resistor banks for traction motor control 1

[berkshire]

I have a job repairing and maintaining resistor packs for traction motor controls. Trolley cars. Is this an apropriate forum for asking questions about this as they arise from time to time.
One question I have now, is that the original maker of some resistors used a copper plated steel attachment terminal on a chrome vanadium resistor band.
The operator uses these street cars near the coast, the salt air eats the copper plating away allowing the steel to rust. This builds between the terminal and the band popping off the spotwelds attaching the terminal to the resistor. The resistance value rises to the point where the band is useless.
I have been replacing some of these terminals with new ones fabricated from 316L stainless steel, the operator of the trolley seems happy with this.
However I now have a beancounter involved with this operation who would like me to use 304 stainless instead.
My question is from an electrical point of view, is there a significant difference between 316L and 304 As a terminal end? The only detriment to the SS terminals I have noted, is that if the operator does not tighten them well, they get some shrinkage and burning at the connection
however I have also noted this with the standard steel connectors.
B.E.

 

[itsmoked]

Fine place to ask this.

I don't have an answer but others will.

 

[UKpete]

From matweb the electrical resistivity of the materials is as follows (all in [Ω]-m x 10^-8, at 20°C):
mild steel 15
316L stainless 74
304 stainless 72

So electrically both grades of stainless are much the same, but notice that (for the same section) stainless has five times the resistance of mild steel, so the stainless terminals will run hotter. If it is getting too hot it could cause creep - resulting in progressive loosening of the terminal; or even melting.

 

[itsmoked]

Any way you could use steel and actually use small fans on the terminals?

 

[berkshire]

Thank you gentlemen,
It appears that there is very little difference between the two grades of stainless. The surface area of the connecting lug can handle the increase in resistivity.
It is bolted to a 35x5mm copper busbar and this particular resistor bank is the first one in the frame against the cooling fan.
B.E.

 

[jraef]

Hmmm... I'd be willing to bet you're working on Cable Cars in San Francisco...

316L or even plain 316 is the proper choice for salt air environs however. Tell the bean counter that if he wants to get cheap on you, maybe he could volunteer to replace them at his own expense when they corrode as well!

Grade 316 is the standard molybdenum-bearing grade, second in importance to 304 amongst the austenitic stainless steels. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments.

That is from

http://www.azom.com/details.asp?ArticleID=863

. If you change the "ArticleID=" at the end of that URL to 965 you will get a similar report on 304, stating that it is suitable for food processing etc., but suggests 316 for marine environments.

Gotta love bean counters doing engineering work!

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[berkshire]

Yes Jraef,
As you correctly deduce the 304 is cheaper than the 316.Which is why the accountant would like me to use it.
The heartburn I have with the 304 is that it tends to corrode around the spotwelds holding the VanChrome resistor bands to the attachment lugs.
A little aside. The cable cars in San Francisco do not have motors in them. They grip on a moving cable running under the street.
The trolley cars I do work for are 500 miles south of there.
B.E.

 

[ccjersey]

That fellow is counting mighty small beans! It's not like you are recovering the whole car in stainless. I think a little partial budget with material and labor costs and compared to replacement cost (probably unavailable anyway)would explain your position well enough.

 

[Warpspeed]

This is not really my area of expertise, but for corrosion resistance I believe 316 would definitely be the grade to go for. It is used extensively in the medical, chemical, and food industries.

304 stainless only has an advantage at elevated temperatures. 304's main application is for exhaust systems, where it resists cracking better than some other grades of stainless. For example, it is used for light weight tubular exhaust manifolds on piston aircraft engines.

 

[jraef]

Doh! Cable cars, I should have known better...



Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[berkshire]

Ok guys,
This is one time when the beancounter wins.
As Uk pete points out, there is not a significant difference between the resistance of the two kinds of stainless, with the 304 being slightly less.
Whilst there may be some corrosion on the SS attachments, the corrosion is considerably less than the copperplated mild steel used before and the temperature rise is acceptable.
The other consideration is, that I had to special order the 316L. We have the 304 in stock and I am being encouraged to use drops ( offcuts)from other jobs which the company has already paid for once.
B.E.