PERFORMANCE ANALYSIS OF CHOPPER CONTROLLED SEPARATELY EXCITED D.C. MOTOR FOR MOTORING AND REGENERATIVE BRAKING OPERATIONS

Abstract

Chopper controlled separately excited d.c. motor drives are now widely used for many industrial applications. A combination of a thyristor chopper and a separately excited d.c. motor can give a wide variety of characteristics to match the load requirements. In addition to speed control, regenerative braking is feasible in such drives. The present thesis gives a comprehensive analysis of chopper controlled separately excited d.c. motor operating under motoring and regenerative braking modes. Two different approaches have been used for modelling the chopper controlled separately excited d.c. motor operating under drive mode. In the first approach, the chopper output load voltage waveform is represented by Fourier series. The differential equations consisting of motor armature circuit equation and equation of motor dynamics with voltage across armature terminals expressed by a Fourier series are solved to give closed form expressions for current and speed. These closed form expressions are then used to predict the performance for motoring operation in quadrant I. Computed torque-speed curves are compared with those obtained experimentally. This approach has some inherent limitations. In the second approach, the system has been modelled by splitting the chopper time period into a number of segments at the points of natural discontinuities. Sets of differential

equations applicable for different segments have been used as the system model. State-space approach is used and closed form solutions for current and speed for various segments of a chopper cycle are obtained. These solutions have been used in a general computer program for computing the performance for known values of forcing functions (chopper input voltage and load torque) and operating parameters (chopper time period and duty ratio). The special features of this analysis and the associated computer program are: (i) Drive performance obtained is more accurate and uses lesser approximations and assumptions than hitherto used by earlier investigators, (ii) A built-in facility in the program allows accurate determination of commutation and free-wheeling intervals, and hence automatically decides whether the mode of conduction is continuous or discontinuous, (iii) It can give steady state as well as transient performance. Variations of current and speed over a chopper cycle can also be obtained, (iv) Effect of source inductance can be considered, (v) The effect of armature reaction and non-linearity of magnetic circuit can be accounted for. (vi) Any load torque profile can be considered. As an application of this analysis, the performance of a typical system has been obtained taking commutation interval into account. The effect of additional inductance in series with armature circuit and chopper frequency on the transient as well

as steady state response (speed and current pulsations, current ripple factor and motor heating) have been investigated and inferences are drawn. Computed torque-speed curves are compared with the experimental results. The effects of source inductance on the drive performance and chopper operation have been investigated. The effect of non-linearity of magnetic circuit and armature reaction on the torque-speed characteristics and transient peak currents have been investigated. Using the same approach, the performance of such a drive with periodically varying load torque has also been investigated, and effects of some parameters on such performance are studied. Thyristor chopper can also be used for electrio-braking of such drives, giving a combination of regenerative braking and resistive braking. The performance of such a system has been analysed for transient as well as steady state conditions. The factor which influence the proportion of energy regenerated current and speed pulsations, steady state speed etc. have been brought out. The cases for which performance has been investi gated include braking to standstill, steady state operation in quadrant II and the transient process of change of speed from one value to another value in the presence of an external forward torque. The thesis thus presents a comprehensive analysis of chopper-controlled separately excited d.c. motor under motoring and regenerative braking regimes, and the influence of various parameters on such drive performance. The analysis provides a step towards improved designs and performance prediction of such systems.