×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

High Current Contactors Failing as Low Current Contact Closure

High Current Contactors Failing as Low Current Contact Closure

High Current Contactors Failing as Low Current Contact Closure

(OP)
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?

RE: High Current Contactors Failing as Low Current Contact Closure

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: High Current Contactors Failing as Low Current Contact Closure

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%20...
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: High Current Contactors Failing as Low Current Contact Closure

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

RE: High Current Contactors Failing as Low Current Contact Closure

>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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources