Abstract :
The calculation of the resistances of the sections of a starting rheostat for a series motor requires a knowledge of the magnetization curve of the motor, as well as a determination of two ¿grading¿ coefficients (of which one depends on the variation of flux, and the other on the variation of current, during starting). A number of graphical and analytical methods of calculation have been proposed, but these give the solution only for a particular number of sections. The method presented in this paper has the same order of accuracy as analytical methods, but it enables the grading coefficients corresponding to several numbers of sections to be obtained simultaneously from a single set of preliminary calculations, which are considerably simpler than those required with other analytical methods. The basis of the method is the use of charts of grading coefficients which are calculated for various numbers of sections, definite values of the percentage pressure-drop in the motor at full load, and given starting conditions. The grading coefficients for any particular motor are obtained by plotting on the appropriate chart a magnetization characteristic of the motor and determining the intersection of this curve with the appropriate curves on the chart. The complete calculation of a starter can be carried out in a few minutes. The method is particularly suitable for rheostat design departments, as the charts are of universal application. Charts of grading coefficients are given for all practical requirements for industrial control and also for seriesparallel control. A number of worked examples are included, illustrating the application of the method.
