Influence of contact dynamics and point-on-wave switching on arc and contact phenomena in medium duty switches
Investigations are made into the effect of contact opening velocity on arc energy and contact erosion in both ac and dc medium duty switches. In addition investigations are carried out into the effect of point-on-wave switching on the erosion of contacts in ac circuits. Theoretical and experimental research is also made into the modelling of the impact event during contact closure. The impact model is used to develop a mathematical model of the dynamics of a contact mechanism which is used in the investigations. The phenomena of pre-impact arcing during contact closure is also studied. The investigations into the effect of contact opening velocity and point-on-wave switching on arc energy and erosion were carried out using a computer controlled experimental apparatus which enables the control of contact opening velocities in the range of medium duty switches (up to 1 m/s). The investigations into the impact and bounce phenomena were made using an existing experimental apparatus. In dc circuits, using currents up to 10 A, the molten bridge length was found to be unaffected by opening velocity indicating that the bridge remains in steady state. For dc currents below 7 A a stepped increase of the arc voltage was observed. The presence of a fourth step and a fifth step are reported in this thesis for the first time. These steps are explained to be due to the quantized nature of the electron energy in the arc which manifests itself at low current. The arc voltage in dc circuits is found to become either higher or lower than its steady state value when the contact velocity increases which was explained to a direct consequence of the energy conservation equation. Arc energy and erosion for a given arc current are found to be inversely proportional to contact opening velocity. Material transfer due to arc erosion was shown to change direction from cathodic to anodic when the circuit current increases above 7A. In ac circuits the arc is observed to extinguish near current zero at all velocities which results in the increase of arc energy and length with opening velocity. This is shown to be predictable from quasi-static arc characteristics. Opening the contacts further from the start of the ac current cycle, i.e. at large point-on-waves is shown to increase the erosion from the anode resulting in the change of the direction of material transfer to become from anode to cathode at a point-on-wave which decreases with the increase of the electrical current. This is explained to be due to the increase of anodic erosion with the increase of arc current. Increasing opening velocity is shown to increase the erosion from anode to cathode for given point-on-wave and current (below 10A)which is due to the increase of the arc length and the anode fall voltage. For currents higher than 10 A erosion due to melting increases significantly with increasing opening velocity leading to material loss from both contacts. Erosion due to random switching of rectified ac current is shown to correlated well with that of fixed point-on-wave switching. It is shown in this thesis that it is possible to model the impact event by a single process of plastic deformation which enables the calculation of impact force and duration. To reduce contact bounce the contact closing velocity has to be reduced. However, the contact velocity has to be high enough to reduce the damage caused by the pre-impact arc whose duration is shown to be inversely proportional to closing velocity. This arc is shown to be caused by field emission breakdown.