This recommended practice proposes a method for determining the efficiency of large, adjustable-speed AC motor-drive systems consisting of a load commutated inverter and a synchronous machine, in which a DC-linked converter is connected between the line and the load. This recommended practice is directed primarily to high-capacity systems (exceeding 1000 hp) and drive systems, which normally include equipments such as transformers, reactors, thyristor power converters, and synchronous machines. The method recommends that the existing test standards for the system equipments be used as reference, supplemented by test and calculations include herein to account for operating characteristics of the adjustable-speed drive systems.<>