A CURRENT SOURCE INVERTER-FED CONSTANT AIR GAP FLUX CONTROLLED SQUIRREL CAGE INDUCTION MOTOR DRIVE.

ABSTRACT

In this thesis, a current source inverter fed squirrel cage induction motor drive at constant air gap flux is presented. The drive scheme is conceived to take advantage of the short circuit withstand capability of the current source inverter and the ruggedness (even under harsh conditions) of the squirrel cage induction motor. The variable inverter input dc link current is derivable from either a controlled ac to dc converter or a controlled dc to dc converter. The constant motor air gap flux

control drive scheme has inner dc link current control loop and an outer motor speed control loop that maintains the motor slip frequency constant for a given motor load torque. The closed loop control parameters are selected such that negligible torque pulsation and relatively fast motor speed response are obtained without the need for an inverter output capacitor filter. A 400V, 10hp, 1440rpm,

50Hz squirrel cage induction motor is used to simulate the motor drive scheme in Matlab-Simulink environment. For step changes in demand (reference) speed at no load torque and at load torque equals to or less than the motor rated torque, the drive response has relatively fast settling time and acceptably low

overshoot/undershoot over the motor speeds not exceeding the rated value.

CHAPTER ONE

INTRODUCTION

1.0 Introduction:

This research work is concerned with the study of current source inverter (CSI)

fed- induction motor drive with constant air gap flux control.

There are many electromechanical systems where it is important to control their torque, speed and position with high level of precision. These machines are used daily to  increase human efficiency in our day to day activities, these include elevators in high rise buildings, mechanical robots in automated factories, which are  crucial  for  industrial competitiveness. Others  are  used  in  general purpose applications of adjustable speed drives, such as pumps, compressor systems and advanced electric drives are also  needed  in wind electric systems to  generate electricity and hybrid electric vehicles and trains which represent an important application of advanced electric drives in immediate future [1,2,3,4].

In the past, many applications requiring motion control utilized DC-motor drives. With the application of solid state industrial drive and the availability of fast signal processing capability, the role of DC- motor drive is being replaced by AC-motor drives [1, 5].

There are three major AC-drives that are widely used today. These are, induction motor drives, permanent magnetic motor drives and switch reluctance motor drives [1].

In this work, induction motor drive is adopted to achieve the constant air gap flux control, due to its ruggedness, low cost of purchase and its robustness.

1.1 Overview of the Study:

In the early drive scheme, AC drives are used for fixed speed operation, because it is not easy to obtain variable frequency supply. The efficiency of the drive was low when fixed frequency and variable voltage supply is used to control the speed of the motors. However, variable DC supply could be easily obtained. Consequently, DC  drives  are  widely  used  for  variable  speed  operation.  Speed  control  is achievable in AC drives because variable frequency can be obtained using power electronic converter. DC drives were replaced with AC in variable speed applications which do not require high performance operations. AC drives uses AC motors hence require less maintenance e.g. squirrel cage Induction motors require minimum maintenance since no contact brushes are used. With the advancement of power  semiconductor devices  and  powerful  microprocessors, it  is  possible  to control the  AC  motors  that will  give  comparable performance to  that  of DC drives.AC drives utilizing control techniques such as field-oriented control (F.O.C)

and Direct Torque Control (DTC) are now gradually replacing DC drives in high performance applications. [6]

Basically, there are two circuits used for power conversion in induction motor drive. These are the voltage source inverters (VSI) and current source inverters (CSI). For reasons such as its stability at no load and open loop control, voltage source inverter (VSI) were used more often than the current source inverter (CSI). Nowadays, the development of power electronics devices has enormous influence on applications of systems based on the CSI and creates new possibilities.

In the 1980s the current source inverters were the main commonly used electric machinefeeding devices [7]. Characteristic features of those drives were the motor electromagnetic torque pulsations, the voltage and current with large content of higher harmonics. The current source inverter was constructed of a thyristor bridge and large inductance and large commutation capacitors. Serious problems in such drive systems were unavoidable overvoltage cases during the thyristor commutation, as the current source inverter current is supplied in a cycle from a DC-link circuit to the machine phase winding. The thyristor CSI has been replaced by the transistor reverse blocking IGBT devices (RBIGBT), where the diode is series-connected and placed in one casing with transistor. The power transistors like RBIGBT or Silicon Carbide (SiC) used in the modern CSIs guarantee superior static and dynamic drive characteristics.

The electric drive development trends are focused on the high quality system. The use  ofcurrent  sources  for  the  electric  machine  control  ensures  better  drive properties than in case of voltage sources, where it may be necessary to use more passive filter at the inverter output. The Pulse width modulation (PWM) with properly chosen DC-link inductor and input-output capacitors result in sinusoidal inverter output currents and voltages.

One advantage of the drive is that regenerative braking is easy because the rectifier and inverter can reverse their operation modes. Six-step machine current, however, causes large harmonic heating and torque pulsation, which may be quite harmful at low-speed operation. Another disadvantage is that the converter system cannot be controlled in open loop like a voltage-fed inverter. [6]

1.2Objectives of the Study.

This drive scheme is aimed at achieving the following

I.     To develop a current source inverter scheme that will drive a squirrel cage induction motor and controlling the speed within a wide range of   value below or equal to the motor rated speed value of 1440rpm at constant rated air gap flux with no additional output filters.

II.     To adopt a control mechanism that will minimize steady torque pulsation within the speed range chosen.

III.     To design a drive that has a fast settling time and low percentage overshoot.

1.3   Thesis Organization.

Chapter 1 is a vivid introduction of the thesis, it also contains an overview to the study and the objective of the study.

Chapter 2 reviewed literatures on previous works done on induction motor drive- especially areas covered by the work.

Chapter 3 looked at the introduction of induction motor, discuss the fundamental equations of the dynamics and steady state analysis as well as the developed mechanical and torque analysis of the induction motor respectively.

Chapter 4 is an overview of current source inverter with the novel method of designing the firing signals of the switches and its MATLAB/ Simulink simulation. The performances of the designed current source inverter on some specified load was also discussed.

In chapter 5, the closed loop current fed speed controlled induction motor was analysed. A detail description of how the work was carried out and the result obtain was discussed

Finally, Chapter 6 offers a conclusion on the work and how the prototype was able to meet the objective, discussions and recommendations.

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