Fast Discharge Mechanism for Ring Motors in Gearless Draglines

In the case of high power applications, or where the machine ratings are in the MW range e.g. mining applications using a AC gearless dragline etc. the level of these currents is extremely high, of the order of 100´s of amps. Such high currents result in huge amount of energy stored in the inductor, which is difficult to dissipate quickly through a small resistance since the time constant given by L/R, for such systems would be too high. Also, if dealing with high power applications, the energy stored in such fields is so high, that dissipating it results in excessive heating ambient and consequently requires additional cooling for the complete system. If instead of dissipating this energy, it is efficiently stored, then in theory it can be used as a backup supply in case of a fault in the system. Traditionally circuit breakers are used to isolate such machines in cases of faults. This paper attempts to provide an alternate method for discharging the field circuit by eliminating the use of circuit breakers. The technique is solely based on high power solid state devices.