Study Of A Voltage /Frequency Ratio Method Of Speed Control For Three Phase Induction Motors

ABSTRACT

Higher percentage of electrical energy is consumed by electric motors in loads like variable torque fan, pump and compressor. Induction motors are the most important workhorses in industry and they are manufactured in large numbers. About half of the electrical energy generated in a developed country is ultimately consumed by electric motors, of which over 90 % are induction motors. When we connect an induction motor directly to the main supply it can run only at its rated speed. However, many operations need variable speed applications. Also we felt that in many applications input power is directly proportional to the cube of motor speed. Today about 90% of all industrial motor applications use three phase induction motor because they are simple in design, easy to maintain and are less costly than other design. There are many induction motor control techniques in practice today, but the most popular control technique is by generating variable frequency supply, which has constant voltage to frequency ratio. This technique is popularly known as V/F speed control technique. The torque developed by the induction motor is directly proportional to the ratio of the applied voltage and the frequency of supply. By varying the voltage and the frequency, but keeping their ratio constant, the torque developed can be kept constant throughout the speed range. This is exactly what VF control tries to achieve.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

• INTRODUCTION
• BACKGROUND OF THE PROJECT
• VOLTAGE/FREQUENCY SPEED CONTROL TECHNIQUES
• PROBLEM STATEMENT
• AIM OF THE PROJECT
• SCOPE OF THE STUDY
• SIGNIFICANCE OF THE STUDY
• MOTIVATION OF THE STUDY
• LIMITATION OF THE STUDY

CHAPTER TWO

2.0     LITERATURE REVIEW

2.1     HISTORICAL BACKGROUND OF INDUCTION MOTOR

2.2     REVIEW OF DIFFERENT SPEEED CONTROL OF INDUCTION MOTOR

2.3     REVIEW PF RELATED STUDIES

CHAPTER THREE

3.0     METHODOLOGY

• DESCRIPTION OF THREE PHASE INDUCTION MOTOR
• OPERATION OF A THREE PHASE INDUCTION MOTOR
  • VARIOUS METHODS OF CONTROLLING THE SPEED OF INDUCTION MOTOR

3.3.1       CONTROL FROM STATOR SIDE

3.3.2  BY CHANGING THE APPLIED FREQUENCY

3.3.2  CHANGING THE NUMBER OF STATOR POLES

3.3.3    CONTROL FROM ROTOR SIDE

3.3.4    SPEED CONTROL BY FREQUENCY VARIATION

3.3.5    SPEED CONTROL BY POLE CHANGING

3.3.6    SPEED CONTROL BY VARYING SUPPLY VOLTAGE

3.4       THREE PHASE INDUCTION MOTOR STARTER

3.5        METHOD OF THREE PHASE MOTOR STARTING

3.6          PROBLEMS WITH ELECTRIC MOTORS

3.7          CHARACTERISTICS OF ELECTRIC MOTORS

3.8          INDUCTION MOTOR DESIGN

3.9          REVERSING THREE PHASE INDUCTION MOTORS

3.10        3 PHASE INDUCTION MOTOR CONSTRUCTION

3.11         PRINCIPLE OF OPERATION OF 3-PHASE INDUCTION MOTOR

3.12         STATIC KRAMER DRIVE

3.13         STATIC SCHERBIUS DRIVES

CHAPTER FOUR

• DESCRIPTION OF V/F SPEED CONTROL
• OPERATION
• SIMULATION MODEL

CHAPTER FIVE

• CONCLUSION
• REFERENCES

CHAPTER ONE

1.0                                                                   INTRODUCTION

1.1                                                   BACKGROUND OF THE PROJECT

Today, three-phase induction motor drives are employed in different industrial areas with a wide power range starting from few 100W to several MW. Many methods are used for the speed control of the induction motor. In the speed control of induction motor the main issue is to achieve maximum torque & efficiency. Smooth speed control of an induction motor had been a major problem in the early industry, difficulty in control has led the industry to go for other options like DC motors. However, with development of microcontroller and power electronic devices there is a scope of better speed control of induction motor. Constant volts per hertz control is the most popular scalar control scheme that varies terminal voltage in proportion to the supply frequency to maintain the air gap flux at approximately the rated V/F ratio especially when the machine operates below its rated frequency[6].

There are two methods to achieve the V/F speed control scalar and space vector PWM M.Reddy [3] and

Zhe Zhang [7] and Pallavi Alagur [4] had discussed several recent design topologies of V/F speed control techniques as well as their applications they have also introduced various techniques related in changes in open loop system. The design improvements in V/F speed control techniques have been described after research and developments in power electronics and PWM because of its quick operation, time high speed performance and lesser overall power requirements.

Researchers have long been trying to optimize and enhance the constant V/F control by compensating the stator voltage drop the simple compensation method of boosting the stator voltage by the magnitude of the current resistance (I.R) drop was proposed in [8]. A voltage auto boost method proposed in [9] provides with optimal V/F pattern based on knowledge of the induction machine equivalent circuit parameters and given load characteristics we have done research on V/F speed control technique and have performed simulations of PWM generation and six pulse inverter output and also analyzed the circuit parameters.

1.2                              VOLTAGE/FREQUENCY SPEED CONTROL TECHNIQUES

Due to the advancement in solid state power devices and microprocessors, speed control of Induction motor controlled by switched power converter are getting popular. Switched  power converters offer an easy way to regulate both the frequency and magnitude of the voltage applied to a motor. As a result higher efficiency and performance can be achieved by these motor drives with less noise. The most common principle of this is the constant V/Hz principle which requires that frequency and the magnitude of the voltage applied to  the stator of a motor maintain a constant ratio. So by this,    the magnetic field in the stator is kept almost constant for all operating points. Thus, constant torque is maintained. Also allows the motor to achieve faster dynamic response.

Fig. 1: Block diagram of closed loop v/f control of induction motor

Ns = 120Fs/p ———————————————————–eqn 1

From eqn (1) we can see that synchronous speed is directly proportional to supply frequency. So by changing frequency  of stator terminal voltage we can control the sped of induction motor. Apart from frequency, the applied voltage should also be varied, to keep constant air gap flux and not let it saturate.

E = 4.44k_wϕ_mf_sT                                                   (2)

1.2                                                            PROBLEM STATEMENT

There are many means of controlling induction motor speed, but the most popular problem discovered in them is that motor used to experience speed instability and the efficiency of the motor drops when they motor speed is varied. Voltage and frequency method of motor speed control came to solve this problem. By varying the voltage and the frequency, but keeping their ratio constant, the torque developed can be kept constant throughout the speed range thereby maintaining the motor efficiency.

1.3                                                             AIM OF THE PROJECT

The main aim of this project is to control the speed of an induction motor by varying the voltage and the frequency. At the end of this work, all the necessary subject related to the speed control for three

1.4                                                             SCOPE OF THE STUDY

In this work, we discuss controlling the speed of 3-phase induction motor using constant volt per hertz profile. The motor is controlled using 3-phase inverter. SPWM technique is used to control the inverter voltage. Modelling of the induction motor is done in stationary reference frame and the system in simulated using MATLAB (simulink) and hardware implementation is done for the same using TIs F28069M launch pad under varying speed and load torque conditions.

1.5                                                      SIGNIFICANCE OF THE STUDY

The advantages of using this method of motor speed control is because it is cheap and efficient means of controlling the speed of an induction motor. Carrying out this project as a student exposes me to the advantages of the voltage/frequency ration method of induction motor speed control over the methods of speed control.

1.6                                                      MOTIVATION OF THE STUDY

Induction motors were used in the past mainly in applications requiring constant speed because conventional methods of IM speed control has either been expensive or inefficient. Variable speed applications have been dominated by DC drives. Availability of thyristors, IGBT, GTO have allowed  the development of variable speed induction motor drives. The presence of commutator and brushes is the main disadvantage of DC motor, which require frequent maintenance and make them unsuitable for environments which involve explosive and dirt. On the other hand, induction motors, particularly squirrel- cage are rugged, cheap, light, small, and more efficient, require lower maintenance and can operate in dirty and explosive environments. Although speed control of induction motor drives are generally expensive than DC drives, they are used in number of applications such as pump, steel mills, cranes, hoist drives, conveyors, traction etc. because of the advantage of induction motor. Dominant of them is the traction system which is used in auto mobiles and locomotives [Gopal, 2001].Induction motor drives have gained equal importance as BLDC motor.

1.8                                                        LIMITATION OF THE STUDY

There are many techniques for controlling the speed of an induction motor both for industrial purposes and locally, but in this work we are focusing only on the voltage/frequency ratio method.