Optimum number of petals for a spiral TMF contact

The simple and easy to machine transverse-magnetic-field (TMF) contact design with a tapered cross-sectional profile toward the outer edge and a swastika-shaped slot design has been successfully used in millions of commercial vacuum interrupters throughout the world, for the last 40+ years. But is it the optimum design? For one example, is 4 the optimum number of petals? And is the answer dependent on the rated voltage or the interruption current? To answer those questions, a series of spiral TMF contacts with different numbers of petals, ranging from 2 to 7, but otherwise identical, were built into commercial vacuum interrupter (VI) envelopes and tested with synthetic testing at 38kV 40kA and direct testing at 17.5kV 63kA. Preliminary results showed that while the 5-petal pair of contacts had a slight edge over the 4-petal pair, interruption performances were very similar among 4 to 7 petals. Even the 2-petal pair managed to interrupt a couple of shots, although arc voltage traces showed evidence of lack of significant arc column motion, due to the large petal size relative to the diameter of the columnar arc ignited in between the contacts. A VI with a higher number of the petals showed more shots with the arc-voltage showing rapid arc motion. Post-mortem examinations, however, revealed a deeper melt depth and rougher surface deformation in contacts with the higher number of petals, due to the smaller cross-sectional size of the petal. This may indicate a higher contact surface temperature and more fluid molten metal flow at current zeros for contacts with a higher number of petals. Taking also into consideration electrical erosion rate and mechanical strength, it is preliminarily concluded that while 4-petal provides a very cost effective solution at the 12~17.5kV level, for higher voltage ratings such as 38kV, 5 appears to be the optimum number of petals.