INFLUENCE OF CONTACTOR CONTACT PRESSURE IN POWER CIRCUITS OF ELECTRIC STOCK ON THEIR PLASTIC DEFORMATION AND FUSING

Dep. «Electric Engineering and Electromechanics», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 47, e-mail baliichukaleksei@mail.ru, ORCID 0000-0003-0119-1446 Dep. «Electric Engineering and Electromechanics», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 47, mona.shelley@gmail.com, ORCID 0000-0003-0319-4544 Dep. «Electric Engineering and Electromechanics», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 47, mona.shelley@gmail.com, ORCID 0000-0003-3602-5851 Dep. «Electric Engineering and Electromechanics», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 373 15 47, ustimenko.1979@gmail.com, ORCID 0000-0003-2984-4381


I and fs
I , taking into account the value of contact pressure, electrodynamic forces of contact repulsion e.d.r F , as well as the pressure influence on the values of such forces.Findings.Taking the contactors 1SVAD8 (locomotive ChS8) and 7SP1 (locomotive ChS4) as an example, the authors conducted the calculations and found that the peak pressure force increases the currents d I and fs I for these contactors by 11.8%; 15.5%; 11.8%; and 15.4% respectively (compared with the specified currents at the minimum pressure force).Influenced by e.d.r F the pressure force of contacts for 1SVAD8 contactor decreased by 13.8…31.8%,and for 7SP1contactor the value e.d.r F may be greater than the pressure force.
This can lead to spontaneous shutdown of the closed contactor that is not provided by the control circuit.Ensuring during operation the maximum allowable pressure force of the contacts c F slightly affects the value e.d.r F , compared with the case when c F has the minimum permissible value (in event of a fault in the heating circuit e.d.r F for contactors 1SVAD8 and 7SP1 is reduced by only 2.45% and 2.59% respectively).Originality.The authors for the first time presented a quantitative assessment of influence of contactor contact pressure in power circuits of electric stock on their plastic deformation and fusing using the proposed engineering method.Practical value.The results obtained during the research allow solving the problem for developing measures aimed

Introduction
Practice shows that there are cases in operation when power contacts of contactors in power circuits of electric stock are fused and sometimes welded.For example, the above effects were recorded during the operation of the electric locomotives ChS8 when the contacts of 1SVAD8 type contactor in the heating circuit of carriages were fused before the short circuit (SC) response time in the given circuit.The time of SC inception until the short-circuit response is 0.08 sec [1,2].

Purpose
The work is aimed at conducting the study to obtain the results that will quantify the influence of contactor contact pressure in power circuits of electric stock on the highest current value d I , that does not cause plastic deformation of contacts; and on the current value fs I -incipient fusion of contacts taking into account the operation conditions.Therefore, the study targeted to establish the contact fusing causes is relevant.

Methodology
We know that contact performance is determined by thermal processes in them [4, 5, 7, 10-12, 1722].Herewith the crucial is relationship between the energy losses in the contact connection l A and the heat dispersion energy hd A , which is output into the environment during the same time.The currents that are larger than the nominal ones significantly hinder the work of contacts.This gives rise to phenomena that can lead to dangerous consequences.The increasing current, for example, in SC mode in the heating coil under consideration, , which can lead to overheating of contacts.A certain value of unbalanced energy A  can initiate plastic deformation of the contact surfaces and even fusing of these surfaces.

The main indicator of the quality of contact connection is voltage drop
where I -current through contacts, c r -contact resistance.
The current rapid changes, peculiar to SC mode, lead to increase of the electrodynamic forces that reduce contact pressure, while this reduction increases the contact resistance c r .Higher c r further increases c U  , caused by the rise in current.
The ratio between the currents d I (the highest current which does not cause plastic deformation of contacts), fs I (incipient fusion current) and the currents at certain modes in the heating circuit of the electric locomotives ChS8, ChS4 are specified in [1,2].This study raises the problem to study quantitatively the dependence of contact resistance c r of the contactors 1SVAD8 (locomotive ChS8) [13] and 7SP1 (locomotive ChS4) [15] on the contact pressure force at different operation modes of heating circuits [9].Let us consider first the influence of the value of the contact pressure force c F , caused by the springs, of 1SVAD8-type contactor on the current value d I in nominal mode.The contact pressure force c F for this contactor may be in the range of 80 N to 100 N [13].Transition resistance at nominal mode [14,16] is as follows:   F lead to contact breaking?These forces are equal to [12]: where D and 0 r are the sizes indicated in Fig. 1; I -current in heating circle at appropriate mode.value without changing the sign (as in the expression (5) the current I is squared).Radius 0 r of the creasing area (cm) is determined by the pressure force in the contacts and resistance cr.s  [12]: The most (shock) values of e.d.r F will be in SC mode when there is no load on the heating coil.[3] For electric locomotive ChS8 (contactor 1SVAD8) the shock currents sh i during short circuit in the heating circle equal to [ U and c I are voltage and current in the traction transformer SC study during short-circuiting of heating coil.We consider the most difficult option when SC for 0 c 2      is at the electric circuit section of from the heating coil clamp to the first electric furnace in the car.Such SC can result from the breakdown of high voltage cable lines, inter-car connection isolation (which is most likely), erroneous actions of service personnel, etc. [8].
For calculations we take the average of the contact pressure force According to (6): Thus, under the influence of electrodynamic repulsion forces in this case the 1SVAD8 contactor contacts will not be disconnected, but the contact pressure is significantly reduced, leading to a substantial increase in contact resistance, instant increase in contact energy losses and therefore to increased contact fusing.
For electric locomotive ChS4 (contactor 7SP1) the shock currents sh i in the same condition, that the short circuit is occurred at time point when ) the 7SP1 contactor contacts of will not be disconnected, but the contact pressure is significantly reduced, leading to a substantial increase in contact resistance, instant increase in contact energy losses and therefore to increased contact fusion.
However the electrodynamic repulsion force e.d.r c 96.5 N 90 N F F     and the contactor 7SP1 can disconnect liberally, bearing SC current.Thus a powerful electric arc arises that first of all will fuse the contacts.It should be noted that the scheme does not provide the circuit breaking by the contactors 1SVAD8 and 7SP1 in current.Given that on the electric locomotives ChS8 and ChS4 the currents sh i , sh i , sh i are equal to several thousand amperes, it can be stated that for the time 0.08 t  sec [2] (until the main switch deenergizes the circuit of the electric locomotive) the fusion process will pass its active phase.The fused metal, spraying in all directions, can cause a fire in the chamber, where contactors 1SVAD8 and 7SP1are mounted.
Let us consider the effect of the contactor contacts pressure on the value of the electrodynamic repulsion forces at SC in the heating circuit.For example, we consider SC at 1  when c F has the minimum permissible value (in event of a fault in the heating circuit e.d.r F for contactors 1SVAD8 and 7SP1 is reduced by only 2.45 % and 2.59 % respectively).

Originality and practical value
The paper for the first time presented a quantitative assessment of influence of contactor contact pressure in power circuits of electric stock on their plastic deformation and fusing using the proposed engineering method.
The results obtained during the research allow solving the problem for developing measures aimed to improve the reliability of the heating circuits of electric locomotives ChS8 and ChS4.The proposed method of determining d I and fs I can be used to quantify the influence of pressure of the main electric contacts mounted in power circuits of electric stock of all series, if during the operation there are cases of plastic deformation, fusing, welding of such contacts, in order to improve the reliability of power circuits.

Conclusions
1.The proposed method of determining the influence of the contact pressure of contactors in the power circuits of electric stock on their plastic de-formation and fusing can be used for all series of electric locomotives and trains.
2. Ensuring the maximum allowable pressure force during operation of the contactors 1SVAD8 and 7SP1 increases the value of current that does not cause plastic deformation of contacts, respectively by 11.8 % and 15.5 %, compared with the value of this current at the minimum allowable value of the pressure force.Herewith the fusion current of the contacts increases respectively by 11.8 % and 15.4 %.
3. The electrodynamic contact repulsion forces during SC in the heating circuits of the electric locomotives ChS8 (contactor 1SVAD8) at the contact network voltage of 19 kV, 25 kV, 29 kV are respectively equal to 12.4 N, 21.5 N, 28.6 N, which leads to a significant increase in contact resistance, instant increase in contact energy losses and therefore to increased contact fusing.These forces in the heating circuit of the electric locomotives ChS4 (contactor 7SP1) are equal to 41.4 N (at 19 kV), 71.73 N (at 25 kV) and at 29 kV the repulsion force is 96.5 N, that is more than the minimum allowable pressure force of 90 N. Therefore, the contactor 7SP1 can disconnect liberally, bearing SC current, and that is not envisaged by the scheme.
4. The results of the research can be used in the development of measures to improve the reliability of power circuits of the electric stock.

LIST OF REFERENCE LINKS
Thus, ensuring of the maximum pressure force c 120 F  N during operation of the contactor 7SP1 increases the fusion current by 2370 2053 100 % 15.4 % 2053    compared to the fusion current value at the minimum allowable pressure force c 90 F  N. Further we define the electrodynamic repulsion forces e.d.r F between the contacts, under which the contact pressure is reduced, resulting in increased contact resistance c r .It is necessary to assess the value of the forces e.d.r F compared to the forces c F .May the effect of e.d.r

Fig. 1 .
Fig. 1.Scheme of current flow lines in the contact