Interesting contactor use...

[olly_k]

Ok sorry to be vague with the following question, as I was not present during the removal of this device, but was an ear shot from the discussion. So, two colleagues were talking about a 3 phase contactor that had been removed from a large industrial oven (paint), and were not sure about the way it was being used.
so they had a standard 3 phase contactor, but instead of being used to switch three separate phases, a single phase was brought in to L1, looped out of t1 in to t2, out l2 to l3 and then out of t3, so effectively three switched contacts in series but obviously switched from the same coil and armature.

My immediate response (after butting in) was to suggest switch redundancy, i.e. if any contacts were at risk of welding shut there would be two more switches that would remain able to open. It would seem a bit odd not to have separate contactor but I suppose this might not actually matter?
My other (weaker) idea was that perhaps by sharing the spark when removing power from an inductive load would help prolong the life span of the contacts, assuming they all left at intervals that would share the burden of spark, would that actually work??

Either way I have not seen this before, although have not had a lot of experience with contactors and heavy duty industrial machines so it was the best guess given the little information he could give at hand!

Any one care to comment?

 

[Tony Stewart]

3 Sequential contactors may give extra longevity for arc breaking, but not much if all the same, maybe triple the life, although I have not seen.

Modern industrial contactors use TRIAC the EM contacts to turn on and EM off then Triac off on next zero crossing of current to eliminate arcing. But high current apps use very long throw solenoid breaking contactors to extinguish the arc or enclosed in SF6.

 

[tcmtech]

Years ago it was not uncommon to do that when dealing with high current resistive loads that have to cycle on and off a lot.

Part of it was to reduce contact welding issues and part of it was for additional arc breaking space being that some high current resistive heating elements can produce substantial inductive pulses in their power systems despite being resistive loads themselves when the circuit is broken at some point other than at or near a zero crossing event.

 

[Tony Stewart]

Yes also considering the cold surge currents heaters with PTC effects and inductive coils of non-bifilar wound heaters which multiplies contact failure rate, may be why that contactor were being replaced.

 

[olly_k]

ok thanks that is very interesting