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 Post subject: EMD SW9 Contactor tip Issue
PostPosted: Wed Jun 24, 2020 2:02 am 

Joined: Tue Oct 30, 2012 11:22 pm
Posts: 60
Has anyone ever seen this before? These contact tips have an odd problem.

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 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Wed Jun 24, 2020 4:00 pm 

Joined: Wed Nov 29, 2017 2:32 pm
Posts: 42
This is an arcing issue that is caused by the contactors not touching the drum (the other side of what ever they touch if not a drum). It means they are not installed tight or the holders or spring tensioners are not tight. Essentially they are allowing small lighting bolts to be created between the contactors and the drum (arcing). It may also mean that the other side (the drum) is dirty, or clogged and or has some other issue that it is not conducting the electricity the correct way. Generally, this issue is simply poor contact and or not tight enough contact.

Hope this helps. If you want to talk this issue out a bit, feel free to call us.

Wasatch Railroad Contractors
Cheyenne, Wyoming

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Wed Jun 24, 2020 9:48 pm 

Joined: Fri Aug 20, 2010 8:25 pm
Posts: 334
Agree, electrical arcing is "eroding" the material each time the contacts are disengaged. Most likely cause is loss of strength in the springs (metal springs do loose strength over decades of repeated cycles). You need good strong springs to "snap" the contacts out of engagement as quickly as possible. Could be fixed by replacing the springs and resurfacing the contacts. Other possible cause is poor (or slipped) adjustment of the contactor mechanism.

Weak springs could also allow the "tips" to bounce along the surface of the mating contact surface, thus allowing arcs to erode the "tips". This usually looks like "fuzzy" contact surfaces with small fine pits. Your photos look like "great big craters" that are more likely from arcs when breaking the electrical connection.

Need to resurface/replace the "tips" and the mating conductor surfaces and check adjustment/action of the mechanism. Operate the mechanism under power (with safety glasses) and see if there is arcing all the time when the tips are in contact, or big arcs only when the tips are "removed" from contact. That will give you a clue to the root cause of the problem.

One other possible cause is the failure of "arc suppression" devices; capacitors, snubbers, diodes, varistors etc. These are used in some systems to suppress arcing. Most high voltage capacitors do have a limited lifetime, if any are in the circuit I would look at them carefully as a suspect.

This might help;

Good luck, Cheers, Kevin.

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Thu Jun 25, 2020 3:10 am 

Joined: Fri Oct 01, 2004 1:33 pm
Posts: 430
Location: Oroville, CA
I would contact (pun not intended) Craig labs and get some anti-arc lube from them. Designed to prevent arcing on contacts, should extend the life of them too. Stuffs not cheap, but then good stuff seldom is.

David Dewey
Hoping for the return to the American Rivers of the last overnight steamboat, Delta Queen!

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Thu Jun 25, 2020 11:49 am 

Joined: Sun Aug 22, 2004 11:06 am
Posts: 504
Location: NE PA
I saw this on a GP9 and when i went to the parts room there was a box of tips that looked like this, the owner said they have needed to be changed on a yearly basis, which I have never heard of. I performed a sequence check on the locomotive and found the power contactors opened every time the throttle was closed. Not proper, they should only open when the reverser is operated. I found a pair of wires on the isolation switch that had been reversed when the switch was changed, corrected the wires and now the same set of tips has lasted more then 10 years.

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Thu Jun 25, 2020 7:32 pm 

Joined: Sat Feb 05, 2005 1:05 am
Posts: 420
I like the way Mike thinks. Don't treat the symptom, solve the problem.
"If all the other SW9s don't have this problem, what is different on this one that is not the same as all the other SW9s."

I learned that way back home on Sesame Street.
"One of these things is not like the others, one of these things is not the same!"

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Fri Jun 26, 2020 9:22 am 
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Joined: Sat Jul 02, 2005 7:16 am
Posts: 1543
On most DC main generator locomotives of the 1940s and 1950s, there will be a large resistor wired in parallel with the main generator battery field. On the SW9, this is usually a 4-Ohm resistor.

It is very important that this resistor and its wiring should be in good condition, and that the resistor has continuity in the circuit. When the BF contactor is opened, the resistor dissipates the residual voltage in the battery field windings. If the resistor is open or not connected, there will be a high voltage spike leading to severe problems with arcing of the contacts.

By the way, HRA has a copy of the EMD guide for inspection of alloy contacts, if it is not posted to the technical library right now, it will be shortly.


Advice from the multitude costs nothing and is often worth just that. (EMD-1945)

 Post subject: Re: EMD SW9 Contactor tip Issue
PostPosted: Sat Jun 27, 2020 4:05 pm 

Joined: Sun Aug 22, 2004 10:18 pm
Posts: 101
Location: Philadelphia, PA
Does the locomotive make transition and is it sequencing properly when the PTR picks up. Also as Preston says the field decay resistors are very important especially when the unit is in parrallel and the throttle is closed and backwards transition takes place.

Also the governor overriding solenoid is in play during transition and wheel slip to drive the load regulator to minimum field to drop the load.


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