Experimental determination of factors influencing contact Experimental contact erosion by the electrical arc

摘要

The objective of this work is the experimental determination of the factors influencing contact erosion by the electrical arc in low voltage switches, in order to characterize and reduce the most detrimental factors. The erosion phenomenon and behavior have been studied using endurance tests performed over up to 30,000 opening/closing cycles on sample switches whose contacts are made of AgNi, for three different values of closing and opening speed, and for three different values of initial gap between the fixed and the mobile contact. The experimental tests have been carried out under an assigned alternative voltage of 250 V, an assigned current of 16 A, and an inductive-load power-factor of 0.6. Tests have been done on real switches from the production line, while the contact gap and opening/closing speed effects have been examined on isolated contacts. Based on contact weight determination and electron microscope images, different erosion behaviors have been observed for AgNi contacts, whether based on mass loss or surface structural changes. Current and voltage monitoring and recording during the tests for different number of cycles (700, 1200, 15600 and 30000) have shown that the electrical arc duration is random and independant of number of cycles. Endurance tests on isolated contacts for different initial contact gap (2 mm, 4 mm, and 6 mm) and for different opening speed (4 cm/s, 5.5 cm/s and 7 cm/s) have featured that the mobility of one contact relative to the other is a very important factor for erosion, and that the mass loss and transfer phenomenon is not equilibrated between the two contacts even though the current is alternating. However the mass transfer is always done from the mobile contact towards the fixed one. In real switch endurance tests, contamination of the contacts by carbonaceous deposits, coming from the electrically insulated wall of the switch apparatus by thermal erosion, have masked the contact mass loss due to arc erosion and have shown the magnitude and importance of these phenomena especially in comparison with results obtained on isolated contacts.