DC Generator Contactors 1

[doubledecker751]

We have a DC generator with potentiometer and when ingaged it has started to weld the contacts together. We have tried things like bigger springs, milling the contact surfaces etc. and it continues. This generator runs a 150-200 hp motor that runs from 400 to 1200 rpm. It has a 200 hp three phase ac motor to run the dc generator. Can someone give us an idea of what we are missing or need to look for to identify why the contacts are welding together?

 

[jraef]

If you are using an off-the-shelf contactor, is it rated for the (unstated) DC voltage you are using?

You cannot take a contactor designed for AC and use it for DC, there are seriously different needs in the design of the contact assemblies between the two.

If you did not know this, then you were not qualified to be doing this work and most likely there are other very serious safety concerns involved. I think that you need an experienced EE to help you before someone gets hurt.

"Will work for (the memory of) salami"

 

[Skogsgurra]

You are somewhat harsh there, Jeff. Rightfully. People approach tasks they do not know much about and then "funny" things happen. This is by no means the worst thing that could happen.

I see this everywhere. AC contactors being used in DC circuits. People simply don't know about this nowadays. And, worse, guys like ABB and Siemens have salespersons that don't have a clue either.

Electricians can be fully qualified (certified by the National Electric Safety Board in Sweden, which I know best) and still not be aware of this. One case: Four Diesel-electric lokomotives with DC generators for traction motors were recently "modernized", mostly because it was difficult (read expensive) to find the correct contactors. Instead, they used standard AC contactors. Much cheaper - but the molten iron that the lokomotives were supposed to bring from blast furnace to LD converter didn't get there. Not so cheap at the end of the day.

This happened in one of the largest steel works over here and the electrical department wasn't understaffed or lacking education. It is just how things are. We experienced engineers find it difficult to believe, but it happens all the time. But good for me - panic pays!

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

ABB's R-Line range of DC contactors is/was one of the more competitively priced types available. Similar ranges are available from Siemens and possibly from Schneider if they still make them.

 

[rasevskii]

@doubledecker:

This sounds like a Ward-Leonard system since you mention a MG set. Therefore likely an older system. Why are you replacing contacters at all? You must use DC contactors as stated above by others. Was there always a problem or is this something new?
More background info is needed. What is the application?

If you are not familier ith DC systems, then get someone else on the site who is, such as a service company or the OEM if they still exist. DC arcs are much worse than AC arcs at the same voltage level.

rasevskii

 

[doubledecker751]

Thank you for the litany of your Engineering background, but I am not looking for words to praise your knowledge in anyother way than to find a solution to our problem. The "contact" I am speaking of is a "DC" contact and we seem to be getting current change that is welding the contact together causing an "unsafe" situation because the operator or electrician has to pass by rotating steel to approach the main power source. So I am looking for ideas that can lead to finding the source of the fluxion. If you may have a solution, I would like to converse with you, if not, thanks anyway! We have a resistor bank that could be the problem and we are presently checking that out, but if someone has an oppinion, idea, method or solution I would really like to intertain it. Thank You in advance.

 

[rasevskii]

@doubledecker:

Please dont be offended by the discussions. We need some kind of circuit diagram or sketch to see where the problem may be.Is this possibly a drum-type control switch rather than DC contactors? Was there always a problem or is this something new? Background info needed. Is it a very old system where there may be insulation failure or earth fault somewhere.

Did you "inherit" this system from someone else? Was something recently replaced with something else that is unsuitable?

Be an electrical detective and investigate who did what in he past...

rasevskii

 

[ScottyUK]

If it is indeed a DC contact then is it well-matched to the current it is breaking? Some magnetic blow-out designs are optimised for certain current ranges and may not be as effective outside this range. Grossly over-sized contactors may struggle to break small highly inductive currents such as the machine field resulting in pole face damage. Once the pole face is disrupted then welding can occur if localised heating occurs due to current crowding.

And generally you will find that being friendly works best here - you have the benefit of having seen this installation, while we only have whatever you bother to tell us about it.

 

[jraef]

And we still do not know what voltage we are discussing here. 24VDC? 240VDC? 500VDC? 5000VDC?



"Will work for (the memory of) salami"

 

[waross]

Many of us have seen contacts weld together. There are a number of reasons. The most common is misapplication. We don't know if you are breaking load current (Maybe not a good idea on an MG driven motor), or control circuit current. Some MG driven motors will stop rapidly if the field is removed from the generator. If you are breaking the generator field current you may have a blown diode.
Cool the attitude. I can't see anything in the posts that warrants the testiness.

Bill
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"Why not the best?"
Jimmy Carter