RESEARCH OF THE VALUE OF CONTACT MELTING DEPENDING ON THE COMPONENTS OF THE CONTACT MATERIAL DURING CURRENT SWITCHING
Authors: Korobskyi V. , Zborshchyk D. 
Published: February 2023

The amount of melting of PML-type starter contacts with current load 4 is investigated; 6.3; 10A. The contact material of the starters used for the production of silver-based serial contacts (СрН-90 and СрМ-0.2+М1) and experimental copper-based contacts is used in the research. The formula for determining the amount (depth) of penetration during a single current switching is given. The electric arc (its burning time) is directly related to electroerosion phenomena (depth of contact penetration). The formula for determining the depth of penetration is obtained by solving the equation of thermal conductivity taking into account the boundary conditions of the II kind and relates the value of the depth of penetration to the energy and thermophysical parameters of the contact material, which is determined by the constituent components of the material.

Difficult operating conditions in agriculture and numerous influencing factors complicate the choice of contact material for starters. In order to ensure the operation of devices with specified reliability indicators, their contact details must resist the influence of an electric arc, a chemically and biologically aggressive atmosphere, and mechanical loads. In general, contacts should be characterized by the following physical parameters: stability of contact transient resistance, high specific electrical conductivity, high erosion resistance and corrosion resistance, high arc resistance and resistance to welding, a combination of mechanical strength and high plasticity.

It is practically impossible to choose a universal material that would meet all the mentioned requirements, therefore, depending on the functional purpose of the contact node, a compromise decision has to be made. Contacts of starters operating in the area of small and medium currents (and infrequent switching on and off of short-circuit currents) must, first of all, ensure the stability of the transient resistance under the influence of aggressive atmospheric impurities in combination with high arc resistance and resistance to welding. Therefore, in recent years, attempts have been made to replace the contact material Ag-Ni and Ag-CdO (used in PML starters from 25 A and above) with other materials.

Transient resistance depends on the actual contact area of the contacts and on the specific resistance of the contact material. The effective area of contact depends on the magnitude of the contact pressure and increases exponentially with its growth until the compressive stress is higher than the yield point of the material. On the other hand, the transient resistance depends on the resistance of the boundary layer, which is determined by its composition (the presence of oxide, sulfide and other films, dust, etc.) and specific electrical resistance.

Taking into account the requirements for switching devices of starters suitable for work in the agricultural sector and in order to save precious metals, it is appropriate to develop contact materials in the direction of improving the properties of multi-component composite materials based on copper and refractory compounds.

Copper is the closest material to silver in terms of physical and mechanical properties. But its main drawback is that it oxidizes in the air and the oxidation process intensifies as the temperature rises. As a result, insulating films appear on the surface of the copper, which disrupt the operation of the contacts and lead to failures.

It is known that copper oxide films are formed even at room temperature and their thickness reaches no more than 5 μm. Such a film protects copper from further oxidation by oxygen. The specific resistance of Cu2O is 10 μΩˑm, but it does not significantly affect the contact resistance of the electrodes, since the film is very thin, so electrons pass through it due to the tunnel effect.