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A high-resistance (R) connection in an induction-motor electrical circuit results in localized overheating and supply-voltage unbalance, which leads to a decreased efficiency and reliability and an increased fire hazard in the electrical distribution system and motor. Therefore, it is important to monitor and correct high-R connections for a reliable, efficient, and safe operation of the industrial facility. This paper focuses on the development of an online technique for detecting poor connections based on monitoring the asymmetry in the voltage and current measurement. The development of the technique begins with the derivation of the dynamic model of an induction motor with high-R connections. Based on the analysis of the model, two approaches for detecting poor contacts using the negative-sequence current and zero-sequence voltage are proposed. In addition to detecting the existence of faults, the location and severity of the fault can also be determined using the proposed method. An experimental study on a 10-hp induction machine under simulated and realistic high-R conditions shows that the proposed techniques provide a simple low-cost solution for reliably detecting poor contact problems at an early stage. It is also shown that the severity and location of the high-R contact fault can be assessed with high accuracy. Index Terms--AC electric machines, condition monitoring, diagnostics, electrical distribution system, high-resistance (R) connection, induction machine, symmetrical components.

The goal of stator-winding turn-fault (TF) detection is to detect the fault at an early stage and shut down the machine immediately to prevent catastrophic motor failure due to the large fault current. A number of TF detection techniques have been proposed; however, there is currently no method available for distinguishing TFs from high-resistance (R) connections (HRCs), which also result in three-phase system asymmetry. It is important to distinguish the two faults, since an HRC does not necessarily require immediate motor shutdown. In this paper, new sensorless online monitoring techniques for detecting and classifying stator TFs and high-R electrical connections in induction machines based on the zero-sequence voltage or negative-sequence current measurements are proposed. An experimental study on a 10-hp induction motor performed under simulated TFs and high-R circuit conditions verifies that the two faults can be reliably detected and classified. The proposed technique helps improve the reliability, efficiency, and safety of the motor system and industrial plant and also allows maintenance to be performed in a more efficient manner, since the course of action can be determined based on the type and severity of the fault. Index Terms--AC electric machines, condition monitoring, diagnostics, electrical-distribution system, high-resistance (R) connection (HRC), induction machine, interturn insulation failure, symmetrical components, turn fault (TF).

A new preventive maintenance strategy for the sensorless condition monitoring of inverter dc-link aluminum electrolytic capacitors based on equivalent series resistance (ESR) and capacitance C estimation is proposed. The main concept of the proposed method is to estimate the ESR and C of the capacitor, using the inverter whenever the motor is stopped. The parameters are estimated from the dc-link voltage and stator current measurements available in the inverter with the switched dc-link voltage applied to the motor stator winding. The temperature of the capacitor can also be estimated under thermal equilibrium based on the stator resistance estimate to take the influence of temperature variation into account. An experimental study on a 250-W permanent magnet synchronous motor performed under accelerated capacitor degradation is presented to verify the proposed technique. It is shown that the proposed method provides a reliable and sensitive indication of capacitor aging without additional hardware requirements for reliable, efficient, and safe operation of the inverter and driven process. Index Terms--Adjustable-speed drive, aluminum electrolytic capacitor, capacitance, condition monitoring, dc-link capacitor, diagnostics, electrolyte, equivalent series resistance (ESR), stator resistance estimation.

In this paper, a novel topology for a photovoltaic (PV) dc/dc converter that can dramatically reduce the power rating and increase the efficiency of a PV system by analyzing PV module characteristics is proposed. Based on the analysis, in the proposed topology, only 30.7% power of the total PV system is needed for a dc/dc converter. Furthermore, the dc/dc converter efficiency curve is flat under wide PV module voltage and all load ranges. In particular, the converter efficiency at the lower duty range is dramatically improved. The total PV system is implemented for a 250-kW PV power conditioning system (PCS). This system has only three dc/dc converters with a 25-kW power rating. It is only one-third of the total PV PCS power. The 25-kW prototype PV dc/dc converter is introduced to experimentally verify the proposed topology. In addition, an experimental result shows that the proposed topology exhibits a good performance. Index Terms--DC/DC full-bridge converter, photovoltaic (PV) power conditioning system (PCS), voltage balance control, zero-voltage switching (ZVS).

The use of the controller area network communication protocol for H-bridge multilevel (HBML) inverters, and localized pulsewidth modulation generation and automated voltage regulation is proposed in this paper. It is shown that the performance and reliability of conventional distributed controllers for HBML inverters can be improved with less communication hardware requirements, when using the proposed method. It is also shown that the expansion and modularization capability of the HBML inverter is improved since individual inverter modules operate more independently. The proposed hardware is described in detail and the validity of the proposed system is verified experimentally on a 6600-V 1400-kW induction motor drive. Index Terms--Controller area network (CAN), H-bridge multilevel (HBML) inverter, pulsewidth modulation (PWM) phase-shift, PWM synchronization, serial communication.

The lifetime of the stator winding insulation is reduced when operated with pulsewidth-modulation inverters, since insulation degradation is accelerated due to increased thermal and electrical stresses on the motor. This paper focuses on a new approach for monitoring the condition of the stator insulation for failure-prone inverter-fed machines. The main concept of the proposed technique is to apply a dc and/or variable-frequency ac test voltage to the stator insulation using the inverter and to perform standard offline insulation tests whenever the motor is not operating. This allows the stator insulation to be tested more frequently as compared to offline tests performed once every three to six years during periodic maintenance. Two methods for testing the insulation with minimal modifications to the inverter circuit are proposed and verified experimentally. The results show that the proposed technique provides a simple low-cost solution for detecting stator insulation degradation at an early stage for reliable operation of inverter-fed machine systems, since the insulation condition can be monitored frequently. This does not only helps prevent forced outages and safety risks due to insulation breakdown but also helps perform maintenance in an efficient manner, since individual motor maintenance can be prioritized and scheduled based on its present condition. Index Terms--AC electric machines, capacitance (C) test, condition monitoring, diagnostics, dissipation factor (DF), insulation resistance (IR), inverters, predictive maintenance, stator insulation.

A stable sensorless controller for DTC of induction motor fed by three-level inverter using the Radial Basis Function Network (RBFN) is presented in this paper. The torque ripple can be drastically reduced and low speed performance can be obtained in the DTC system for high performance induction motor drives. However, speed control performance is still influenced by the lumped uncertainties of the system such as parameter variations, external load disturbances, and unmodeled dynamics which make it difficult to obtain an exact mathematical model. In this paper, the lumped uncertainties are estimated on-line by the RBFN. Simulations as well as experimental results are shown to illustrate the performance of the proposed system. Index Terms--AC motor drives, Direct Torque Control (DTC), neural network applications, observers.

A torque ripple reduction technique of direct torque control (DTC) for high power induction motors driven by three-level inverters with the inverter switching frequency limited around 0.5-1 kHz level is presented. It is noted that conventional two-level DTC algorithms to reduce torque ripple are devised for applications with relatively high switching frequency above 2-3 kHz and cannot accomplish satisfactory torque ripple reduction for three-level inverter systems with such lower switching frequencies. A new DTC algorithm, especially for low switching frequency inverter system, illustrates quite reduced torque ripple characteristics all over the operating speed region. Simulation and experimental results show effectiveness of the proposed control algorithm. Index Terms--Direct torque control, induction motor, three-level inverters, torque ripple reduction.

In this paper, a novel speed estimation method of an induction motor using neural networks (NNs) is presented. The NN speed estimator is trained online by using the error backpropagation algorithm, and the training starts simultaneously with the induction motor working. The estimated speed is then fed back in the speed control loop, and the speed-seusorless vector drive is realized. The proposed NN speed estimator has shown good performance in the transient and steady states, and also at either variable-speed operation or load variation. The validity and the usefulness of the proposed algorithm are thoroughly verified with experiments on fully digitalized 2.2-kW induction motor drive systems. Index Terms--Induction motor, neural networks, speed estimation, speed-sensorless vector control.

A direct torque control algorithm for three-level inverter-fed induction motors is presented. Basic voltage selection methods similar to two-level inverter provoke some problems such as stator-flux drooping phenomenon and undesirable torque control deterioration appeared, especially at low-speed operation. To overcome these problems, an algorithm with the basic switching sectors subdivided and intermediate voltage vectors applied is proposed in this paper. This algorithm basically considers applications in which direct torque-controlled induction motors are fed by three-level inverters with maximum switching frequency lowered around 1 kHz. An adaptive observer is also employed to bring better responses at the low-speed operation, by estimating some state variables and motor parameters which take a deep effect on the performance of the low-speed operation. Simulation and experiment results verify effectiveness of the proposed algorithm. Index Terms--Direct torque control, induction motor, low-speed operation, three-level inverters.

An attempt to degrade volatile organic compounds (VOCs) and sterilize air simultaneously is highly desirable to improve indoor air quality. With the help of deep ultraviolet (UVC) lighting, harmful bacteria that exists in the air can be destroyed. Thus, a new photocatalytic substance that can break down VOCs under UVC irradiation is required. Here, we demonstrate the photocatalytic activity of β-Ga2O3 nanostructures, synthesized via the solvothermal method for removing formaldehyde (HCHO) under deep ultraviolet irradiation. The Raman and XRD results indicated that as-synthesized nanostructures showed β-crystalline phase with a monoclinic structure. The photoluminescence spectrum exhibited a broad and strong green emission peak at 510 nm, which was likely attributed to a considerable amount of oxygen and gallium vacancies formed during the calcinating process. The photocatalytic efficiency for decomposing HCHO at room temperature under deep ultraviolet irradiation (278 nm) of the synthesized β-Ga2O3 nanoparticles is higher than that of the β-Ga2O3 nanorods. Both nanoparticles and nanorods obeyed the pseudo-first-order Langmuir-Hinshelwood kinetic model with a degradation rate constant of 0.057 and 0.033 min−1, corresponding to the efficiency of 82% and 62% in the formaldehyde removal, respectively. [ABSTRACT FROM AUTHOR]