I just finished cleaning the contacts on a 180A motor starter. It's running a 100HP 600V 3ph motor on a granulator (grinds up plastic... sporadic very heavy loading, then load reduces until it gets slammed again). The starter is a pretty good one - an Allen Bradley bulletin 100 B180 - replacement cost $1700. A new set of contacts for it is $400, so I just filed them down and re-used them. There was still enough silver left on them.
The question is what might cause the contacts to be heavily pitted in just 2 or 3 years. I've got contactors on machines, here, that are 40+ years old and still working fine. I've got one particular 37 year old AB contactor on an air compressor that I _know_ has cycled at least 200,000 times, and it still works great. It looks like a reject prop from a low-budget Frankenstein movie, but it still works. This newish contactor on the granulator is rated for 180A (resistive) and it's only running 90A (inductive) so I'm not sure why it's getting hit so hard. Is there anything I can do about it? The contactor can't be taken apart... at least, not that I was able to find. You can remove the contacts and the coil, but the springs and the rest of it are all glued/snapped together, so I can't do much about alignment etc.
Any thoughts?

Some thoughts:

1 – Contactors in an area of vibration may chatter. This causes pitting of the contacts, reduction in effective area, increase in current density and temperature.

2 – Low voltage can cause the contacts to chatter... same result as above.

3 – External contamination can accelerate contact degradation.

4 – Improper adjustement – as you know alignment, spring tension etc are important.

5 – The contactor may not be rated for the job. You said "newish contactor on the granulator is rated for 180A (resistive) and it's only running 90A (inductive)". The 180A resistive caught my attention because as you know a motor is not purely resistive. The inductive portion of the current causes arcing during circuit interruption and the contactor needs to be rated for that. But it does look like this particular contactor is rated for motor duty up to 90 KW (around 90HP) at 380/415 volt and higher horsepower at higher voltage based on the info here:
http://www.ab.com/en/epub/catalogs/12768/229240/229254/229461/229477/tab5.html
Why they called it "resistive" if it's a motor rated contactor doesn't make sense to me.

You might try this question on the motor forum at www.eng-tips.com

quote:

Originally posted by electricpete:
Some thoughts:

1 – Contactors in an area of vibration may chatter. This causes pitting of the contacts, reduction in effective area, increase in current density and temperature.

That could be it right there. This is definitely a high-vibration area. The floor shakes when you drop a roll of scrap into this granulator, and you can feel it hit your chest from 12 feet away. Maybe I should mount the contactor on vibration isolators.

quote:

2 – Low voltage can cause the contacts to chatter... same result as above.

Another possibility, I suppose... I haven't tested that, but the coil is rated, if I remember correctly, for anything from 90V to 130V for a nominal 120V coil, so the chances of it dropping below its holding voltage seems unlikely, even with the huge draw of the motor, but it is something that I will check with a minimum-holding meter.

quote:

3 – External contamination can accelerate contact degradation.

Yet another possibility. The area is heavily (!) contaminated with 'angel-hair' from grinding up plastic, but the electrical box is dust-proof and I see no evidence of infiltration, and didn't notice any when I had the contactor as disassembled as I could.

quote:

4 – Improper adjustement – as you know alignment, spring tension etc are important.

I wish I could open it up, but neither I nor the electrician at my supply store could figure out how, and the original instructions didn't mention a way, either.

quote:

5 – The contactor may not be rated for the job. You said "newish contactor on the granulator is rated for 180A (resistive) and it's only running 90A (inductive)". The 180A resistive caught my attention because as you know a motor is not purely resistive. The inductive portion of the current causes arcing during circuit interruption and the contactor needs to be rated for that. But it does look like this particular contactor is rated for motor duty up to 90 KW (around 90HP) at 380/415 volt and higher horsepower at higher voltage based on the info here:
http://www.ab.com/en/epub/catalogs/12768/229240/229254/229461/229477/tab5.html
Why they called it "resistive" if it's a motor rated contactor doesn't make sense to me.

That page that you reference rates the contactor at 150HP at 575 volts (the nominal voltage that it runs at) so 100HP shouldn't be a problem for it. As to why they rate it for resistive loads... I can only guess that they like to put a bigger number on it when they can, so in the interest in calling it a 180A unit instead of 100A they rate it for resistive loads. Helps to justify the exorbitant price, maybe.

quote:

You might try this question on the motor forum at www.eng-tips.com

I'll try that... thanks!

Sounds like a lot of possibilities. I saw your post show up at eng-tips. Those guys definitely can give you a good answer on whether that contactor is applied correctly.

I said "90 KW (around 90HP)" implying a 1:1 ratio, which is not right. Just to clarify, 1kw is around 1.34 horsepower.