Why is looping serially all the poles on a DC contactor that switches DC useful in increasing the amperage ratings in DC-1 utilization category?

 

My guess would be that you are improving the interrupting rating by lengthening the arc path. But it's just a guess. :^)

Regards
cww

 

Hi, in response to your question regarding series connection of poles of a multi-pole contactor it is important to understand the concept of switching DC current as opposed to AC.

In an AC application the purpose of opening the contacts is obviously to isolate the connected load from the mains supply, at the instant of opening the contacts if sufficient current is flowing an arc will form between the opening contacts, this arc may be worsened when switching and inductive load. As the supply is AC the arc will normally extinguish as the AC cycle passes through 0.

In the case of a DC supply there is no current zero as the current is always in one direction so no natural arc extinction properties exist. By connecting multiple poles in series this effectively increase the available contact gap between the supply an load and so improves the ability of the contactor to interrupt the load current flowing through the formed arc, in most cases there is no improvement in its current handling ability but it generally will help improves contact life.

Also in most cases the manufactures recommend connecting two pole in series in the positive lead with the third pole interrupting the negative (in the case of a 3 pole contactor).

Hope this helps.

 

Responding to Anonymous' Sep 5, query and Curt Wollet's Sep 7 comment on a "double-break" application... directionally Curt is right.

The 'ROT' is that current increase could be as much as 2 to 3 times the contact's nominal DC rating. But, from a practicality standpoint it is
difficult to achieve. There are many factors to consider. A very important one is, "Will the contacts open at the same instant?" Another is, "What is the circuit's L/R ratio?"

Thus, unless you have the relay manufacturer's concurrence, or tested it yourself, then I would not recommend it!

BTW, of the many manufacturers I have used, only one would guarantee their relay's performance in a "double-break" application!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

They will (almost) never open at the same instant. And if they did, next time is a crap shoot. This from hours in a darkened lab with a scope and hood. But you do get the advantage of the gap increasing x times as much in the same time. When you do failure analysis, you get to know a lot about relays. While simple in principle, they fail in incredibly diverse and complex ways. Millions of man-hours have been expended on relay reliability and failure modes and mechanisms. It is almost always worthwhile designing them out of equipment. But they are almost unavoidable with the current state of automation.

Regards
cww