Starting and Speed Control of a.c. Motors

Abstract

Comparisons are occasionally made between the starting currents taken by a.c. motors with cage-type rotors and those taken by d.c. machines, generally to the detriment of the a.c. machine. Although it is true that cage motors generally take starting currents in the range 4 to 7 times full load current whilst those of d.c. machines are normally much less, the comparison ignores one of the principal virtues of the a.c. machine, its very robust construction, which permits it to be started by a direct connection of the motor windings to the supply system; the lower starting currents of d.c. machines are almost always associated with multi-step resistance starters. Steps taken to reduce the starting currents of cage machines are generally due to the need to reduce these currents to values which the supply system can handle, rather than because of any limitations within the motors themselves.