A grid-connected inverter is equipped with an LCL filter at its output. This filter has a resonant characteristic and many active damping methods have been researched to suppress the resonant effects. The inverter is often controlled digitally and there is a control time delay, which is inevitable due to A/D conversion, computation times, etc. This delay degrades the stabilities and damping effects of the inverter because the frequency characteristics are different from the designed one, especially around the resonant frequency. This paper proposes a simple and effective compensation method of the control time delay to improve the damping effects. It is based on an estimation method of future state values advanced by the delay time. The proposed method is applied to the grid-connected inverter control and its effectiveness is investigated and validated through determining transfer functions of the regulated current errors by experiment.
This paper presents a straightforward modeling method for the complex permeability of common mode chokes. The proposed model is obtained as an RL ladder network from a single RL parallel equivalent circuit by repeated manipulation called circuit dividing. Circuit dividing is the transformation of a single parallel RL circuit into two series-connected parallel RL circuits, and is used for expressing the complicated characteristics of inductive and resistive elements such as frequency-dependent complex permeability. The proposed model can be made to basically repeat the parameter fitting up to two parameters, so that the fitting procedure is straightforward. This is a great advantage for engineers because the model can be obtained without the need for special additional optimization programs, which were required in previous works. By using this model, the common mode impedance of a choke is fitted precisely, compared to the conventional single parallel RLC equivalent model. Moreover, not only the insertion loss of the EMI filter but also a time waveform is simulated precisely by both frequency and transient analysis with the proposed model.
In recent decades, the use of induction motors in industrial environments has increased, and the demand for both maintenance management and condition monitoring has expanded. A literature survey shows that the inter-turn short-circuit insulation failure of stator windings is one of the most likely faults to occur in motor-drive systems. This short-circuit fault is mainly due to the damage or deterioration of electrical insulation. Moreover, the short-circuit failure of one turn will initiate stator winding insulation failure, leading to the breakdown of the entire system. Hence, identification of one-turn insulation failures in early stages is essential. Thus, the present paper deals with such failures of the induction motor. First, the frequency spectrum of the load current is analyzed by fast Fourier transform, and the characteristic frequency components are extracted. Next, the distortion ratio of the load current is calculated using the above characteristic frequency components. Finally, a new diagnostic method applying a support vector machine is proposed, and its advantages are described. The experimental and diagnostic results are presented to validate the proposed analytical procedure using the distortion ratio.
This paper proposes a novel multicellular ac-dc transformer (ADX). The proposed ADX consists of sensorless isolated ac-dc cell converters based on the multicellular converter topology and a non-isolated dc-dc converter for power factor correction (PFC). A single ac-dc cell converter is composed of a diode rectifier and a dc-dc transformer (DCX) with no feedback controllers. These cell converters are connected in input-series-and-output-parallel (ISOP) and input-parallel-and-output-series (IPOS) configurations to achieve various voltage transformation ratios. One of the features of the proposed ADX is the high scalability with no master-slave control. Each power converter in the ADX requires its own local information, and no additional auxiliaries are required for the global control of the multicellular converter, because the ISOP and the IPOS connection topologies in the multicellular converter inherently achieve balanced voltage and equalized current among the cell converters. The proposed ADX has a simplified control system and accomplishes high scalability without requiring additional components. A simplified circuit analysis is carried out to show the feasibility of the proposed ADX, and an ac 60V - dc 40V laboratory prototype, with three pairs of cell converters connected in ISOP, is fabricated to verify the analysis. The proposed approach contributes to realizing future dc distribution systems in data centers, taking into account the prevalence of the standardized high-power-density converters.
This paper presents a controller design approach considering robust vibration suppression against resonant frequency variations in piezo-actuated systems. In piezo-actuated systems, the improvement of suppression performance with respect to external disturbances and nonlinearities such as hysteresis and creep by the expansion of feedback control bandwidth is indispensable for achieving high-precision positioning. The vibration suppression approach is a key technology for the expansion of bandwidth. From a practical point of view, a minor feedback loop including a simple compensator with a few parameters to increase the design freedom is added to the general feedback control system. In the minor-loop design, the reduction of sensitivity gain at around the resonant frequencies is considered to suppress the residual vibration of the references and disturbances, and the loop-shaping approach based on the frequency domain is adopted to provide an intuitive and practical design guideline. A major-loop for an augmented plant, including the minor-loop, is designed by considering the system stability, servo bandwidth, and settling performance. The proposed approach is verified by conducting experiments using a commercial piezo-actuated stage system.
Conventional motion copying systems are capable of performing repetitive fixed objectives. Motion copying systems can be used to handle different objects and tools. The inertia of the handled object affects the total system inertia of a motion coying system. If the total system inertia of the motion saving phase and motion reproduction phase are different, it affects the force and position tracking performances during motion reproduction. Inertia estimation should be performed in order to expand the applications of motion copying systems. Precise inertia estimation of a motion copying system is a challenging task when the actuator grasps different objects. This paper presents a method to estimate the inertia variation defends on the grasped object and the inertia compensation of a motion copying system while reproducing the recorded motion with different objects. The inertia of the grasped object is estimated by using a simple harmonic motion based method. The performance of the proposed method verified and discussed based on the experimental results.
With the increasing penetration of renewable energy in power systems, the requirement of fault ride through (FRT) capability becomes imperative for the stability of the grid. However, the topology and operation of the power system during the fault and fault clearance, especially during single phase disconnection, is seldom taken into account in most existing control schemes for grid-connected inverters. In this paper, a novel seamless positive-sequence current control method based on phase voltage with equal 120 degree dephasing and zero-sequence voltage decomposition is proposed. It realizes a smooth and rapid transition between normal operation, single phase fault, single phase disconnection without mode change, and injects no negative-sequence current to the grid during single phase disconnection which may last long and cause dangerous overheating of the rotor of conventional generators in a power system. The results obtained from the simulation and experiment demonstrate the effectiveness of the proposed method.
Interior permanent magnet synchronous motors (IPMSMs) are widely used because their size can be reduced and their motor efficiency can be improved by utilizing the residual flux of permanent magnets. The reduction of flux leakage from the bridge parts of a rotor is one of the important issues to enhance merit. In this paper, two structures to reduce flux leakage were proposed: 1) lightening of the rotor cores by using a multi-holes structure and 2) applying non-magnetic materials for reinforcing the mechanical strength of the bridge parts. Measurement of a prototype applying the above structures was also performed in a no-load and load condition. The no-load test showed a 3.1% increase in no-load back EMF, and the load test clarified that the motor loss at 8.1Nm and 1000min-1 was decreased by 1% as an effect of bridge narrowing compared with a conventional motor.