DESIGN AND CONSTRUCTION OF A THYRISTOR SPEED CONTROL OF A DC MOTOR
DC motors are widely used in industries because of its versatile characteristics and because of power electronics devices its controlling is becoming sophisticated and precise, but on the other hand because of the power electronics devices power factor and total harmonics distortion problem is becoming more prominent. In this paper DC motor control methodology is proposed which includes thyristor based converters but with improved power factor and total harmonics distortion.
CHAPTER ONE
INTRODUCTION
The thyristor is a four-layered, three terminal semiconductor device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across all four layers. The control terminal, called the gate, is attached to p-type material near the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause a self-latching action:
Because of low cost, less complex control structure and wide range of speed and torque, DC motor are popular in industry. Available methods of speed control of DC drives are field control, armature control and armature resistance control methods.
Speed of a DC motor can be controlled easily in both the ranges above and below the base speed. Most of the time, DC motors are customized at the time of installation as per the need which makes them irreplaceable. Armature voltage control method of speed control of DC drive is popularly used in which controlled rectifier or chopper are involved, though due to power electronics elements, an undesirable nonlinear torque speed characteristics are observed
In past, many researchers worked on various converter topologies of DC motor control for different industrial applications but all of them are thyristor based.
A thyristor drive is a motor drive circuit where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor.
Thyristor drives are very simple and were first introduced in the 1960s. They remained the predominant type of industrial motor controller until the end of the 1980s when the availability of low cost electronics led to their replacement by chopper drives for high performance systems and inverters for high reliability with AC motors.
They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristors and the simplicity of the design can make them a more attractive proposition than transistor based controllers.
A derivative of the thyristor drive is the DC phase controller. This uses a single phase controlled triac to provide a variable voltage AC output for regulating a universal motor. This is the type of motor speed control most commonly used in domestic appliances, such as food mixers, and small AC powered tools, such as electric drills.
The speed of dc motor is directly proportional to the supply voltage, if we reduce the supply voltage from 12V to 6V, the motor will run at half SCR “phase-angle controlled” drive -By changing the firing the speed. Here we are using angle, variable DC output voltage can be obtained. For this we are using an IC to generate pulse and by using this we are setting the speed of motor
1.1 OBJECTIVE OF THE PROJECT
The objective of this work is to design a device that uses a thyristor to control the speed of a dc motor
1.2 SCOPE OF THE PROJECT
The only really satisfactory way of obtaining the variable-voltage d.c. supply needed for speed control of an industrial d.c. motor was to generate it with a d.c. generator. The generator was driven at fixed speed by an induction motor, and the field of the generator was varied in order to vary the generated voltage.
The motor/generator (MG) set could be sited remote from the d.c. motor, and multi-drive sites (e.g. steelworks) would have large rooms full of MG sets, one for each variable-speed motor on the plant.
For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used to supply the field of the motor: the power is much less than the armature power, so the supply is often single-phase
It is accepted practice for motors supplied for use with thyristor drives to have laminated construction, but older motors often have solid poles and/or frames, and these will not always work satisfactorily with a rectifier supply. It is also the norm for drive motors to be supplied with an attached ‘blower’ motor as standard. This provides continuous through ventilation and allows the motor to operate continuously at full torque even down to the lowest speeds without overheating.
Low power control circuits are used to monitor the principal variables of interest (usually motor current and speed), and to generate appropriate firing pulses so that the motor maintains constant speed despite variations in the load.
1.3 LIMITATION OF THE PROJECT
Drawback of this method of dc motor speed control is that they are not able to control the motor speed smoothly at lower levels, and as the desired speed is decreased, the torque of the motor also decreases proportionately. Due to this, at any unpredictable point the motor may just halt very abruptly. Also, during power ON, the motor may just not start up at lower speed settings and may require an initial boost by increasing the setting. Such situations are pretty undesirable and do not constitute an ideal speed control.
1.4 APPLICATION OF THE PROJECT
This device is mostly use in controlling the movement and speed of other devices in industries such as in conveyors.
1.5 PURPOSE OF THE PROJECT
The purpose of a motor speed control is to take a signal representing the demanded speed and drive the motor at that speed.
1.6 ADVANTAGES OF THE PROJECT
This work is a Simple control of motor speed, it has:
- High reliability and
- Low initial cost of production
1.7 PROJECT WORK ORGANISATION
The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:
Chapter one of this work is on the introduction to a dc motor using thyristor. In this chapter, the background, significance, objective limitation and problem of a dc motor using thyristor were discussed.
Chapter two is on literature review of a speed control of a dc motor using thyristor. In this chapter, all the literature pertaining to this work was reviewed.
Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.
Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.
Chapter five is on conclusion, recommendation and references.
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