Contactor Uneven Wear 1

[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.

 

[jraef]

It depends upon the contactor design, and in the case of a Y-D starter, which contactor it is.

Some contactors are designed to do just that. If the relationship between the stationary and moveable contacts resemples a wedge, they are supposed to hit on one surface first, then "wipe" across to full contact. That first surface to hit will show more pitting that the rest, but it usually is not a problem unless the contacts show other signs of excessive heat, such as carbon buildup or severe discoloration.

In an open transition Y-D starter too, the "Run / Delta" contactor sometimes is subject to high voltage and current spikes because it has to contend with regeneration from the motor being out of phase with the line power when it closes. That can cause a lot of extra arcing in the contacts. "Rolling" the motor leads connected to that contactor can often help with that. It changes the phase shift relationship during transition.


"Venditori de oleum-vipera non vigere excordis populi"

 

[noel0589]

Yes, my understanding is that during y-d starting there is always a current spike to contend with. Would you or someone please explain what is meant by "rolling" the motor leads? Thanks a lot,

Noel

 

[jraef]

If you want to give your brain some serious exercise, take a look at this paper. It explains the basics of the transition spike problem, but in the end it is leaning towards a solution of using a Closed Transition Y-D starter, which is an expensive solution compared tousing a solid state starter now.

http://www.mastercontrols.com/EngInfo/Articles/Wooddall/TA_RWood.htm

This same company used to discuss the cheaper solution of rolling the conductors, and they sold a cheap little device called a "Leading Phase Indicator" to help you do it. It appears that since they have started selling solid state starters they no longer support providing a way to help mitigate the transition spike on Open transition Y-D any more.

Rolling means taking the three leads and shifting them over one terminal, yet keeping the same rotational relationship. For example, move A to B, B to C, and C to A. It will still rotate A-B-C. Here is a link to Franklin Electric's instruction sheet on doing this. They promote it as a way to try to match natural motor winding unbalances to voltage unbalances in order to have a more balanced current, but the principal is the same.

http://www.franklin-electric.com/Manual/pdf/AIM_33.pdf

Rolling changes the relationship to the inherent phase shift that happens in Y-D transition. So if your voltage was lagging the current, which may have been contributing to your spike, you can make it lead the current during transition. You still get a spike, and if you read that paper it will tell you that it can be as high as roughly 3x the LRC, but you may minimize it to a lower value. Unfortunately they stopped publishing the older paper that gave the calculations for the lower value, but it was better.

Note that this is not 3x FLC, it is 3x LRC, which is usually already 6x FLC! So that means that you can have a transition spike of up to 1800% FLC when switching from Y to Delta. That can give the contacts (and motor windings for that matter) a really hard time.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

Thanks for the tips, jraef. This also means that a reversing wye-delta starter could be a sticky wicket.

Mike Cole, mc5w@earthlink.net

 

[busbar]

 
On the seemingly deteriorating contactor, it may be helpful to do AC millivolt-drop tests like that described in

http://ecmweb.com/mag/electric_testing_contact_quality/index.html

[Aside — years ago when silver contacts were new in motor control—beginning to replace copper contacts, Allen-Bradley would instruct users NOT to file silver, and that “mountains” on one contact face would mesh with the “valleys’ opposite them. I believe they also noted that silver contacts were usable as long as they had at least 10% silver remaining on pole faces.]

The motor-lead rolling/matching trick has been around for quite awhile, such as in §10 of

http://www.pge.com/docs/pdfs/custom...ces/greenbook/service_requirements/054619.pdf

It’s important to note that rolling is intended for and most effective on single transformer-bank/single-motor installations, and it effectiveness degrades (and is sort of ‘fluid’ or dynamic) for multi-motor applications.