ANALYSIS AND PERFORMANCE OF SEMICONVERTER FED DC DRIVES

Abstract

DC drive systems continue to provide flexible, reliable and low-cost solution to the vast majority of variable-speed industrial applications. Their use in conjunction with thyristor schemes has further upgraded the utility of such drive systems. One such scheme often used in industry requiring first quadrant operation con sists of a semiconverter controlled dc motor. A precise analysis of such a drive system is therefore desirable as it leads to a better understanding of the system op eration and may enable improvements in the system design. The present study is an effort in this direction. Performance of an electromechanical drive system is not only affected by its electrical parameters, but also by various mechanical factors such as shaft elasticity, mecha nical misaligament, pulsating load etc. Influence of these mechanical factors on the performance cf semiconverter con trolled drives, therefore, needs to be investigated. No analytical methods have been reported so far, for computing the transient and steady state performance of single-phase or three phase semiconverter drives with speed treated as a variable. The solutions of such systems have been obtained either through the use of numerical methods or through the use of analytical methods, which require iterative steps and assume speed to be constant over a converter cycle. General closed- form solutions applicable for tran sient as well as steady state conditions with speed treated as a variable are not available. The aim of the present thesis is to overcome these shortcomings. In order to achieve this objective, a new technique for the analysis of separately excited dc motor fed through single-phase and three-phase semiconverters is presented in this dissertation. For the purpose of analysis, the system is taken to consist of a separately excited dc motor controlled through a single-phase or three-phase semiconverter. The motor is coupled to the load by two elastic shafts and the speed-reduction gears. The load is considered to have a periodic torque profile. The output voltage of single phase semiconverter and three phase semiconverter with three-pulse mode of opera tion, which are discontinuous but periodic, are expressed as a superposition of a number of sections of sine waves with appropriate time displacements. The output voltage of a three phase semiconverter with six-pulse mode of operation is expressed as a sum of two component voltages and each of these components is further expressed as a sum of a number of sine wave components with appropriate time displacements. The mechanical system is represented by an equivalent two-mass system for the purpose of analysis. With such representations of the electrical and mechanical systems, the equations des cribing the system are solved to obtain closed-form expressions for the system variables for transient and steady state conditions. VI For three phase semiconverter with six-pulse mode operation, two sets of solutions obtained for each of the two component voltages are superimposed to yield the complete solution. The analysis is carried out first with constant and periodically varying load torque but with rigid coupling and subsequently the effect of shaft elasticity is included. The special features of the presented technique are: (i) Only one set of differential equations, as compared to two sets used by earlier investigators, is needed to describe the system during conduction and freewheeling periods. (ii) Values of system variables corresponding to steady state conditions can be computed without passing through the transient period, (iii) Transient solution during any converter cycle can be obtained without working through the preceding converter cycles. (iv) Closed form expressions give the values of system variables directly without iterative calculations. (v) The solutions for system variables, being in closed form, provide better insight into the system perfor mance, (vi) Solutions are more accurate as speed is not assumed to be constant.

The proposed techniques are illustrated by examples and the results are verified by numerical (Runge-Kutta fourth order) method. Performance of single-phase and three-phase semiconverter drives without shaft elasticity is studied for constant and periodically varying load torques and inferences regarding current and speed pulsations as well as settling time of the system are drawn. The techniques presented above have also been used for computing the performance of semiconverter dc drives under conditions of step load changes and pulse type periodic load torques. Effects of some parameters on the performance of such drives are discussed. The method presented in this dissertation has the limitation that it is applicable for continuous conduction of the armature current. Typical performance studies on a separately excited dc motor with elastic coupling and periodically varying load torque are carried out and useful inferences are drawn. It is shown that when the alternating component of load torque has a frequency approaching the system natural frequency associated with low damping ratio, the system experiences resonance. Suggestions are made so as to avoid the occurrence of such situations. It is hoped that the work presented in this thesis will be useful for predicting the performance of such drives more precisely and will help in improving the system design.