EVALUATION AND OPTIMIZATION OF STEADY-STATE PERFORMANCE OF AN INVERTER-FED INDUCTION MOTOR DRIVE SYSTEM

Abstract

Variable frequency inverters are being widely used to obtain step-less speed control of three phase cage rotor induction motors. The increasing use of inverter-induction motor drive systems has resulted in a number of interesting and challenging problems pertaining to the performance and design of the drive system. The pulsating torques acting on the rotor of the induction motor result in harmonic variation in the rotor speed and the harmonic components of stator current cause variations in the magnitude and phase angle of the voltages applied at the motor terminals. In the commonly used open-loop mode of operation the breakdown torque of the motor is considerably depleted and the perfomance of the motor is deteriorated as low frequencies are approached. The effects of rotor speed and terminal voltage variations is to aggravate the situation. Consequently conventionally designed induction motors will not operate satisfactorily when fed from a variable frequency static power source and the motor needs to be redesigned. Very little has appeared in the literature on the subject of performance optimization of the drive system from the machine point of view. The present work is concerned with optimizing the steady-state performance over a wide frequency range of (iv) a rectifier-inverter induction motor drive for minimum torque pulsations and minimum cost of active materials of the drive motor by considering a set of machine design variables. Such an approach will prove to be economical since the additional costs involved in modifying the machine to match to a standard inverter circuit \^ill be a minor fraction of the overall costs of the drive system. A simple graphical method has been developed to evaluate the steady-state performance of thyristor powered a.c. drives and has been applied to obtain feasible points for initiating the search process. This method is applic able to constant-torque as well as constant -horse power modes of operation and also permits synthesis of rotor networks to obtain desired torque-speed curves on variable frequency. A comprehensive investigation of the effects of various machine design variables, filter parameters and applied voltages has been carried out to form a basis for the choice of independent deterministic variables and a prelude for the process of optimization. An analysis of the computed results presented is oriented towards re design aspects of the drive motor suitable for different industrial application requirements. A modified version of Rosenbrock's hill-climb technique suitable for large scale optimization problems

has been developed and coded in FORTRAN to evaluate the optimal machines on a digital computer. A variety of 2 -pole, h -pole and 6 -pole machines of different horse pox^er ratings have been considered to illustrate the method of approach in evaluating the optimal machines with satisfactory steady-state performance over a wide frequency range with minimum torque pulsations suit able for applications such as radar tracking systems, antenna positioning etc., where uniform speed of rotation down to very low speeds is mandatory. Additionally, since cost economised drive motors find attractive commercial value, this aspect has also been considered and cost minimized drive motors have also been evaluated for each of the machines considered. Results for each machine for both the cases of optimization have been presented. Based on a comparative study of these results, the different application possibilities of the optimized drive motors have been explored, design recommendation presented and scope for further work indicated.