A Pwm Strategy For Ac Voltage Controllers

Published Date: 02 Nov 2017

Abstract — Ac voltage controllers has been widely developed to a matured level with improved power quality in terms of power factor correction, reduction in harmonic distortion .In this paper a PWM techniques for AC voltage controller has been designed for single phase converters. This paper presents a different approach which is based on equal area criteria. The method eliminates the harmonics of output voltage up to specified level, enabling linear control of the fundamental component of output voltage. Using digital simulation the proposed method is investigated with existing methodologies, experimental values are calculated using proper mathematical equation, results found feasible performance over wide range using MATLAB 9.1

Index Terms — Equal Area Criteria (EAC), Phase Angle Control (PAC), Symmetric Angle Control (SAC), Pulse Width Modulation (PWM), Raising and falling edges time in PWM pulse (Tr & Tf)

Introduction

Ac Voltage controllers are widely used in power electronics system in order to control the output Ac voltage. Industrial applications like soft starting of induction motors, lighting. Among the basic types available the simplest one is AC voltage controller allowing without change in supply frequency feeded to loads. On the other hand use of use of matrix converters provides output voltage 15% lower than the input voltage. Thyristors and triacs pair connected in antiparallel is usually employed .As it provides simplicity and reliability to control voltage over wide range economically. The coast, reliability and compact in size adds advantage to these types of controllers.

Symmetric angle control, Phase angle control and integral cycle control of Thyristors have been traditionally used over the last decade. These control techniques suffer from optimal performance such as delay in firing angle (α) leads to lagging power factor at the input side .A large firing angle α also enhances injection of higher order harmonics contents in load and supply voltage /current [1].

The reason behind the generation of harmonics is discontinuous operation of switches, delay in firing angle leads to lagging power factor at the input side of the load. The recent development and design technology made it feasible to improve the efficiency their by reducing losses in the form of harmonic elimination. Line commutated ac controllers provides better overall improvement and it has been operated in chopped mode [2]-[6].Both input and output voltage is chopped in segments and the output voltage is controlled by duty cycle of controlled switches.

The use PWM technique leads to get a near sinusoidal output voltage/current waveform with improved power factor, eliminating lower order harmonics[7]-[10].The input power factor must be taken in to account in order to meet the requirements at load side .[9] –[11] have not considered the changes in power factor at input side of controllers. Instead of that, the input power factor can be made in phase with output, independent of delay angle with the load power factor [11].

Switching control method is often used in drives which leads to losses such as switching losses their by leads to heating of induction motor drives. The reduction in switches aids to compact control and reliability and high switching frequency with acceptable power factor [12].

This paper produces a new configuration by PWM technique enhanced through equal area criteria. Under any sample of required sinusoidal fundamental output and the area of the Pulse occur during the same sampling time in the actual output are made equal [13].The chopped each sine wave symmetry is calculated using Fourier converter harmonic analysis [14]. The evaluated values are compared with existing decades. Proposed strategy aims to lowering the output voltage total harmonic distortion less waveform. Mathematical fourier analysis completely eliminates certain higher order harmonics spectrum as analyzed, their by improving power factor sufficiently high.

Circuit description and principal of operation

The proposed power circuit is as shown in Fig.1 The circuit has only two switches and a parallel connected diode to the source side, a resistive load is considered for analysis purpose. The circuit fallows Single phase fully operation, the series connected switches s1 and s2 used to connect periodically to the load, continues operation is considered by providing less interval time to the switches. The gating signals are continuously provided by pwm methods. The basic reason to use diode to enable in a circuit where a reverse voltage is encountered. The main theme behind this paper is gating of switches is based on equal PWM technique / continuous constant pulse widths.

Fig. 1 Proposed Circuit configuration

The switching patterns of the controlled switches are decided by the gate signal and polarity of the source voltage and load current. When the supply voltage and load current are of equal polarity, the normal switching takes place either S1 or S2 will conduct for a certain interval of time. Diodes will conduct in parallel with the thyristor switches D1, S2 turned on or D2 , S1 vice versa. By switching patterns continuous patterns of gating signal provided to lead continuous conduction mode of operation.

PROPOSED EQUAL AREA CRITERIA METHOD

Principal of the proposed method

The proposed method is entirely different from other approach it’s a combination of PWM and EAC, complete elimination of lower order harmonics is possible using EAC .Fourier theory for converter harmonic analysis is applied for mathematical analysis. The conceptual diagram of EAC is shown in fig 2

Fig 2 Equal area criterion for AC voltage controllers

The diagram is comprehensive for any number of samples in which the target waveform is chopped in to ‘m’ number of samples per half cycle, the actual output has only one pulse in every sampling that spreads between the two limits to and the pulse width during the kth sampling is ‘δk’.The pulse widths of the output waveform in sequence are directly proportional to the corresponding instaneous magnitude of the modulating waveform as seen in fig 2.The width of the PWM pulses is determined by equating the area under PWM signal to that under the reference waveform for every sampling interval. The pulse width is directly proportinal to the output voltage and inversily proportational to the output frequency.The width of the PWM pulse is determined by equating the area under the PWM signal to that under the refernce waveform for every sampling interval.Therfore pulse width are directly proportional to output voltage.

Fourier analysis is performed for each chopped waveform and the result is summened to give the alalysed origal waveform at the output terminals.

In this equation n is the nth harmonic, js is the sign and magnitude of sth pulse,xs is the angle at which the gate occurs.

For PWM type waveform equ(1) and equ (10) can be simplified for analysis.

Simulation and experimental results

The results presented in this section are specially chosen to provide a comparative basis for assessing the performance of propose strategy. The comparison is made between existing symmetrical angle control and Equal area criteria .The simulation results were obtained using the MATLAB software Package 9.1

Prosed Simulink Circuit and input/output waveforms

The propose Simulink circuit for single phase converter is a is shown in fig 3.

Fig. 2 Proposed Simulink model

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