GEAR TO PROTECT ELECTRIC NETWORKS Russian patent published in 2003 - IPC

FIELD: relay protection, protection of three-phase electric motors against current overload. SUBSTANCE: gear includes current transformer, voltage rectifier, overload test unit, thermal simulator of electric network, comparator and final-control relay. Voltage exceeding rated voltage emerges across input of rectifier in case of overload in electric network, overload test unit generates signal proportional to effective current in electric network and voltage which value is proportional to heating energy of electric network is formed across output of simulator. If this voltage exceeds some value specified by reference voltage electron key of comparator closes, switches on final-control relay and electric network is disconnected from system voltage. Overload test unit incorporates field-effect transistor showing quadratic dependence on initial section of volt-ampere characteristic which contributes to enhanced accuracy of operation and to simplified design of gear. EFFECT: enhanced accuracy of operation and simplified design of gear. 2 dwg