Commutating-pole saturation in D-C. machines

The paper discusses the importance of obtaining the characteristic curve of the commutating-pole useful flux, especially in regard to machines designed to withstand heavy overloads. It is pointed out that this curve must not depart from the curve representing the flux required to neutralize the reactance volts of the machine by more than an amount corresponding to a reactance voltage which experience has shown can be successfully taken care of by the resistance of the brush A method of measuring the useful and total commutating-pole flux is described. The results of tests on two machines are given, together with curves showing the useful and total fluxes obtained. The importance of the ratio of leakage flux to useful flux at low saturations, in regard to maintaining and determining the ultimate point of reversal of the useful flux, is shown. An approximate formula for obtaining the useful flux from this ratio is given. An attempt has been made to analyze the magnetic leakages due to the main and commutating-pole ampere-turns and to show that the paths taken by and the dimensions of these fluxes are to a certain extent dependent on the relative values of the main and commutating-pole ampere-turns. The variation in the relative values of the main and commutating-pole ampere-turns and the absence of uniformity in the magnetic potential of the commutating pole introduces some degree of error into the formula given. To obtain a greater degree of accuracy, corrections have to be made for the varying initial leakage ratio and for the change of magnetic density existing at different points in the length of the commutating pole.