Power Electronics

publications.muet.edu.pk

AC voltage controllers are used to vary the output ac voltage from a fixed ac input source. They are also commonly called ac voltage regulators or ac choppers. The output voltage is either controlled by PAC (Phase Angle Control) method or on-off control method. Due to various advantages of ac voltage controllers, such as high efficiency, simplicity, low cost and ability to control large amount of power they efficiently control the speed of ac motors, light dimming and industrial heating, etc. These converters are variable structure systems and generate harmonics during the operation which will affect the power quality when connected to system network. During the last couple of years, a number of new semiconductor devices and various power electronic converters has been introduced. Accordingly the subject of harmonics and its problems are of great concern to power industry and customers. In this research work, initially the simulation models of single phase unidirectional and bidirectional ac voltage controllers were developed by using MATLAB software. The harmonics of these models are investigated by simulation. In the end, the harmonics were also analyzed experimentally. The simulated as well as experimental results are presented.

Proceedings of 5th International Conference on Power Electronics and Variable-Speed Drives, 1994

IEEE, 2014

This paper describes the development of a prototype of a microcontroller-based phase angle controlled single-phase AC voltage controller that can efficiently control AC voltage and also accommodates soft start capability for single-phase induction motors. The output voltage of the controller is regulated to maintain a desired fixed RMS value and provide stabilized output by implementing a feedback control system. One microcontroller generates PWM signals in synchronism with the supply voltage to control the firing angle of thyristors while a second microcontroller remains dedicated for measuring the RMS value of the output voltage and sending that to the main microcontroller for the purpose of feedback control. The provision for soft starting of a load is also incorporated into this prototype. Index Terms-AC voltage controller, Automatic voltage regulation, Soft start I. INTRODUCTION PWM-based AC voltage controllers are widely used in UPS and high power flexible AC transmission systems. This varying voltage output is used for dimming street lights, varying heating temperature in homes and industry, speed control of fans and winding machines and many other applications. These systems need switching elements which can bear high voltage. Frequently, high power MOSFETS are used as the switching element. Their advantage is that they generate less lower-order harmonics. They have the physical limitation of maximum blocking voltage; excessive heating is also an issue for MOSFET based controllers. Amongst other options, an anti-parallel pair of SCRs has the advantage over a TRIAC in controlling highly inductive loads. Although TRIACs have the advantage of a comparatively simpler gate circuit, TRIACs have lower dv/dt ratings than SCRs and are available in only small ratings. Moreover, the reliability of a SCR is more than that of a TRIAC [1]. SCR is a reliable solution when it comes to AC voltage control at high power as they are available in higher ratings. However, SCR based system can only be controlled by phase angle control method. This method generates more harmonic distortions [2]. Moreover synchronization with AC supply is necessary when the control mechanism is provided by microcontrollers. The gating circuit required is relatively more complex. The provision of feedback allows output voltage to be stable and maintain the same RMS value of voltage while there is change in the supply voltage [3]. The gain of the error signal of the feedback control system can be varied to obtain a slower or faster response of the system thus allowing soft start.

Three-phase controlled rectifiers have a wide range of applications, from small rectifiers to large High Voltage Direct Current (HVDC) transmission systems. They are used for electro-chemical process, many kinds of motor drives, traction equipment, controlled power supplies, and many other applications. From the point of view of the commutation process, they can be classified in two important categories: Line Commutated Controlled Rectifiers (Thyristor Rectifiers), and Force Commutated PWM Rectifiers.

Scientific Research and Essays, 2010

AC voltage controllers as power converters used in many application between power ranges from few watts up to fractions of megawatts, such as light dimmers, induction motor soft starter, industrial heating and cycloconverters. Overall efficiency of power system controlled by some advanced methods in power converters is very important which covers supply and load power factor, harmonic distortion at both supply and load side. One of these advanced methods that is best used is the conventional controller, with phase angle control (PAC) of thyristors and it is a known method in contolling AC voltage applied to specific loads. However, these controllers are found to create some problems whose main problems are introduction of high level low order harmonics in current at the load and supply side, poor supply power factor and also low efficiency. This paper presents a proposed three phase AC voltage contoller scheme which allows the application of extinction angle control (EAC) to overcome these problems. Also, a comparison is given between proposed and conventional controllers on the basis of simulation under this viewpoints. The improvements are obtained in terms of high supply power factor, low harmonic content in the load current and high efficiency by proposed controller on comparing to conventional controllers.

The harmonic analysis of output voltage of single phase AC voltage controller was well known. But, it has been found that less harmonic analysis and comparison between voltage dimmer and thyristorised AC voltage controller. This paper presents such an analysis on the AC voltage controller using TRIAC, thyristor and voltage dimmer circuit. Results are obtained from simulations as well as hardware implementation and results were compared.

Proceedings of the Institution of Electrical Engineers, 1976

The paper describes a controlled rectifier with firing pulses supplied from a separate oscillator locked to the convertor a.c. supply. The phase difference between the pulse oscillator and the a.c. supply can be controlled from 5° to 175°, giving a full control range for the thyristor convertor. A multiplexer, supplied with the convertor input voltage and gated with signals from the firing pulse oscillator, gives an output waveform identical to the thyristor convertor and is used as a phase detector for the feedback signal to the pulse oscillator. The circuit is insensitive to distortion on the supply net. List of symbols /, = supply frequency, usually 50 Hz fi = frequency of v.c.o. 0i-f\t = accumulated supply phase angle,cycles A0 = 0 i-02 = phase difference between supply and v.c.o. U d = output d.c. voltage of convertor U pd = output d.c. voltage of the phase detector = feedback signal for the v.c.o. Uref = reference signal for the v.c.o. Updmax = maximum value of the U pd =/(A0) U dio = maximum value of the U d = f(a) a = firing angle of the thyristors U v = supply a.c. voltage K pd = phase-detector constant, V/cycle A^c o = oscillator constant, cycle/V /o = corner frequency of the filter, Hz £ = damping constant of the filter R(s) = transfer function of the regulator and filter of the p.1.1.