Note on the grading of d.c. starters

The geometric principle used in grading d.c. shunt starters is extended to the general case by the introduction of a constant p, which is shown to equal (Vb)/a, where V is the line voltage and the flux/current relation in the relevant region is expressible as ¿ = a + bI. Then if R is the total resistance in circuit, the progression ratio k of the steps r1, r2, etc., is the same as that of R1, R2, etc., where R¿ = R + p. From this the grading follows as in the shunt case (for which P = 0). The treatment is exemplified for a single motor and for the series-parallel starting of a pair of similar machines.