10-03-2011, 03:01 PM
Presented By
A.ARTHI JANNIE
ME project report.docx (Size: 3.78 MB / Downloads: 207)
HARMONIC ANALYSIS OF SEPARATELY EXCITED DC MOTOR DRIVES FED BY SINGLE PHASE CONTROLLED RECTIFIER AND PWM RECTIFIER
ABSTRACT
PWM rectifier is an ac to dc power converter which is implemented using power electronic semiconductor switches such as MOSFET, IGBT. Different from diode bridge rectifiers, PWM rectifiers achieve bidirectional power flows,which can be extensively used in battery charger, UPS system, regulated dc voltage source. SCR converters provide low power factor and also result in higher order harmonics. Previously, in most of the cases, the ac-dc power conversion was extensively carried out using passive techniques. These include diode bridge rectifiers and phase controlled thyristor rectifiers with suitable passive filters at the output. With the advent of fast semiconductor devices such as MOSFET & IGBT and the development of various pulse width modulation techniques, passive filters are increasingly replaced with PWM rectifiers.
This project presents the design and harmonic analysis of Single Phase PWM rectifier fed separately excited DC motor drives. The scope of this paper is to reduce the current harmonics at the input side of the PWM rectifier. A PWM based switching technique is proposed to reduce the harmonics. Single phase PWM rectifier and Single phase controlled rectifier comparison on the basis of the input current harmonic analysis is presented with the help of MATLAB simulation and experimental values. The programming is done in FPGA to generate gating signals.
1. INTRODUCTION
1.1 POWER ELECTRONICS
At the beginning of the 19th century, electric energy came into use in many technical fields. From the beginning ways were sought to change parameters such as voltage, frequency and current. The converters of electric parameters can be divided into two main groups. The first group uses for change the Faraday’s law of induction e.g. the Ward-Leonard drive. The second group includes converters that work on the controlled switching principle, i.e., semiconducting rectifiers, inverters, etc. These power semiconductor switching systems are high efficiency due to low loss in power semiconductor device and high reliability. The key element is the switching converter. A converter is a static device that converts DC to DC, AC to AC, AC to DC, and DC to AC. In general, a switching converter contains power input, control input ports and power output port. The raw input is processed as specified by the control input yielding the conditioned output power. In an AC to DC converter the AC input voltage is converted into DC output voltage.
1.2 PHASE CONTROLLED RECTIFIER
The electric energy conversion made by semiconducting converters is being used more and more. This had led to the growth of negative phenomenon that appeared negligible, when only a few converters are being used. However the development of semiconductor structures has enabled higher power to be transmitted and has also led to wide spread of converters. In this way, converters have a negative effect on the supply network. The regressive effects of overloads with harmonics and reactive power consumption are becoming major disadvantages of phase controlled (mostly thyristor) rectifiers. These side effects need to be compensated by additional filtering circuits with capacitors or inductances. However, such circuits raise the costs and also increase
material and space requirements for the converter. Phase control and commutation of semiconducting devices impact the phase displacement between the first harmonics of the consumed current and the first harmonics of the supply voltage. This displacement leads to power factor degradation and to reactive power consumption. The consumed current harmonics cause non-sinusoidal voltage drops on the supply network impedances and lead to supply voltage deformation. This may cause malfunctions of other devices that are sensible to the sinusoidal shape of the supply voltage (e.g. measurement apparatus, communication and control systems). The reactive power rises with longer control angle delays, so the rectifier acts as time variable impedance that is nonlinear and causes deformed current consumption.
1.3 PWM RECTIFIER
In order to suppress these negative phenomena caused by the power rectifiers, use is made of rectifiers with a more sophisticated control algorithm. Such rectifiers are realized by semiconductors that can be switched off MOSFET transistors. The rectifier is controlled by pulse width modulation. A rectifier controlled in this way consumes current of required shape, which is mostly sinusoidal. It works with a given phase displacement between the consumed current and the supply voltage. The power factor can also be controlled and there are minimal effects on the supply network. Harmonics produce electromagnetic distortion, and the network will be loaded with reactive power. The PWM rectifier aims to consume sinusoidal current and to work with given power factor.
Main features of PWM rectifiers are: bi-directional power flow, nearly sinusoidal input current, regulation of input power factor to unity, low harmonic distortion of line current (THD below 5 %), adjustment and stabilization of DC link voltage (or current), reduced capacitor (or inductor) size due to the continuous current. With the advent of high power, inexpensive, fast switching devices, line commutated rectifiers have been gradually replaced by pulse width modulated (PWM) switch mode rectifiers.