SELF

44

and S.B. Karavashkin

Despite at this stage the effect of spark-extinguishing circuits was confined only to considerably lower showering arcs, contact elements became much better working. This has been corroborated in comparative experiment at the Sokol mill, Belgorod, Russia, on the relays which at that time were commercially produced there. In Fig. 29 we can see the images of contact pairs that have worked out their standard resource under speed-up regime and nominal load.

a) C = 0,033 F , R_k = 0 kOhm

b) C = 0,037 F , R_k = 0,33 kOhm

c) C = 0,037 F , R_k = 1,6 kOhm

d) C = 0,037 F , R_k = 2,2 kOhm

e) without spark-extinguishing circuit

Fig. 29. Contacts of relays BM MKC #3 after comparative test for resource of reliable operation with different protecting RC spark-extinguishing circuits and without them

In the images we see, a capacitance inserted to the spark-extinguishing circuit considerably increases the contacts reliability and longevity, due to less number of bridge formation periods. But especial effect has the resistance R_k that provides the selection of circuit optimal parameters achieved under R_k = 1,6 kOhm .

We have to underline, the suggested method to obtain the breakdown voltage curves is shaking down. So it is efficient to obtain such curves statistically at the mills manufactured contact-having products, on the basis of selected lots. The presence of such curves in certificates will facilitate the users to easily select by known methods the optimal protection for contacts in accordance with the used inductive loads, switched currents and other specific conditions and limitations.

On the basis of new understanding of erosion, more perfect methods to lessen it by way of selection of contact materials, speed of contact divergence etc. will be developed.

The general practical result of the supposed discovery will be higher quality, reliability and longevity of contact products, and therefore of all products containing contacts. Noting that about 60% of contacts existing all the world round commute just inductive loads, it is hard to over-estimate the economy that follows from this discovery.

Conclusions

We have completed the experimental and theoretical study of contact breakage of inductive load, and established the following basically new physical regularities:

1. The unstable discharge is the alternation of relaxations each of which consists of two stages: monotonous exponential growth of voltage in limits 500 V and high-frequency oscillation process with the amplitude in limits (2000 - 2500) V.

2. At the monotonous sections of relaxation, contacts are shunted by metallic bridges and parasitic capacitance of contacts.

3. At the sections of high-frequency oscillation process, contacts are shunted by parasitic capacitance of contacts and discontinuous short arc alternating in sign; under considerable power accumulated in the inductive load, it transforms into long arcs which break the sequence of unstable discharges.

4. Metallic bridges are formed in each period of unstable discharge relaxations.

5. Unstable discharge duration is fully determined by the duration of bridge formation (contacts ability to form new bridges).

6. Most probably, large emission currents arise during short arcs because of autoelectronic emission caused by large voltage applied to the contacts.

7. Most general reconstruction of oscillogram characterising the process of inductive load broken by contacts has a shape shown in Fig. 25.

Of course, the study stated in this paper cannot give an answer to all questions - such as the cause of bridge formation, physical processes occurring in the short arcing, the causes and conditions of bridge breakage etc. We have developed many techniques for this study and began to fulfil them, but the work was interrupted by reasons out of our control.

1976 - 1994 - 2005