It is described that the three phase series commutator motor of the doubly feed type is valuable to b used in practice. The adjustment of the speed in such a type of these machines, in mos cases, is done by the shifting of the brushes automatically or by hand. The stiffness, in this paper, means the ratio of the magnet motive force of the roter to that of the stator and is shown by<br>μ=[MmrWmrfr/MmsWmsfs][MtsWts/MtrWtr.]<br>The important points to be noticed for the vector analysis of this motor are explained tanding upon the considerations for the stiffness. The states of the variation of the current locus which is obt ined by the inversion of the voltage vector diagram are considered under constant value of stiffness, but on the variable brush angle and also the limit of stable operation. Then after the effect of the variation of the stiffncss under constant impre sed voltage and constant brush angle for the states of the current locus are considered, the comparison of the results, obtained experimentally and that obtained graphically, mainly on the effect of the saturation of the magnetic circuit have been explained.<br>The variation of the quantity of the main flux and its stat s are th fundamental factors for the charasteristics of the motor on the design considerations of the stiffncss and the saturation. The curves which show the variation of the main flux under various condition of magnetic saturation, various stiffness, and varicus brush angle are calculated point to point. The main flux is deromposed into two components, one of which is effective for the motor output and the other for the phase compensation and this decomposi ion is dspended upon to the stiffness and the brush angle. From these conslderations the output coefficient of this type of the mach ne must be explained by<br>r=.116BgACfs√1+2μcosρ+μ2S210-11.<br>The relation between the stiffness and the brush angle on the phase compensation are explained generally. The experi ental results are shown about the satu ation effect of the rotor transformer under constant stiffness and constant brush angle =145°, and next the cffect of stiffness under constant angle =145° on the various characteristics of the machine.