Patent GB-465761-A (GB)
Title: An improved method and apparatus for testing high potential insulation
IPC: F42B30/04
Filed: 1936-03-16
Abstract: 465,761. Phase-difference measurements. SIEMENS & HALSKE AKT.-GES. March 16, 1936, No. 7888. Convention date, March 15, 1935. [Class 37] A method of testing high potential insulation consists in recording the course of the loss factor or loss current of the insulation first in dependence upon potential to which the insulation is subjected with continuous or stepwise increase of the potential up to a prescribed test value and directly following thereon during a prescribed test period in dependence upon time while maintaining the insulation subjected to the prescribed test potential. The insulation resistance 3 to be tested forms one arm of a Schering bridge to which a voltage is applied from a transformer 1, the value of which increases from zero to a predetermined maximum and is then maintained constant for a predetermined time. The recording instrument 6 may have one coil included in the diagonal of the bridge, the rectifier 7 being connected in series, while the other coil 12 is connected across a tapping on a resistance 8 connected across the primary or secondary winding of the transformer 1. The recording instrument then produces a curve of which the ordinates are proportional to the current flowing through the resistance 3 and the abscissµ to the voltage. When the maximum voltage is attained, the voltage applied to the coil 12 of the measuring instrument is arranged to increase steadily with time by means of a contact member which short-circuits a resistance 13 and is moved by a synchronous motor 14. The motor is started automatically when the voltage reaches its maximum value and at the end of the testing period actuates a contact 16 which switches off the applied voltage. When the instrument 6 records the ratio of the current flowing in the diagonal of the bridge to the applied voltage, the sensitivity of the instrument is varied with the applied voltage by connecting a resistance in series with the current measuring coil which is gradually short-circuited by a slider as the voltage applied to the bridge is increased, the slider being electrically or mechanically coupled to the slider for varying the transformer voltage. If the recording instrument is not provided with a coil such as 12 for providing the abscissµ of the record, the paper on which the record is to be made may be moved first at a rate proportional to the voltage until the maximum is attained and then at a steady rate. This may be effected by using a transformer having a rotary member, the position of which determines the voltage output. The rotary member also drives a gear coupled to a generator which produces a voltage proportional to the position of the rotary member of the transformer, this voltage being applied to a motor which drives through a ratchet the paper-feeding drum at a speed proportional to the voltage. An adjustable contact on the rotary member of the transformer switches on a synchronous motor when the test voltage has been attained, which drives the feed drum through a second ratchet mechanism for a predetermined time, the voltage being switched off automatically. In a modification, when the voltage produced by the transformer increases steadily as the rotary member moves, the feed drum is rotated steadily by a synchronous motor. In order to indicate the point at which the voltage attains its maximum on the record, a resistance connected in series or shunt with mechanism 6 and normally short-circuited is momentarily switched into the circuit, thus giving rise to a downwardly pointed peak.