ANALYSIS OF CHOPPER CONTROLLED D.C. MOTOR DRIVES INCLUDING THE EFFECTS OF SOME MECHANICAL FACTORS

Abstract

The d.c. motor is a widely used type of motor in industry. The bulk of d.c. drives find their application in variable speed drives. The use of thyristors has further increased the scope of d.c. motors as it has led to the development of a variety of effi cient techniques of speed control. One such frequently used tech nique is chopper control which converts a constant d.c. voltage to a pulsed type of voltage.. Considerable interest has been shown in the last few years to upgrade the methods of analysis to predict the performance of chopper controlled drives more accurately, and to incorporate improvements in design based on such accurate ana lyses. The work presented in this thesis is an effort in this direction. The performance of an electric drive not only depends upon its electrical components but is also significantly affected by its mechanical features such as elasticity of shaft, misalign ment, backlash, etc. Extensive work is available in literature on the performance analysis of electrical drives but without including the effect of these mechanical factors. The development of high performance d.c. drive systems requires a precise analysis of such systems, in which case these factors can no longer be ignored. The basic aims of this work are (i) to develop better techniques of analysis for chopper controlled drives, (ii) to determine the effects of mechanical factors and to establish the importance of including such factors in the analysis, and

(iii) to suggest changes required in design in order to improve the drive performance. The work presented deals mainly with the analysis of separately excited d.c. motor drives, fed by a chopper as well as ordinary d.c. supply. The analysis includes the effects of mecha nical factors associated with drives, such as elasticity of coup ling and periodic variation of load torque. The effects of these mechanical features on the performance of d.c. series motor drives are also investigated and compared with those in separately excited d.c. motor drives. The work presented in this thesis is summarized below* The performance of an electro-mechanical system consisting of a d.c. motor fed by a constant d.c. voltage and coupled through an elastic shaft to a load with periodic torque variation is ana lysed (Chapter-2). A mathematical model of the system is given and equations are solved using State Space techniques. Closedform solution is obtained to give the system performance under transient as well as steady state conditions. The analysis is illustrated by an example and inferences drawn. It is observed that the performance is significantly affected by elasticity of shaft, specially when the load torque is pulsating in nature. Some suggestions are given to improve the performance. A new technique for the analysis of chopper fed d.c. motor drives using pulse width control is presented (Chapter-3) • The methods of analysis of such drives given by earlier authors involve a progressive step-by-step solution of system differen tial equations. Closed-form solutions using such techniques are not available and the computation efforts are large. The proposed analytical technique which overcomes these limitations is superior because of the following advantages* (i) A single set of equations is needed to describe the system in duty as well as freewheeling intervals, (ii) The performance in terms of current and speed at any instant, in transient as well as steady state conditions, can be directly obtained without using step by step methods starting from switching-in condition. The computational efforts are therefore, greatly reduced, The solutions are in closed-form, and therefore provide an insight into the transient and steady state performance of the drive, (iv) The solutions are more accurate, as speed over a chopper cycle is not assumed constant. The frequent use of chopper controlled d.c. drives makes it imperative to investigate the effects of mechanical factors on performance of such drives. An analysis of chopper fed d.c. motor drives with elastic coupling and pulsating load torque is presen ted in Chapter-1*. The equations governing the performance of such a system are expressed in State model form. The analytical technique of Chapter-3 is used to obtain closed-form solutions for system performance. The effects of mechanical factors is observed to be more severe when the motor is fed through a chopper. The conditions leading to resonance are investigated and sugges tions are made to avoid such situations. Results are illustrated by an example and Inferences drawn therefrom.

PWM control is commonly used in closed-loop d.c. drives for obtaining the desired control of speed. The influence of mechanical features on performance of such drives has not been attemped so far. A system consisting of a d.c. separately excited motor with load coupled through an elastic shaft, and fed through a PWM supply obtained from a controller having a speed feed-back is analysed (Chapter-5). A mathematical model of the system for constant as well as pulsating load torque conditions is presented and the transfer function obtained. The effects of some of the system parameters on the dynamic stability of drive are studied using 'parameter-plane technique'. A set of values of system parameters to give stable operation and minimum settling time is determined, and performance of the system obtained using numerical techniques. The value of amplifier gain required to give minimum settling time is observed to be affected by elasticity of shaft. The effects of variation of system parameters on pulsations of current and speed are studied and conditions of resonance investi gated. The work discussed above (Chapters 2,U-,5) deals with the analysis of linearised systems assuming the frequency of load torque equal to average steady state motor speed. For certain types of driven mechanisms, this frequency is proportional to instantaneous value of motor speed and leads to non-linear system equations. Such non-linearity is accounted for in the analysis (Chapter-6) and the system performance is determined using numeri cal techniques. The assumptions made in ignoring such nonlinearity are shown to be valid.

D.C. series motors are also used in a variety of industrial drives and, therefore, it is worthwhile to analyse the effects of mechanical factors on performance of such drives. The influence of elasticity of coupling and periodic variation of load torque, on the performance of d.c. series motor drive fed by a chopper as well as constant d.c. voltage, is investigated (Chapter-7). The performance of this type of drive is compared with that of a simi lar motor when excited separately in order to identify the diffe rences in the behaviour of these two types of drives. The important results are summarized in