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

Description

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.

CHAPTER ONE

1.0                                                                   INTRODUCTION

1.1                                                   BACKGROUND OF THE PROJECT

Three-phase induction motor drives are employed in different industrial areas with a wide power range starting from few 100W to several MW. In modern industrialized countries, more than half the total electrical energy used is converted to mechanical energy through AC induction motors. Induction motors are extensively used in industrial and household appliances and consume more than 50% of the total generated electrical energy. Single-phase induction motors are widely used in home appliances and industrial control. During the last few years, the concept of speed and torque control of asynchronous motor drives has gained significant popularity. This way, it has been possible to combine the induction-motor structural robustness with the control simplicity and efficiency of a direct current motor. This evolution resulted to the replacement of the dc machines by induction motors in many applications in the last few years. Earlier only dc motors were employed for drives requiring variable speeds due to facilitate of their speed control methods [J. Zubek, 2010]. The conventional methods of speed control of an induction motor were either too extravagant or too inefficient thus limiting their application to only constant speed drives. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc. This type of electric motor is so popular due to its simplicity, reliability, less maintenance and low cost. Today, with advancements in power electronics, microcontrollers, and digital signal processors (DSPs), electric drive systems have improved drastically. Initially the principle of speed control was based on steady state consideration of the induction motor. V/f control was the commonly used one for the open-loop speed control of drives with low dynamic requirements.

There are different methods of controlling induction motor for industrial application. However, the main aim of this work is to carry out a research on the voltage frequency ratio method of induction speed control technique. This method offers 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 (F. Blaschke, 2010).

The work is organized as follows: chapter one discuses the introductory part of the work,   chapter two presents the literature review of the study,  chapter three describes the methods applied which includes the simulation using MATLAB, chapter four discusses the results of the work, chapter five summarizes the research outcomes and the recommendations.

1.2                                                            PROBLEM STATEMENT

Controlling the speed of a motor using traditional methods involve running the motor at full speed and then using mechanical means such as gears, hydraulic couplings or pulleys. This is not only expensive, but also consumes a lot of energy. 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 the motor speed is varied. Voltage frequency ratio method of motor speed control came to solve this problem. By varying the voltage and 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 AND OBJECTIVE OF THE PROJECT

AIM

The main aim of this project is to control the speed of an induction motor by varying the voltage and the frequency.

OBJECTIVE

The objectives are:

1. To develop a model for implementation of V/f control scheme in a three phase induction motor.
2. To enhance the efficiency of motor control techniques by using PID microcontroller for a three phase induction motor speed control.

• To show the performance of the V/F control technique by controlling the current of the induction motor.

1. To model and simulate V/F technique in a MATLAB.
2. To perform a simulation of an induction motor speed control using the model and V/F control technique.

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

This research work will throw more light on the best techniques for controlling the speed of electric motor. This study will also be designed to be of immense benefit to all the users of electric motor most especially in industries.

It will also serve as a guide to whoever that wants to purchase speed controller for induction motor.

Finally, it will also serve as a useful piece of information for both producers and users of electric motor speed controller.

1.6                                                      MOTIVATION OF THE STUDY

Controlling the speed of an induction motor is necessary for both industrial and local applications in that it gives adjustable Speed Drives include energy savings, efficient motor starting, reversing, and speed and position control. 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. However, advantages discussed above were points that motivate the carrying out of this research work.

1.8                                                        LIMITATION OF THE STUDY

As we all know that no human effort to achieve a set of goals goes without difficulties, certain constraints were encountered in the course of carrying out this project and they are as follows:-

1. Difficulty in information collection: I found it too difficult in laying hands of useful information regarding this work and this course me to visit different libraries and internet for solution.
2. Difficulty in parts gathering: I found it too difficult when gathering electronics parts used for the prototype and also difficulty in getting the system simulated.