Engelbert Hetzmannseder

The influence of bounce parameters on the make erosion of silver/metal-oxide contact materials

Make erosion of contacts made from powder metallurgically produced Ag/CdO and three differently produced Ag/SnO/sub 2/ materials was investigated employing both a commercial contactor and a new bounce model switch. Additional tests were executed in a fixed gap arrangement in order to study separately the influence of the thermal stress due to the bounce arc only. The bounce model switch was developed to simulate contact bounces at well-defined and reproducible but independently variable values of bounce duration, bounce height, and contact force. The influence of the bounce parameters (bounce arc duration, bounce height and static contact force, current and magnetic induction) on the make only erosion was investigated by systematic variation of each single parameter value in the bounce model switch. The Ag/SnO/sub 2/ materials suffered either three times higher or about 50% lower erosion losses than powder metallurgically produced Ag/CdO, depending on the manufacturing process (internally oxidized, powder metallurgically produced) and/or the additives of the materials, and therefore on the structure of the materials rather than on the kind of the main metal oxide component (CdO or SnO/sub 2/. The different make erosion rates of the materials investigated have been correlated with the micro structure of their stressed surfaces. >

Asynchronous modular contactor for intelligent motor control applications

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.

Arc mobility on bouncing contacts

The arc mobility on eroded Ag/CdO and three different Ag/SnO/sub 2/-contacts during bounces simulated in a model switch was investigated with the aid of a two-dimensional optical arrangement. The high fictitious arc mobility observed on two of the Ag/SnO/sub 2/-materials investigated was caused by arc commutation inside the contact gap. Ag/CdO and another Ag/SnO/sub 2/-material showed both arc creeping and arc commutation-but only very close to the site of arc ignition. No continuous fast arc motion occurred in short bounce gaps (0.18 mm, corresponding to bounce values in modern contactors) whether or not a magnetic blast field has been applied. There was no correlation between the amount of contact erosion at make and the fictitious arc mobility observed on certain materials.

Electrical Arcing and Material Ignition Levels

Experimental data and analysis show the relationship between arcing watts, the time of the arc exposure and the degree to which common polymers experience damage. Arcing test apparatus for 125V DC and 48 volt DC are shown. Arc gap control allows 125-volts to mimic arcs in lower voltage systems. Arcing current waveforms depict the chaotic behavior of arcs and differences due to anode / cathode materials. Levels of degradation are defined and used for “Constant Damage Contours” for Vinyl, Carpeting and Sound insulation. The polymers were exposed to conditions of constant arcing watts for increasing times. Connecting the points of “first flame” establishes a “contour of constant damage”. The data indicates that a “3 second flame free” limit is between 50 and 100 watts. 4000 watts of arcing takes 1/4 to ½ second to cause burning. The influence of circuit resistance on the maximum arc power transfer is presented.

Make-and-break erosion of Ag/MeO contact materials

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/SnO/sub 2/ 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/SnO/sub 2/ 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 SnO/sub 2/) 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 are 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.