Abstract: Described is an operating system for a plurality of poly phase alternating current motors having a common load, said alternating current motors including a first motor which is a synchronous induction motor and a second motor which is a synchronous induction motor or a synchronous motor, and driving said load in synchronism through respective connecting means connecting said alternating current motors to said load, each of said alternating current motors having respective phase field windings excited variably by direct current supplied by separate excitor circuits. The system comprises antiparallel connected thyristor means provided to one of said exciter circuits of said first motor and means for sensing the gap between the internal phase angles of said first and second motors and supplying gate control signals to said thyristor means provided to said exciter circuit of said first motor.