Phase breakdown- and null current-detector, especially for an electronic

Abstract

A phase breakdown- and null current detector, especially in or for an electronic motor protection relay with current-dependent response or triggering, which is controlled by detection alternating-current voltages proportional to current values in the motor outer conductors and contains an electrical motor simulator representative of the thermal or heating behavior of the motor due to the charging operation of a capacitive storage. The motor simulator can be adjusted by a controlled switching device to different time-constants corresponding to the operating conditions of the motor. At the input side of a voltage comparator-circuit arrangement there are electrically connected circuit elements for comparison of the mean or average value of a measurement voltage obtained by rectification of the detection alternating-current voltages with at least one comparison voltage in such a manner that in the presence of an asymmetry of the detection alternating-current voltages greater than a predetermined asymmetry of such detection alternating-current voltages there is delivered by the voltage comparator-circuit arrangement an output signal as a phase breakdown-reporting or indicator signal and with detection alternating-current voltages smaller than a predetermined minimum amplitude there is delivered an output signal as the null current-reporting or indicator signal. Further, the voltage comparator-circuit arrangement contains a first output channel embodying a peak value-rectifier arrangement for the phase breakdown-reporting signals and a second direct output channel for the null current-reporting signals.