DESIGN, DEVELOPMENT AND PERFORI 1 ANCP' EVALUATION. OF MICROCOMPUTER CONTROLLED PWM CONVERTER FED DC DRIVE

Abstract

DC motors are widely used for variable speed drives, due to ease in control. The advent of thyristor and MOSFET has brought a virtual revolution in the field of converter fed drives. Attempts are being made to control the converter operation, so as to reduce the low order harmonics and to improve the fundamental power factor and the input power factor. To achieve the above aim, the present scheme utilizes a single phase PWM converter to fed the DC motor. The fundamental component of the supply current is always in phase with the supply voltage in PWM converter thus, resulting in unity fundamental power factor and improved input power factor. The PWM converter consist of two MOSFET's in the upper arm and two thyristors in the lower arm, connected in the bridge configuration. The firing pulses for the thyristors and the MOSFET's are generated using dedicated 8031 microcomputer software. The developed system is tested with R--L load and with DC motor in open loop as well as in close loop.The close loop control is achieved by incorporating a speed feedback loop using PT controller. Mathematical analysis for the transient and steady state load current and input supply curfent is carried out for R-L load and DC motor load . Chapter 1 includes the introduction to variable speed drives, the drawbacks in the conventional converters and the power factor improvement techniques. Literature review is also discussed here. Chapter 2 deals with the principle of operation, design and development of single phase PWM converter. Chapter 3 discusses the system hardware which consists of synchronizing circuit, dedicated 8031 circuit and pulse amplification and isolation circuit. Chapter 4 deals with the system software in detail. The flowcharts of the main program and the associated subroutines are provided. Chapter 5 gives the mathematical analysis of the present system and the results obtained therefrom. Chapter 6 gives the experimental results of the system with R-L load, open loop and close loop operation of DC motor. Chapter 7 gives the conclusion and the future scope of the present work.