Mains failure monitor used with three=phase sync. machine - has transformer between star point and neutral and frequency relay detecting voltage zero crossings for signalling too low frequency

摘要

A self-excited or externally excited three-phase synchronous machine is connected to the mains and to an immediately available emergency unit with an energy store. The monitoring device controlling a switch is inserted between the neutral of the synchronous machine and the mains neutral. The monitoring device (11) has a current transformer (12) followed by a load (13) and a frequency relay (14) detecting zero crossings of the voltage drop across the load (13). A drop in the zero crossings frequency below a certain value serves as a singal for mains failure, for the mains switch (6) operation and/or for other switching processes.