Series stability windings and their dependence upon the working flux

The speed/load curve of the shunt-characteristic direct-current motor worked with the increased loading constants common to contemporary design frequently departs from the legendary form. In this paper the shape of the curve which is realized in practice is stated and discussed, and instability is defined. The stabilizing and de-stabilizing influences are enumerated, and their resultant is studied as a function of the working flux. A method of approximate predetermination of the effect of the load currents in the armature is presented, and the applicability of it to the generator action of the same mechanism is pointed out. Finally, methods are discussed of suppressing excessive speed regulation with load at weak fluxes. The paper treats of machines with symmetrical main pole air-gaps; distributed compensating windings are not considered.