calculation method is documented elsewhere [5].
Figure 7 shows the calculated current criterions
versus the main current. The current criterion means
instantaneous current when the temperature at the
cathode spot lowering under the melting point of the
contact material before the current zero. The current
criterions are supposed to be related to the
chopping-current levels. From Fig. 7, the general
tendency that the chopping current level decreases
versus the main current could be obtained. However, the
current levels in the inverter circuit are around 20%
lower than those in the AC circuit. This is because of
the larger current decline (-dIldt) before the current zero.
The high chopping current in the inverter circuit could
not be explained by this thermal model.