Make-only erosion, break-only erosion, and the combined make-and-break erosion of contacts made from powder metallurgically produced Ag/CdO and three differently produced Ag/SnOz materials were investigated employing both a commercial contactor and a model switch. The test conditions were chosen according to IEC H947-4 AC3 duty, yielding make and break arc energies of the same order of magnitude.The model switch simulates both make and break operations with the same parameter values occurring in modern contactors, but in a much better reproducible way, and with the opportunity to vary the single parameter values in order to gain information about its influence on the erosion behavior.Make-only erosion tests demonstrated that Ag/SnOl materials suffered either three times higher or about 50% lower erosion losses than Ag/CdO depending on the manufacturing process and/or the additives of the materials. Therefore, the structure of the materials rather than the kind of the main metal oxide component (CdO or SnOz) is decisive.The combined make-and-break tests yielded significantly lower erosion rates than make-only tests. This unexpected result was reproducible for all contact materials investigated, although the arc energy per operation was more than doubled due to the additional break operation.Inspection of the micro-structure of the stressed contacts suggested that this reduction of the erosion rates is due to the motion of the break arc and its effect on the eroded surfaces.
R&D activities have been carried out for decades by many researchers in order to improve electromechanical contactor performance employing electronic control. This includes electronic controlled contactor opening to minimize contact erosion, and closing to reduce contact bounce, but the improvement is limited with significant increase of cost and size due to the inherent design structure of contactors: simultaneously switching of all three poles. Meanwhile, solid-state motor starters have shown great benefit with point-on-wave (POW) switch-on to minimize transient current and torque stress of induction machines during motor starting process. However, cost and size is a premium. A novel technology that employs asynchronous modular concept and POW switching has been developed. This improves motor starting and contactor switching performances in respect to switching transients and contact erosion while utilizing small size and low cost electromechanical contactors. These contactors are assembled into a flexible modular contactor assembly that allows each pole to be switched independently with precise POW switching control. This paper will focus on motor starting applications to achieve significant reduction of motor starting transients and contactor switching capability improvement. Asynchronous modular contactor prototypes with newly developed 24-VDC controlled contactors have been developed. The evaluation tests of the prototype proved significant transient current and torque stress reduction during motor starting.
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