INVESTIGATIONS ON VARIABLE SPEED LOAD COMMUTATED INVERTER FED INDUCTION MOTOR DRIVE SYSTEM

Abstract

The advancement of semiconductor technological culture has made it possible to develop a variety of static power converters for supplying the three-phase a.c. power at variable frequency and variable voltage. The advent of these sources have enhanced the potential of the variable speed a.c. drives in general and cage induction motor drive in particular. There is evidence in the literature on adjustable speed a.c. drives about the unique features of the load commutated inverter (LCI) fed drive. It is inherently capable of being built with more simplicity, higher efficiency and reliability, particularly for large power requirement. In the present investigation, a variable speed cage induction motor drive fed from LCI is considered. This system makes use of capacitor bank across the motor/inverter terminals. The terminal capacitor helps in commutation of the inverter thyristors. It also meets the system lagging reactive power requirement and plays a role in deciding the speed of the motor in conjunction with the inverter input voltage. In the present work, an attempt is made to develop a computer based analytical tool to evaluate the characteristics of the proposed drive under steady state, dynamic and transient operating conditions. In addition, an attempt is also made on the stability analysis of the proposed system. In order to value the confidence of the computed results, an experimentation is carried out on a laboratory model. The entire work done is presented in the following format. A concise review of the available literature on the LCI fed induction motor drive is made. It reveals the existence of a scope for the detailed and precise analysis of the load commutated inverter fed induction motor as a variable speed drive. This has motivated the author to investigate the steady state, dynamic and stability analysis of LCI fed induction motor drive for its variable speed operation. The author's contribution is followed by the thesis organization. The schematic configuration of the drive is given with relevant description. The drive system employs a variable d.c. voltage source, a d.c. link inductor, a fully controlled thyristor bridge (LCI), an associated control circuit, a bank of terminal capacitor and a cage motor. The operating principle of the LCI, the power flow, the power balance in the system and the functions of the terminal capacitors are explained in detail. From the basics of operation, the significant control parameters for the drive are identified. A laboratory size drive unit is developed to obtain its performance under different operating conditions of interest. For the purpose of the steady state performance analysis, the system is represented by an equivalent circuit and a corresponding vector diagram for better understanding of the drive. A basic model for the operation of the drive is evolved. A computer algorithm is developed using the presented model of the drive which is capable to check the feasibility of LCI operation, to find the system frequency and the drive performance (ii) characteristics for the specified operating conditions. Moreover, the method for determination for the terminal capacitor value is also developed for the required speed and the mode of motor peration (constant torque/constant power). The steady state performance characteristics are computed by implementing the algorithm on Dec 20 mainframe system, for wide range of speed control. Tests are conducted on the developed drive unit to obtain its performance. The steady state computed performance along with the corresponding experimental results are presented and discussed. In order to study the system dynamics, the complete drive system is described by means of a mathematical model in terms of a set of first order differential equations. The differential equations are developed using the d-q variables in stationary reference frame. The non-linear effect of the magnetic saturation in the machine is also considered in the dynamic model. The inverter, d.c. link and the capacitor bank are also suitably modelled in terms of differential equations. The set of differential equations are integrated by using the fourth order Runge Kutta (R.K) method to obtain the dynamic response of the system. The computed results under the perturbations of input and output parameters are presented and discussed along with the corresponding test results. In the last part, stability analysis of the drive operation is dealt. The linearized perturbation equations are obtained from the dynamic equations of the system, which are valid around the o

stable operating point. The stable operating condition of the drive is perturbed by introducing a sudden small change in the value of any one parameter e.g. load torque, terminal capacitors etc. The coefficients of the characteristic polynomial representing the set of linearized equations are determined by making use of the numerical method based on Leverrier's algorithm. From the coefficients obtained, all the roots of the polynomial are determined by a numerical method based on Newton-Raphson's algorithm. All the roots (eigen values) of the system characteristic equation are referred to decide the system stability status by using Lyaponov's first method. The stability results are verified by the transient response exhibited by drive motor. Moreover, under identical conditions considered, the transient response of the drive /s also recorded. At the end, the main conclusions based on the results of the present investigation are given and discussed. The salient features of the system are highlighted to evolve potential for the possible applications of the proposed system. On the other hand, the limitations of this scheme and the possible scope for improvement are also suggested.