Contact melting depth as a function of energy and heat physical parameters of contact material

Abstract

When switching alternating current with electromagnetic starters in the contact node, the electromagnetic energy W is released, while an electric arc may be observed. Thermal energy Q. while consuming heating of the material of the contacts and partly on the melting of the metal on the working surface (Q1), and the other part (Q2) - to heat the mass of the molten metal to the boiling point and the evaporation of the metal part. On the other hand, this energy W has two components: one is the directly proportional combustion time of the arc t0, which is determined by the active load of the circle - WR, and the second - the constant - WL, which is equal to the energy of the magnetic field accumulated in the inductance of the electric circuit and consumed in the switching devices when switching

The difficult conditions for the operation of electric vehicles in agriculture and the numerous factors of influence make it difficult to choose the contact material for switching devices. In recent years attempts have been made to replace the Ag-CdO contact material with a less toxic one. Eco-friendly copper-based contact materials have been created for apparatus operating in electrical installations of agriculture, at a low cost with a given level of reliability with respect to transient resistance [5].

The amount of heat required to heat the contact element to the melting point Q1 operates on the working surface [2, 3]. The heat comes perpendicular to the surface and the thermal conductivity in all directions is the same. We assume that the duration of the fluxes of heat Q1 is equivalent to the duration of the burning of the electric arc between the electrodes t0 ( the time of single-circuit or the opening of contact details). Therefore, for the heat equation, the process will have the character of a stationary inhomogeneous one-dimensional one.

The solution of this equation makes it possible to determine the temperature at any point of the contact detail along the x-axis (longitudinal axis) at each specified time t, including the points located on the working surface (x = 0) at the end of the switching interval ( t = t0) (Fig. 1).

The aim of the work is to establish the relationship between erosive phenomena during switching (the depth of melting) and the energy parameters of the contact material.

The objects of the experimental study are the contact details of the electromagnetic actuators PML-1100 and PML-2100 made from serial (on the basis of silver) and environmentally safe research (on the basis of copper) contact materials.

Oscillograms of voltage and current obtained during the study are made using a universal two-beam memorandum oscilloscope C8-14. The parameters of switching (the time of burning the arc and the average value of current in the arc) was determined by oscillograms.

For calculations with a sufficient probability it is assumed that this heat capacity c1 is equal to the heat capacity of the non-molten (cold) metal, that is, c=c1.

If the metal weight of the contact component m1 is brought to the melting point Tll and the melting process is added, then it can be considered equivalent to bringing the total melting temperature Tll and the effective Tef temperature to the total molten mass m1.

For real contact materials (copper, silver, Srn-90), the coefficient of thermal conductivity is a = (1,164 ... 1,689) 10-4 m2 / s.

The time of combustion of the electric arc t0 in the low-voltage electromagnetic actuators of the alternating current is not more than one half-period of the sinusoid (Fig. 2), and the time of uncoupling of the contact parts is within the limits of t = 0,02c = 20mc. According to the technical data of the documents [1], the full time of activating the triggers PML-1100 is approximately t1 full = 17 ms, and PML-2100 - t2 full = 22 ms.

Conclusions and perspectives. 1. Erosion stability and magnitude of the depth of contact mellows h is a function of the energy, thermal and physical characteristics of the contact material, the design temperature of melting, boiling, evaporation of the contact material.

2. The depth of contact melting depends on the energy characteristics of the electric arc, primarily the value of the commutated current (4; 6.3; 10A) and the burning time of the arc.

3. The calculation of h is made for a contact material that has a heterogeneous structure. Therefore, for contacts of a homogeneous structure, the picture may change and in calculations it is necessary to take into account the factor of heterogeneity. The results of calculating the heat emission in the process of switching the electric current, the thermal regime and the depth of the melting of contacts of starters quite well coincide with the experimental data of the depth of the melting (in the study of longitudinal cross sections of electrodes differ by 4 ... 5%).

Keywords: amount of melting, ecologically safe contact material, electromagnetic starter, electric arc, thermal conductivity, temperature, coefficient of temperature conductivity, heat capacity