(US4812730-A)      The generator/motor system comprises a generator/motor having a primary winding and a secondary winding installed on the motor rotor coupled to a prime mover/load. A main circuit is interposed between a power system and the primary winding of the generator/motor. An excitation circuit including a frequency converter is inserted between a part of the main circuit and the secondary winding of the generator/motor. A frequency difference detector determines the difference between the frequency of the power system and a frequency dependent on the rotational speed of the load. A phase-shift control device controls the firing phase of the frequency converter in the excitation circuit by use of a signal from the frequency difference detector as a reference signal.    In normal operation, a frequency signal following the frequency of the power system is produced, while at the time of a disturbance of the power system, the same frequency signal of the power system as before the disturbance continues to be produced thereby to keep the system in stable operation. For this purpose, a starting electric machine, a synchronous motor and a synchronous generator are aligned, and an output of the synchronous generator is applied to the frequency difference detector.    (DE3711657-C)      The generator-motor system with variable rpm has a 3-phase AC machine (1) with a mains fed stator winding (1P) and a rotor winding (1S), and a frequency changer (4) arranged between the mains (5) and the rotor winding (1S). A model machine (2) has a 3-phase stator winding (2P) fed with a main synchronised voltage and an AC rotor winding (2S) which transmits a rotor synchronous signal (132). The rotors (1S,2S) are mounted on a common shaft.    A unit (3) is provided for controlling the phase angle of the frequency changer (4) relative to the position specified by the rotor synchronous signal (132). A synchronising unit (7) has    an electrical starting machine (31) and a mains fed synchronous motor (32) synchronous generator (33) which are arranged on the same shaft, so that the synchronous generator (33) produces the mains synchronous voltage.    USE/ADVANTAGE -   Water power plants. Coupled to pump-water turbine to give reversible power-load combination. With short term mains failure frequency changer controlled without interruption. (8pp)