Stability analysis of PWM inverter-fed synchronous motors

Abstract

AC drives with variable loads operate at maximum efficiency when they are operated at adjustable speed. Speed variation of ac drives can most effectively be attained through change of supply frequency. Normal electromechanical frequency conversions do not meet the criteria of stiff frequency operation (i.e. frequency deviates with drive condition without any desirable control). Modern static converters, specially the inverters and the cycloconverters provide solution to the stiff frequency source. Pulse width modulated (PWM) inverters in this respect also allow one to change voltage and frequency of the supply in one conversion stage requiring minimum filters or no filters at all between a supply and the motor. This thesis aims at the stability analysis of conventional synchronous motors fed from PWM inverters. Particular emphasis is given to the stability study of motors supplied by inverters having open loop v! f control. This thesis contributes in two major areas. These are the determination of switching points of PWM waveforms using a simplified technique and formulation of a new operating strategy for maintaining stability of synchronous motors fed from pulse width modulated inverters. This strategy at the same time enhances the load bearing capability of synchronous motors without loosing the synchronism. New formulations in the form of algebraic equation have been proposed to find the switching waveforms of PWM converters. The approach is completely new and different from conventionally used simulation and numerical techniques, and provides a method of easier and computationally fast analysis of sine PWM waveforms. In the present thesis the proposed technique is utilized in the analysis of triangular sine PWM and delta sine PWM inverter waveforms for subsequent use in the stability analysis of inverter-fed synchronous motors. Stability analysis of synchronous motors has been carried out extensively in this work considering the variable-voltage variable-frequency supply. The analysis is based on the small signal perturbation and eigenvalue search techniques. In the formulation of linear differential equations small signal perturbation of all the possible variables are considered. It has been established in this thesis that normal control schemE'with rated vIf to ensure constant air gap flux is not appropriate for maintaining stability of synchronous motors in low frequency range. This research proposes a new control scheme with vIf ratio other than its rated value to maintain stability of synchronous motors fed from variable-voltage variable-frequency sources. The results of the analysis have also been substantiated by time domain analyses and experiments. A suitable modulation scheme is proposed in the thesis for an inverter-fed synchronous motor to obtain the desired vIf ratio in open-loop control to maintain the motor stability. The scheme has been developed from the results of inverter waveform analysis and the stability analysis of the synchronous motor. Accurate representation of a machine by its parameters is necessary for the stability investigations. During this research, problems were encountered in measuring the required parameters from tests. It was found that available test methods were not equally applicable for large and small machines. Parameter estimation of synchronous machines is a topic of continuous research even today. Many of the methods proposed in recent years are computationally involved, requires costly Instrumentation and are based on assumptions which are not valid under various operating conditions. A simple test method based on input impedance measurement has been developed. This method is applicable for finding the parameters of, synchronous machines of any size.