High Current Contactors Failing as Low Current Contact Closure 1

[JohnMcNutt]

This came to the forefront of my mind again today because of a recent failure I am having. This is not the only time I have had something like this happen.

I recently installed a VFD in a machine that is simply on or off. For the start/stop signal I simply used a set of contacts from the original contactor that started the motor DOL so that the VFD starts and stops the same way the control circutry is expecting. The current across it is simply the control closure to run the VFD. I actually ended up replacing the contactor with a brand new one that was 40a, 240v due to a misdiagnois I made in a different failure to start, so the present contacts have never seen actual motor current. Now I estimate it has seen 50-100 cycles at the most. With the machine running I discovered that these contacts were not passing enough current to start the VFD and that by shorting across them it started right up. Having done that I pulled the coil current and reenergized it and I found that I would see the VFD run about 1 time out of 10. Once I got a good closure I left it connected and it ran all day just fine.

I know the right solution is to replace the big contactor with a simple low current relay with the right specs but I was wondering if anyone can explain why this happens?

 

[xnuke]

A high current will destroy the less-conductive oxide layer that forms on certain types of metals used in contacts. A low current allows the oxide layer to continue to develop until the contacts can't conduct anymore. This is why gold contacts are used for many low current relays. Gold doesn't have the oxide problem.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Skogsgurra]

Agree with xnuke.

There may also be dust (not only oxides) that makes dry circuit applications difficult.

The Appendix 3 in this report

http://www.gke.org/rapporter/files/SpecialPaper Mill report may 2013.pdf

may be overkill, but there are a few pictures that show part of the problem. There's also a discussion on twin contacts and other such tings.

Please note that the report is "de-identified". All names and places are fake.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Many small relays are designed to close and open with some sort of a wiping action to keep the points clean. Not so on most contactors. Cheaper than gold for frequently operated relays. After a long time idle, these relays may have to be cycled a number of times to remove accumulated oxides before they again become reliable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

>that makes dry circuit applications difficult.

Hmmm, low signal currents passing or not through high current contacts.

That warrants a term like 'damp' circuit; not wet, not dry, just damp.