Erosion and electrode energy distribution in switches with silver-cadmium-oxide contacts
The cathode and the anode fall of the DC arcs are measured by fast oscilloscope for Ag-CdO contacts over a range of gap-lengths from 0.05 mm to 1 mm, and currents of 4-10 Amps at atmospheric pressure, with a known electrode closing speed, using the Moving Electrode Method. It was observed that the anode fall can occur in a few places within the arc voltage waveform, and is dependent of the electrodes' surface condition. Both cathode and anode falls increase with gap-length and decrease with current. It was found that when arc length is shorter than electrode separation, discontinuity within the arc voltage waveform during closure is caused, in many cases, by vapourisation of the first point of contact or by a high electric field set up between the two electrodes. These discontinuities are named as Voltage Step Phenomena. These voltage steps are related to the cathode and anode fall voltages, and their regular occurrences are a function of surface roughness. The fluctuations in the arc voltage waveform are thought to originate mainly from the cathode. A technique has been developed to measure the temperature of the electrodes accurately by using a T-type thermocouple, 0.075 mm diameter, in conjunction with a DC amplifier with a gain of 247. The thermocouple is placed as close as possible under the electrode surface (200 µm). This enables the temperature of the contact to be measured, after breaking contact, for an arc-duration even as short as 1ms. The time-constant of the probe (contact containing the thermocouple) is measured to be approximately 18ms. With this technique the temperature of the electrodes are measured for currents and gap-lengths ranging 4-10 Amps and 0.05-1 mm respectively. The effect of contacts being new and change of polarity have been investigated. From these results it is concluded that the co-existence of layers of foreign material on one, or both, surfaces causes the temperature of the electrodes to be high for the first 50-100 operations, before reaching to steady-state conditions. Change of polarity suggests that the moving electrode, either anode or cathode, due to the effect of air movement over its surface, is cooled relative to when fixed. The power transfer to the electrodes is calculated for various currents and gap-length using thermal analogue formulae derived from the transient response of an RC circuit to a d.c. pulse. The results show that below 0.2 mm the sum of the anode and cathode power is approximately equal to the arc power, and hence losses are negligible. At around 0.125 mm, for currents of about 6A and 12A, they both receive an equal amount of power from the arc. This has been related to the thermal energy of the electrons being negligible, at such separation, at the anode end of the plasma column. The power balance equations are solved to calculate the positive ion current to the cathode, and the thermal energy of the electrons in the plasma column, under various test conditions. In the investigation of erosion, the S.E.M. studies show that most of the power dissipated on the surface of the electrodes is used in melting and evaporation. The x-ray analysis shows that the melted metal is composed mostly of Silver. To operate the test rig and collect the generated data automatically, a computerised test system, with a mini data acquisition system, has been designed and constructed here.