Protection device for detecting earth shorts in energising circuit for electrical machine

摘要

The circuit contains a square wave generator (2) of a very low frequency voltage symmetrically coupled to the energising circuit via high impedance resistances on one side and on the other side via measurement shunt (10) to earth. The voltage drop across the shunt is used to check the measurement circuit contg. the energising circuit, square wave generator and measurement shunt for open circuits. An integrator (16) connected to the shunt generates a measurement parameter and related output corresp. to the voltage-time area during a half period of the square wave voltage (Ug(t)) from the voltage drop across the shunt. A limiter stage (18) connected to the integrator output generates an alarm signal (S) if the output value (A) is below a defined level. The limiter thus generates low output voltage alarm