P: Foreword Electrical Drives play a vital role in engineering and industry both in this country and abroad. It is therefore essential that students of electrical engineering have a proper grounding in this subject. Conventional courses in Electrical Machines, however, are not adequate for the purpose as clec- tric motors do not by themselves constitute an electrical drive and their characteristics have to be studied keeping in mind the types of control schemes such as those using thyristor circuits and the dynamics of the load. It is thus necessary to have a course on the fundamentals of electrical drives, suitable for study by undergraduate students of electrical engineering.
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P: Foreword Electrical Drives play a vital role in engineering and industry both in this country and abroad. It is therefore essential that students of electrical engineering have a proper grounding in this subject. Conventional courses in Electrical Machines, however, are not adequate for the purpose as clec- tric motors do not by themselves constitute an electrical drive and their characteristics have to be studied keeping in mind the types of control schemes such as those using thyristor circuits and the dynamics of the load.
It is thus necessary to have a course on the fundamentals of electrical drives, suitable for study by undergraduate students of electrical engineering. This book A First Course on Electrical Drives—is designed to meet the need for a text- book in English for sucha course. The book gives a comprehensive introduction to the dynamics of drives, the characteristics, starting and braking of de and ac motors, as also their loading conditions, ratings and heating.
There are separate chapters devoted to solid state controlled drives and industrial applications. The author, Dr. Pillai, has over twenty years experience of teach- ing and research in electrical engineering, and he has developed the material of this book over the past ten years while conducting lecture, tutorial and laboratory classes for final year undergraduate students of Electrical Eagi- neering at the Indian Institute of Technology, Bombay.
The style and organization of the work reflects the discerning insight of a teacher into the requirements of a student and each topic is developed step by step in a clear and cogent manner. I am confident, therefore, that this book will be welcomed by students and teachers alike. April 16, R. Director Indian Institute of Technology Bombay Preface to the First Edition Electrical drives offer a convenient means for controlling the operation of different equipment used in industry.
The high reliability and great ver- satility of electrical drives, especially of those controlled by solid state devices, have resulted in their wide application. In fact, the growth and developments of electrical drives have been closely in parallel with those of automation in industry. One of the reasons may be the non- availability of suitable text books on the subject in English language. This volume is primarily written with the aim of providing a text book for the undergraduate students in Electrical Engineering on the fundamentals of electrical drives.
The contents of this book have gradually evolved over the last ten years from the notes used by the author in teaching final year undergraduate students of I. They not only indicate the extent of indebtedness of the author to those authors and publishers, but also provide the student with material for further reading. Most chapters include, in addition to a number of solved examples, problems to test how well the student has grasped the subject matter presented.
I gratefully acknowledge the assistance and encouragement given to me by several of my past students and colleagues. A special word of thanks is due to Prof. Bedford, who kindly agreed to write a Foreword for the book. Bombay for giving me the necessary financial support to prepare the first version of this book. Grateful acknowledgement is due to the Indian Standards Institution for granting permission to use the definition and the associated figures relevant to the different classes of duty indicated in LS.
I also wish to thank the production department of Wiley Eastern Limited for the great care with which they have worked in bring- ing the book to its present form. Last but not least, I wish to record my gratitude to my wife Vijaya and sons Sivan and Kumar for the sacrifices they have made that this work could be completed.
Bombay April S. K, Puat Preface to the Second Edition The present edition, essentially, introduces the students of Electrical Engineering to the recent additions in solid state controlled drives viz. The author would like to acknowledge the valuable advice and sugges- tions of many instructors and students who used the earlier edition.
LT, Bombay S. The function of the first two components is to impart motion and operate the third one. The most commonly used prime mover is, of course, an electric motor, since it is far superior in performance to steam, hydraulic, diesel and other types of engines.
Electric motors are, often, operated directly from a supply line, under their own inherent speed- torque characteristics and their operating conditions are dictated by the mechanical loads, connected to them.
However, in many applications, the motors are provided with a control equipment by which their characteristics can be adjusted and their operating conditions with respect to the mechanical load varied to suit specific requirements. The most common control adjust- ment is of motor speed, but torque and acceleration or deceleration can also be adjusted. The control equipment usually consists of relays, con- tactors, master switches and solid state devices such as diodes, transistors and thyristors.
The aggregate of electric motor, the energy transmitting shaft and the control equipment by which the motor characteristics are adjusted and their operating conditions with respect to mechanical load varied to suit particular requirements, is called an electrical drive. The drive. Classification of Electrical Drives In general, electrical drives may be classified into three categories: Group drive, Individual drive and Multimotor drive.
Group drive consists of a single motor which actuates several mechanisms or machines by means of one or more line shafts supported on bearings. It is also called a line shaft drive, The line shaft fitted with multistepped pulleys and belts that connect these pulleys and the shafts of the driven machines serve to vary their speed. But, seldom is the group drive used, nowadays, due to the following disadvantages : Any fault that occurs in the driving motor renders all the driven equip- ment idle.
In the individual drive, an electric motor is used for transmitting motion to various parts or mechanisms belonging to a single equipment. For example, such a drive in a lathe rotates the spindle, moves the feed and also with the help of gears, imparts motion to the lubricating and cooling pumps of the lathe. In many applications, the individual drive consists of motor, which is specially designed to form an integral part of the equip- ment, In the case of individual drive too, the energy is transmitted to the diffe- rent parts of the same mechanism by means of mechanical parts like gears, pulleys etc.
Hence, there occurs some power loss, This disadvantage is removed in the case of multimotor drives. In multimotor drives, separate motors are provided for actuating different parts of the driven mechanism. For example, in travelling cranes, there are three motors: one for hoisting, another for long travel motion and the third for cross travel motion. Paper mills, rolling mills, rotary printing machines, metal working machines etc. The use of individual drives and multimotor drives has enabled intro- duction of automation in production processes, which in turn has consider- ably increased the productivity of different industrial undertakings.
Complete or partial automation helps to operate various mechanisms at optimum conditions and to increase reliability and safety of operations. In other words, only static resisting torques, commonly called as load torques, are to be counterbalanced, if the motion were uniform. Types of Loads Loads can be of two types—those which provide active torques and those which provide passive torques. Active torques are due to either gravitational force or deformation in elastic bodies. The active torques due to gravitational pull are obtained in case of hoists, lifts or elevators and railway locomotives operating on gradients, Such torques are also developed during compression or release of springs.
Since the functioning of hoisting mechanisms, operation of locomotives on gradients and compression or release of springs are all associated with a change in potential energy of the drive, active torques are also closely connected to the potential energy. When a load is moved upwards or a spring is compressed, the stored potential energy increases and the active torque developed opposes the action that takes place, i.
On the other hand, when a load is brought downwards or a spring is released the stored potential energy decreases and torque associated with it aids the action. Thus, it can be seen that the active torques continue to act in the same direction even after the direction of the drive has been reversed. Passive torques are those due to friction or due to shear and defor- mation in inelastic bodies lathes, fans, pumps etc.
They always oppose motion, retarding the rotation of the driven machine. Moreover, with change in direction of motion, the sense of torque also changes. Further, in many applications, the motor may be required to run in both directions. Therefore, in sketching the speed torque characteristics of either the load or the motor, it is prefer- able to use all four quadrants of the speed-torque plane for plotting, rather than to confine the characteristics to the first quadrant alone.
When drawn in this manner, the diagram is referred to as quadrantal digaram. The conventions used for positive and negative values of speed, motor torque and load torque in a diagram of this type must be understood very clearly. In the case of reversible drives, the positive sign for speed may have to be assigned arbitrarily either to anticlockwise or clockwise direction of rotation. The motor torque is said to be positive if it produces an increase in speed in the positive sense. The load torque is assigned a positive sign when it is directed against the motor torque.
Figure 2. The arrows in this figure indicate the actual directions of motor torque, load torquc and motion in the four quadrants. It can be easily seen that they correspond to the sign conventions stated earlier for speed, motor torque and load torque. The load torque of the hoisting mechanism may be assumed to be cons- tant, i. Therefore, the speed torque curves ofa hoist load can be represented by means of vertical lines passing through two quadrants.
The speed torque characteristic of a loaded hoist is shown in Fig. Since the counter- weight is assumed to be heavier than the empty cage, the inherent tendency of the load, viz.
Four quadrant operation of a motor driving a hoist load, of rotation. Hence, to drive the loaded hoist up, the developed torque in the motor Ty must act in the same direction as the speed of rotation, i.
Since the speed is also of positive sign, being an upward motion, the power will also have a positive sign, i. The hoisting up of the unloaded cage is represented in the second quadrant. Since the counterweight is heavier than the empty cage, the speed at which the hoist is moved upwards may reach a dangerously high value.
In order to avoid this, the motor torque must act opposite to the direction of rotation, i. Note that 7, will have a negative sign and speed still has a positive sign, giving power a negative sign, corresponding to the generator or brak- ing operation. The downward journey of the cage is prevented by the torque due to the counterweight and friction at the transmitting parts.
Therefore, in order to move the cage downwards, the motor torque must act in the same direction as the motion of the cage. The motor torque has a negative sign because it causes an increase in speed in the negative sense and the speed also has a negative sign being a downward motion. Power, thus, bas a positive sign.
The downward motion of the loaded cage is shown in the fourth quad- rant. The motion can take place under the action of load itself, without the use of any motor.
But, the speed of downward motion can be dangerously high. Therefore, in this case, the electrical machine must act as a brake limiting the speed of the downward motion of the hoist.
The motor torque has a positive sign since it causes a decrease in speed in the downward motion. The speed, of course, has a negative sign, being a downward journey. The power, thus, acquires a negative sign, corresponding to the braking operation of the motor. A second basic type of loading that occurs is the one characterized by dry friction.
This type of load presents to the motor a passive torque, which is essentially independent of speed. It is characterized also by the require- ment of an extra torque at very near zero speed. In power applications it is, often, called as the break away torque and in control systems, it is referred to as stiction derived from sticking friction.
s.k.pillai a First Course on Electrical Drives
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