JPS6425068A - Insulation-resistance measuring method, wherein fluctuation in circuit constant is compensated - Google Patents

Abstract

PURPOSE:To measure the insulation resistance of an electric path accurately, by always correcting deviations in amplitude and phase with an amplitude correcting circuit and a phase control circuit. CONSTITUTION:A piece of conductor 20, to which a switch SW is connected, is inserted in a current transformer ZCT. The switch SW is opened and closed with a signal having a repeating frequency fs from a phase control circuit PC1. A current, whose phase is shifted by 90 deg. from a low frequency signal voltage for measurement having a frequency f1, is made to flow through the conductor 20. The voltage is outputted to a winding N2 of a transformer OT. Meanwhile, the output of the current transformer ZCT is made to pass through a filter FIL. The commercial frequency component is removed. The output is supplied to an adder ADD12 and an amplitude correcting circuit 30. The output through the circuit 30 is supplied to one terminal of the adder ADD12. The signal is corrected with the circuit 30 so that the amplitudes of both components of the frequencies f1-f2 and f1+f2 included in the output of the filter FIL become equal. The corrected current is inputted into a synchronous detector MULT1. A signal voltage having a frequency f1 from the circuit PC1 is inputted into one input end. Synchronous detection is performed. The phase of the signal voltage having the frequency f1 is automatically adjusted so that the component of the frequency fs in the output of the synchronous detection becomes zero. Thus the insulation resistance can be accurately measured.