Effects of modern variable frequency drive on the reliability of an old crane induction motor

The work aims to analyze the possibilities of utilizing old crane driving AC induction motors in modern pulse-width-modulated variable frequency drives. Bearing currents and voltage stresses are the two main problems associated with modern IGBT inverters, and they may cause premature failure of an old induction motor. The origins of these two problems are studied. An analysis of the mechanism of bearing failure is proposed. Certain types of bearing currents are considered in detail. The most effective and economical means are chosen for bearing currents mitigation. Transient phenomena of cables and mechanism of over voltages occurring at motor terminals are studied in the work. The weakest places of the stator winding insulation system are shown and recommendations are given considering the mitigation of voltage stresses. Only the most appropriate and cost effective preventative methods are chosen for old motor drives. Rewinding of old motors is also considered.

A multi-cell variable frequency interleaved ZCS boost rectifier digitally controlled by FPGA

This paper presents a multi-cell single-phase high power factor boost rectifier in interleave connection, operating in critical conduction mode, employing a soft-switching technique, and controlled by Field Programmable Gate Array (FPGA). The soft-switching technique is based on zero-current-switching (ZCS) cells, providing ZC (zero-current) turn-on and ZCZV (zero-current-zero-voltage) turn-off for the active switches, and ZV (zero-vohage) turn-on and ZC (zero-current) turn-off for the boost diodes. The disadvantages related to reverse recovery effects of boost diodes operated in continuous conduction mode (additional losses, and electromagnetic interference (EMI) problems) are minimized, due to the operation in critical conduction mode. In addition, due to the interleaving technique, the rectifier's features include the reduction in the input current ripple, the reduction in the output voltage ripple, the use of low stress devices, low volume for the EMI input filter, high input power factor (PF), and low total harmonic distortion (THD) in the input current, in compliance with the IEC61000-3-2 standards. The digital controller has been developed using a hardware description language (VHDL) and implemented using a XC2S200E-SpartanII-E/Xilinx FPGA device, performing a true critical conduction operation mode for all interleaved cells, and a closed-loop to provide the output voltage regulation, like as a preregulator rectifier. Experimental results are presented for a implemented prototype with two and with four interleaved cells, 400V nominal output voltage and 220V(rms) nominal input voltage, in order to verify the feasibility and performance of the proposed digital control through the use of a FPGA device.

A phase-shifting control for variable frequency multi-cells interleaved boost pre-regulator based on FPGA device

In this paper were investigated phase-shift control strategies applied to a four cells interleaved high input-power-factor pre-regulator boost rectifier, operating in critical conduction mode, using a non-dissipative commutation cells and frequency modulation. The digital control has been developed using a hardware description language (VHDL) and implemented using the XC2S200E-SpartanII-E/Xilinx FPGA, performing a true critical conduction operation mode for a generic number of interleaved cells. Experimental results are presented, in order to verify the feasibility and performance of the proposed digital control, through the use of a Xilinx FPGA device.

Six-element circuit for maximum power point tracking in photovoltaic-motor systems with variable-frequency drives

A low-cost circuit was developed for stable and efficient maximum power point (MPP) tracking in autonomous photo voltaic-motor systems with variable-frequency drives (VFDs). The circuit is made of two resistors, two capacitors, and two Zener diodes. Its input is the photovoltaic (PV) array voltage and its output feeds the proportional-integral-derivative (PID) controller usually integrated into, the drive. The steady-state frequency-voltage oscillations induced by the circuit were treated in a simplified mathematical model, which was validated by widely characterizing a PV-powered centrifugal pump. General procedures for circuit and controller tuning were recommended based on model equations. The tracking circuit presented here is widely applicable to PV-motor system with VFDs, offering an. efficient open-access technology of unique simplicity. Copyright (C) 2010 John Wiley & Sons, Ltd.

Algorithm for Estimation of Electricity Consumption Reduction by Using Variable Frequency Drive of Main Line Pumps

The article is about using of variable frequency drives for reduction oil pumping main line pumps energy consumption. Block diagram of developed computer program is shown in the article. The computer program allows to determine the reduction of energy consumption and to estimate payback period of variable frequency drives.

A linearly-tunable OTA-C sinusoidal oscillator for low-voltage applications

A low-voltage, low-power OTA-C sinusoidal oscillator based on a triode-MOSFET transconductor is here discussed. The classical quadrature model is employed and the transconductor inherent nonlinear characteristic with input voltage is used as the amplitude-stabilization element. An external bias VTUNE linearly adjusts the oscillation frequency. According to a standard 0.8μm CMOS n-well process, a prototype was integrated, with an effective area of 0.28mm2. Experimental data validate the theoretical analysis. For a single 1.8V-supply and 100mV≤VTUNE≤250mV, the oscillation frequency fo ranges from 0.50MHz to 1.125MHz, with a nearly constant gain KVCO=4.16KHz/mV. Maximum output amplitude is 374mVpp @1.12MHz. THD is -41dB @321mVpp. Maximum average consumption is 355μW.

A low-voltage low-sensitivity sinusoidal VCO for DPLL realizations

A quasi-sinusoidal linearly tunable OTA-C VCO built with triode-region transconductors is presented. Oscillation upon power-on is ensured by RHP poles associated with gate-drain capacitances of OTA input devices. Since the OTA nonlinearity stabilizes the amplitude, the oscillation frequency f0 is first-order independent of VDD, making the VCO adequate to mixed-mode designs. A range of simulations attests the theoretical analysis. As part of a DPLL, the VCO was prototyped on a 0.8μm CMOS process, occupying an area of 0.15mm2. Nominal f0 is 1MHz, with K VCo=8.4KHz/mV. Measured sensitivity to VDD is below 2.17, while phase noise is -86dBc at 100-KHz offset. The feasibility of the VCO for higher frequencies is verified by a redesign based on a 0.35μm CMOS process and VDD=3.3V, with a linear frequency-span of l3.2MHz - 61.5MHz.

Radial basis function network applied to the linearization of a voltage controlled oscillator

An analog circuit that implements a radial basis function network is presented. The proposed circuit allows the adjustment of all shape parameters of the radial functions, i.e., amplitude, center and width. The implemented network was applied to the linearization of a nonlinear circuit, a voltage controlled oscillator (VCO). This application can be classified as an open-loop control in which the network plays the role of the controller. Experimental results have proved the linearization capability of the proposed circuit. Its performance can be improved by using a network with more basis functions. Copyright 2007 ACM.

An ice machine adapted into an autonomous photovoltaic system without batteries using a variable-speed drive

The adaptation of a commercially available ice machine for autonomous photovoltaic operation without batteries is presented. In this adaptation a 1040 W(p) photovoltaic array directly feeds a variable-speed drive and a 24 V(dc) source. The drive runs an induction motor coupled by belt-and-pulley to an open reciprocating compressor, while the dc source supplies a solenoid valve and the control electronics. Motor speed and refrigerant evaporation pressure are set aiming at continuously matching system power demand to photovoltaic power availability. The resulting system is a simple integration of robust, standard, readily available parts. It produces 27 kg of ice in a clear-sky day and has ice production costs around US$0.30/kg. Although a few machine features might be specific to Brazil, its technical and economical guidelines are applicable elsewhere. Copyright (C); 2010 John Wiley & Sons, Ltd.

Components and circuits for tunneling diode based high frequency sources

Terahertz (THz) technology has been generating a lot of interest because of the potential applications for systems working in this frequency range. However, to fully achieve this potential, effective and efficient ways of generating controlled signals in the terahertz range are required. Devices that exhibit negative differential resistance (NDR) in a region of their current-voltage (I-V ) characteristics have been used in circuits for the generation of radio frequency signals. Of all of these NDR devices, resonant tunneling diode (RTD) oscillators, with their ability to oscillate in the THz range are considered as one of the most promising solid-state sources for terahertz signal generation at room temperature. There are however limitations and challenges with these devices, from inherent low output power usually in the range of micro-watts (uW) for RTD oscillators when milli-watts (mW) are desired. At device level, parasitic oscillations caused by the biasing line inductance when the device is biased in the NDR region prevent accurate device characterisation, which in turn prevents device modelling for computer simulations. This thesis describes work on I-V characterisation of tunnel diode (TD) and RTD (fabricated by Dr. Jue Wang) devices, and the radio frequency (RF) characterisation and small signal modelling of RTDs. The thesis also describes the design and measurement of hybrid TD oscillators for higher output power and the design and measurement of a planar Yagi antenna (fabricated by Khalid Alharbi) for THz applications. To enable oscillation free current-voltage characterisation of tunnel diodes, a commonly employed method is the use of a suitable resistor connected across the device to make the total differential resistance in the NDR region positive. However, this approach is not without problems as the value of the resistor has to satisfy certain conditions or else bias oscillations would still be present in the NDR region of the measured I-V characteristics. This method is difficult to use for RTDs which are fabricated on wafer due to the discrepancies in designed and actual resistance values of fabricated resistors using thin film technology. In this work, using pulsed DC rather than static DC measurements during device characterisation were shown to give accurate characteristics in the NDR region without the need for a stabilisation resistor. This approach allows for direct oscillation free characterisation for devices. Experimental results show that the I-V characterisation of tunnel diodes and RTD devices free of bias oscillations in the NDR region can be made. In this work, a new power-combining topology to address the limitations of low output power of TD and RTD oscillators is presented. The design employs the use of two oscillators biased separately, but with the combined output power from both collected at a single load. Compared to previous approaches, this method keeps the frequency of oscillation of the combined oscillators the same as for one of the oscillators. Experimental results with a hybrid circuit using two tunnel diode oscillators compared with a single oscillator design with similar values shows that the coupled oscillators produce double the output RF power of the single oscillator. This topology can be scaled for higher (up to terahertz) frequencies in the future by using RTD oscillators. Finally, a broadband Yagi antenna suitable for wireless communication at terahertz frequencies is presented in this thesis. The return loss of the antenna showed that the bandwidth is larger than the measured range (140-220 GHz). A new method was used to characterise the radiation pattern of the antenna in the E-plane. This was carried out on-wafer and the measured radiation pattern showed good agreement with the simulated pattern. In summary, this work makes important contributions to the accurate characterisation and modelling of TDs and RTDs, circuit-based techniques for power combining of high frequency TD or RTD oscillators, and to antennas suitable for on chip integration with high frequency oscillators.

Estudo dos desvios detectados pelas imagens portais no cancro da próstata em radioterapia conformacional (3DCRT)

Mestrado em Radioterapia

Projecto e concepção de instalação frigorífica a NH3, para uma unidade de congelação de frangos em contínuo, na Venezuela

Trabalho Final de Mestrado para obtenção do grau de Mestre em Engenharia Mecânica

Simulation of offshore wind system with two-level converters: HVDC power transmission

A new integrated mathematical model for the simulation of offshore wind energy conversion system performance is presented in this paper. The mathematical model considers an offshore variable-speed turbine in deep water equipped with a permanent magnet synchronous generator using full-power two-level converter, converting the energy of a variable frequency source in injected energy into the electric network with constant frequency, through a high voltage DC transmission submarine cable. The mathematical model for the drive train is a concentrate two mass model which incorporates the dynamic for the structure and tower due to the need to emulate the effects of the moving surface. Controller strategy considered is a proportional integral one. Also, pulse width modulation using space vector modulation supplemented with sliding mode is used for trigger the transistor of the converter. Finally, a case study is presented to access the system performance. © 2014 IEEE.

Simulation of offshore wind system with two levels converters: HVDC power transmission

A new integrated mathematical model for the simulation of offshore wind energy conversion system performance is presented in this paper. The mathematical model considers an offshore variable-speed turbine in deep water equipped with a permanent magnet synchronous generator using full-power two-level converter, converting the energy of a variable frequency source in injected energy into the electric network with constant frequency, through a high voltage DC transmission submarine cable. The mathematical model for the drive train is a concentrate two mass model which incorporates the dynamic for the structure and tower due to the need to emulate the effects of the moving surface. Controller strategy considered is a proportional integral one. Also, pulse width modulation using space vector modulation supplemented with sliding mode is used for trigger the transistor of the converter. Finally, a case study is presented to access the system performance. © 2014 IEEE.