Contactor contacts uneven wear

[noel0589]

Hi all,

I'm a recent graduate so I have a simple question. What should uneven wear/pitting on a contactor's contact surface face for wye delta starting tell me? It seems that only the upper portions have wear while the bottom 1/3 to 1/2 is unscathed. What should this tell me if anything at all. Is it normal? Thanks in advance for any response.

 

[mc5w]

Uneven contact wear can have several sources:

1. Motors never ever have perfect current balance because of mechanical imperfections and supple voltage imperfections. For that matter, metal corrosion in s fuse clip burned up one of my customer's motors and after a rewind it would not come up to speed.

2. One contact will close first and open last because of mechanical imperfections. This will have the least wear.

3. When doing a wye-delta start, the delta contactor will have one contact more in phase depending on time delay.

Mike Cole, mc5w@earthlink.net

 

[raisinbran]

Most contactors (and even control relays and switches) employ "contact wipe". This is when the contacts roll against one another both during the make and break operations. This wiping effect is more pronounced on the older vertical mount contactors. If you can operate the contactor slowly by hand (power removed of course), you can usually see the wiping action. The idea on the break cycle is to move the point (line) of contact away from the contactor, and when the contacts finally part, the direction of arc travel is already established. This also keeps 1/2 of the contact looking new for minimum contact resistance while the contacts are closed. In lower power applications, this wiping action helps to penetrate any oxides that may have built up.