Short-circuit monitor for machines - has resistor switching transistor which controls power switch upon short-circuit

摘要

The conductor short-circuit monitor, for electrical machines, has a transistor (13) coupled by its emitter and collector across the output of a rectifier bridge supplied via a transformer from the machine's star-coupled windings (i.e. supplied from two phase lines). The transistor's base is connected to its emitter via a high value resistor (15) whose resistance is chosen so that the base current is practically equal to the short-circuit current. The ends of this resistor are connected over separate diodes, one to neutral and one to earth. The monitor still detects a fault even if the earth line is defective.