An Overview Of Variable Frequency Drive Working Principle And Application

ABSTRACT

When Tesla first introduced the 3-phase alternating current (AC) induction motor in 1888, he knew that his invention was more efficient and reliable than Edison’s direct current (DC) motor. However, AC motor speed control requires either varying the magnetic flux or changing the number of poles on the motor. Even decades after the induction motor gained widespread use, changing the frequency for speed control remained an extremely difficult task — and the physical construction of the motor prevented manufacturers from creating motors with more than two speeds.

As a result, DC motors were necessary where accurate speed control and significant power output were required. In contrast to AC motor speed control requirements, DC motor speed control was achieved by inserting a rheostat into the low-power DC field circuit, which was feasible with available technology. These simple motor controls varied the speed and torque, and were the most economical way to do so for a number of decades.

By the 1980s, AC motor drive technology became reliable and inexpensive enough to compete with traditional DC motor control. These variable-frequency drives (VFDs) accurately control the speed of standard AC induction or synchronous motors. With VFDs, speed control with full torque is achieved from 0 rpm through the maximum rated speed and, if required, above the rated speed at reduced torque. VFDs manipulate the frequency of their output by rectifying an incoming AC current into DC, and then using voltage pulse-width modulation to recreate an AC current and voltage output waveform.

 COMMON VFD TERMS

 There are several terms used to describe devices that control speed. While the acronyms are often used interchangeably, the terms have different meanings.

Variable Frequency Drive (VFD)

This device uses power electronics to vary the frequency of input power to the motor, thereby controlling motor speed.

Variable Speed Drive (VSD)

This more generic term applies to devices that control the speed of either the motor or the equipment driven by the motor (fan, pump, compressor, etc.). This device can be either electronic or mechanical.

Adjustable Speed Drive (ASD)

Again, a more generic term applying to both mechanical and electrical means of controlling speed.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

COMMON VFD TERMS

TABLE OF CONTENTS

CHAPTER ONE

INTRODUCTION

1.1      BACKGROUND OF THE PROJECT

• AIM OF THE PROJECT
• OBJECTIVE OF THE PROJECT
• PURPOSE OF THE PROJECT
• APPLICATION OF THE PROJECT
• SCOPE OF THE PROJECT
• BENEFIT OF THE PROJECT
• PROBLEM OF THE PROJECT

CHAPTER TWO

LITERATURE REVIEW

• OVERVIEW OF AC INDUCTION MOTOR DESIGN
• THE PURPOSE OF INDUCTION MOTORS
• INDUCTION MOTOR CONSTRUCTION
• OPERATING PRINCIPLES

CHAPTER THREE

METHODOLOGY

• BASICS OF THE SYSTEM
• SYSTEM BLOCK DIAGRAM
• DESCRIPTION OF SYSTEM BLOCK
• BASIC OPERATION OF A PWM INVERTER (VFD)
• THE IMPORTANCE OF MAINTENANCE

CHAPTER FOUR

• VARIABLE SPEED CONCERNS
• TROUBLESHOOTING DRIVES

CHAPTER FIVE

• CONCLUSION
• REFERENCES

 CHAPTER ONE

1.0                                                        INTRODUCTION

Electric AC motor works at fixed speeds and are ideally suited to applications where a constant output speed is required. However around half of all motor applications have some kind of varying speed demand and this includes processes such as moving air and liquids (fans and pumps), winding reels and precision tools.

Historically in applications requiring precise speed control such as paper winding reels expensive direct current (DC) motors or hydraulic couplings were used to regulate the machine speed, whereas in other applications the processes have been controlled by opening and closing dampers and valves, or changing output speeds with gears, pulleys, and similar devices whilst the motor works at constant speed.

In the 1980’s and 1990’s, variable frequency drives started appearing on the electric market offering an alternative method of control. A variable frequency drive, also called frequency inverter, adjustable speed drive, the basic working principle is adjusting the electrical supply to an AC motor with a corresponding frequency and voltage change in the motor’s speed and torque output.

By implementing this type of control a very close match between motor speed and the process requirements of the machine it is driving may be achieved.

Variable frequency drive technology is now mature and enjoying widespread adoption and use with AC motors; Variable frequency drives are extremely versatile and offer a high degree of motor control where motor speeds can be accurately varied from zero rpm through over 100% of the rated speed, whilst the torque is also adjusted to suit.

Different options are available to suit a variety of applications; basic variable frequency drive designs are used in simple applications such as fan and pump control whereas more complex versions might be used for very precise speed and torque control in for example multiple winders or materials forming applications.
Sizes of variable frequency drive capacity range from 0.2kW through to several MW; they are usually available as standalone devices and are connected to the motor’s electrical supply, however on some smaller motor designs, usually under 15kW the variable frequency drive may be built onto the motor and is available as an integrated motor-drive product.

1.1                                                   AIM OF THE PROJECT

The main aim of this work is write on the device that is used in electro-mechanical drive systems to control AC or DC motor speed and torque by varying motor input frequency and voltage.

1.2                                             OBJECTIVE OF THE PROJECT

The objective of this work is to provides variable speed to an electric motor

1.3                                              PURPOSE OF THE PROJECT

The purpose of the work is to convert the incoming electrical supply of fixed frequency and voltage into a variable frequency and variable voltage that is output to the motor with a corresponding change in the motor speed and torque.

1.4                                          APPLICATION OF THE PROJECT

In many applications variable speed control can lead to a substantial reduction in energy cost. The use of variable frequency drives is particularly effective in fan and pump applications where they might be used to replace traditional methods of output regulation; here an exponential relationship exists between the machine speed (and output) and the energy used.

1.5                                                 SCOPE OF THE PROJECT

The only really satisfactory way of obtaining the variable-voltage electric supply needed for speed control of an industrial electric motor was to generate it with a electric 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.

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.

1.6                                                      BENEFITS OF VFD

As VFD usage in HVAC applications has increased, fans, pumps, air handlers, and chillers can benefit from speed control. Variable frequency drives provide the following advantages:

• energy savings
• low motor starting current
• reduction of thermal and mechanical stresses on motors and belts during starts
• simple installation
• high power factor
• lower KVA

Understanding the basis for these benefits will allow engineers and operators to apply VFDs with confidence and achieve the greatest operational savings.

1.7                                              PROBLEM OF THE PROJECT

Drawback of this method of an electric 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.