Some Electromagnetic Field Problems in D. C. Machines Supplied from Thyristors

Abstract

This thesis presents a study of two electromagnetic problems in a d.c. machine. Part (a) Field analysis in the interpolar region of a d.c. machine operating on smooth voltage supply, using Schwarz~Christoffel transformation. Part (b) Experimental and theoretical study of eddy currents induced in the solid shell of a d.c. machine when carrying a pulsating component of a.c. flux which is the case when the d.c. machine is supplied fromthyristors. The work under part (a) is given below. Chapter 1 outlines the principle of commutation process and reviews the problems of commutation analysis of a d.c. machine supplied from smooth voltage source, and discusses the work done by various investigators in this field. in Chapter 2 theoretical flux density distribution and toothpitch flux distribution under the compole, which are very important so far as the estimation of speed induced voltage in a commutating coil is concerned, are obtained in terms of machine dimension and field quantities, by solving the Laplacian field equation in the interpolar region with the help of Schwarz-Christoffel transformation. Theoretical investigation is made for three models, but none of them considers the armature slots. ‘he effect of slots is taken into account with the help of a factor k∝ obtained experimentally and discussed in Chapter 7. Non-linear multivariable optimisation technique is adopted to find out the five w-plane constants from equations containing elliptic functions and elliptic integrals. This has been discussed in detail and its computer programme is given in the appendices. Part (b) of the thesis is a different problem from part (a). After an introductory chapter which reviews the problems in the process of commutation arising from the eddy currents induced in the solid part of the magnetic circuits of a d.c. machine fed from controlled rectifiers, details of the test rig and instrumentations are written in Chapter 4, and discussion of the results of a series of experiments on a specially designed test machine are given in Chapters 5 and 7. Chapter 6 presents the theoretical analysis for the eddy currents induced in the solid shell due to the pulsating a.c. flux. Variable μ is taken into consideration with the help of Davies's formula H( µrµ0) ¼ H= kHm. The discovery of power relationship between the fundamental flux reduction due to the induced eddy currents and the frequency of pulsation of a.c. flux within the range (25Hz  to 600 Hz) is written in Chapter 7. It is also discovered that the index 'm' is a function of the d.c. level of flux in the shell. Comparison of theoretical and experimental results are also given in Chapter 7.