" that may occur in a long-distance distribution feeder having many capacitors for power factor correction. The whack-a-mole is that installation of an active or passive filter on the feeder makes voltage harmonics increase on some buses whereas it makes voltage harmonics decrease on other busses, especially at the point of installation. The distributed-constant circuit theory is applied to a simplified feeder in order to perform analysis of the whack-a-mole, thus reaching such an effective way as to avoid the whack-a-mole. Moreover, both theory and experiment clarify that installation of the active filter acting as a harmonic terminator on the end bus of the feeder can damp out harmonic propagation throughout the feeder without causing any whack-a-mole.

This paper describes a next-generation 6.6-kV motor drive employing a bi-directional isolated dc/dc converter instead of a line-frequency transformer. It consists of cascaded six "converter cells" in each phase. Each converter cell includes a bi-directional isolated dc/dc converter and two single-phase PWM converters. On the focus is dc voltage control by the bidirectional isolated dc/dc converter, together with the implementation of the dc/dc converter. Small-signal analysis reveals that the control system can be described as a second-order system. The control scheme is confirmed by both computer simulation and experiment based on a dc/dc converter rated at 360 V, 10 kW, and 20 kHz. Simulation and experimental results verify effectiveness of the control method in both steady-state and transient characteristics.

This paper presents a zero-current-switched voltage-fed inverter equipped with resonant circuits on the ac side. The current flowing through a switching device, that is, IGBT, is the sum of the load current and the resonant current. When the amplitude of the resonant current is larger than that of the load current, the current in the switching device becomes zero at an instant in each resonant cycle. This allows the switching device to be turned on or off at the zero current. The zero-current switching makes a significant contribution to reduction of switching losses and electromagnetic noises. In this paper, the principle of zero-current-switching operation, along with a novel control scheme, is described from a theoretical and practical point of view. Experimental results obtained from a laboratory system of 5 kVA verify the practicability. Moreover, the switching and conduction losses of the proposed soft-switched inverter are compared with those of a conventional hard-switched inverter. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 85–95, 2000

A modular multilevel converter (MMC) is one of the next-generation multilevel PWM converters intended for high- or medium-voltage power conversion without transformers. The MMC consists of cascade connection of multiple bidirectional PWM chopper-cells and floating dc capacitors per leg, thus requiring voltage-balancing control of their chopper-cells. However, no paper has been discussed explicitly on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of modular multilevel PWM converters with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the MMCs to achieve voltage balancing without any external circuit. The viability of the MMCs as well as the effectiveness of the PWM control method is confirmed by simulation and experiment.

This paper presents dynamic control and performance of a unified power flow controller (UPFC) intended for installation on a transmission system consisting of two sets of three-phase transmission lines in parallel. When no UPFC is installed, interruption of either three-phase line due to a fault reduces an active power flow to half, because the line impedance becomes double before the interruption. Installing the UPFC makes it possible to control an amount of active power flowing through the transmission system. The validity of the theoretical analysis developed in this paper is verified by experiments using a 10-kVA laboratory setup, as well as a computer simulation.

This paper proposes a new inverter-fed induction motor drive system with a slip-frequency estimation circuit, in which the slip-frequency can be accurately estimated even under transient state without any speed sensor. The authors clarify the definition of "the instantaneous slip-frequency under transient state", and show some experimental results attained by the instantaneous slip-frequency estimation circuit.