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Chief Flunky
Field Service Engineer
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2,078 Posts
Discussion Starter · #1 ·
Ok the situation is it’s a plant with a couple refrigeration techs on staff located in another stare...4 hour road trip each way. They called reporting “motor undercurrent trip” on a soft started motor. Ok so this sounds like just adjusting the undercurrent trip which we could do over the phone. They insisted on the road trip.

Got there. Starter panel was wicked hot and so was motor. Multiple overload trips. Undercurrent is OFF. So where is “undercurrent”? Well the control panel on the chiller only looks at current feedback so it rightly reported undercurrent once the overload function tripped.

Oh yeah we forgot to say we cranked up the controls to push production. Looking like a wasted trip here.

Oh and by the way can you test a couple fan motors on the radiators that dump all the heat, half their cooling capacity?!? Well one “runs then trips”. The other “trips out instantly”. Meggered both. “Runs then trips” was obviously burned up so it tripped in a few seconds. “Instantly trips” was a good motor (even did a PdMA test because I had one and to save time on what might be an obscure issue. Turns out it had a jammed overload...it never reset no matter what it felt like. It also NEVER closed the contactor but the PLC just looks at an aux contact for feedback. Standing by it, it was very obvious. Took 5 minutes to pop out the heater and fix the jammed trigger.

So...big thing here is although PLCs can be good at indicating there is SOME problem basic troubleshooting is still needed. Each of these is easily diagnosed if you treat it like a non-PLC system.

So the road trip was probably worth it for both of us.
 

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Bilge Rat
motors and controls.........
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7,948 Posts
One of the best troubleshooting skills is to figure out how something works. Also, always be ready to look at things that 'never fail'......like an O/L. Just because the computer says this, this or this doesn't mean that's what's actually happening.

I was called to a rock crushing plant a few years ago for a 10HP conveyer motor controlled by an MCC that blew the fuses on the bucket.

I got out my trusty megger and yes, it failed the test. The circuit went underground to a can with lug blocks but of course, nothing was labeled. All the motors in the can were started across-the-lines and nothing was energized so I found which block it was with the merger. I took the tires out from the lug block and was surprised that the fault was toward the MCC.

I figured the underground wire had a fault but decided to pull the wire off the bottom of the starter. It megged onto the can. I reconnected the lug block and it still megged ok.

So I megged the starter lugs. It failed.

Basic Square D size 1 starter with heater type O/Ls.

Pulled the heaters and the bottom lugs still failed. Pulled the starter (On Square Ds, you have to pull the starter in order to remove the O/L block; the screws are in the back, not the front.......grrrrr.........).

I couldn't see anything wrong with the O/L block. Replaced starter, passed the megger test and it ran fine.

I've never seen an O/L block short to ground before. After it was out, I set it on a steel plate and it failed the megger test.
 

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Registered
Power distribution and controls
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546 Posts
I have had a plethora of problems with Triac contacts. I typically use relay contacts and some of the issues go away. Most of the time as you so elegantly stated the problem is not in the PLC.

Good read thanks for sharing
 

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Chief Flunky
Field Service Engineer
Joined
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2,078 Posts
Discussion Starter · #5 ·
I have had a plethora of problems with Triac contacts. I typically use relay contacts and some of the issues go away. Most of the time as you so elegantly stated the problem is not in the PLC.

Good read thanks for sharing
Triacs don’t survive inductive kick very well. Those cheap little surge protectors on the coils will stop this from happening. No inductive loads are fine.

The advantage of solid state is no moving parts so it doesn’t wear out. Instead of say a million cycles or less, you can do as many as you want. Great for twitchy operators. The downside is it’s electronic so you have to protect it from surges.
 

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Chief Flunky
Field Service Engineer
Joined
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2,078 Posts
Discussion Starter · #6 ·
One of the best troubleshooting skills is to figure out how something works. Also, always be ready to look at things that 'never fail'......like an O/L. Just because the computer says this, this or this doesn't mean that's what's actually happening.

I was called to a rock crushing plant a few years ago for a 10HP conveyer motor controlled by an MCC that blew the fuses on the bucket.

I got out my trusty megger and yes, it failed the test. The circuit went underground to a can with lug blocks but of course, nothing was labeled. All the motors in the can were started across-the-lines and nothing was energized so I found which block it was with the merger. I took the tires out from the lug block and was surprised that the fault was toward the MCC.

I figured the underground wire had a fault but decided to pull the wire off the bottom of the starter. It megged onto the can. I reconnected the lug block and it still megged ok.

So I megged the starter lugs. It failed.

Basic Square D size 1 starter with heater type O/Ls.

Pulled the heaters and the bottom lugs still failed. Pulled the starter (On Square Ds, you have to pull the starter in order to remove the O/L block; the screws are in the back, not the front.......grrrrr.........).

I couldn't see anything wrong with the O/L block. Replaced starter, passed the megger test and it ran fine.

I've never seen an O/L block short to ground before. After it was out, I set it on a steel plate and it failed the megger test.
I’ve seen breakers do this. Once it arcs and sprays metal on the insulator or out the back, it’s done. Time to replace.
 

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Registered
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66 Posts
Had one of these today. A recycling facility called. “The main cardboard baler is down, HMI says there is an E-stop” I’m like did you check the the E-Stops? Yup, everything is fine. Did you check ALL of the E-stops? Yes, we opened everyone and checked the contacts with a ohm meter. We need you to come out, there’s something else wrong.

I show up, E-stop circuit is open. I start tracing it out, there’s something on the other end of this conduit that’s not closed, do you have another button over there? Walk under the conveyor, twist the knob... Snap! And the error clears.

Then of course... We checked that one! It was working before! Sure, sure, could be, I left them a spare button.
 

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Registered
industrial E,I&C
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3,339 Posts
It always amazes me when people are un-willing to follow the meter and have already ruled out a problem with out testing.
 

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Tool Fetish
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813 Posts
I went to a marine fuel tank fabricator last week. Foot pedal on 1950s Cincinnati 50 ton break wasn't working. All were positive it was a problem with the foot pedal. OK, opened up the foot pedal; all OK. Did anybody body touch anything?? NO, just stopped working while we were using it. Waited for 45 minutes while they cleared away a pile of new and scrap aluminum plates so I could lay in the oil on the floor and see in the starter panel. All wires oil soaked and number tags laying on floor of panel. All fuses OK. Jumped out motors one at a time, all OK. No power coming back from operator panel. Traced power wire back from key switch. Hey, what is this key switch for? I don't know, we never touch it. Turned key switch and AHHH works like new. Oh, I bumped the key switch while setting table measurements this morning but I put it right back where it was.
 

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Band Member
DIYer Extrodinaire
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7,475 Posts
Back when I was automating water/sewage plants, I'd get called out because "our RTU" wasn't working. (Remote terminal unit, very much like a PLC)

Get there, they'd show me motor doesn't start .... SEE !!!

I'd short out the Digital Output, still wouldn't start. That's all the RTU would do, close that contact, so no way was it the problem. Then explain to the plant electrician how to troubleshoot HIS controls.

Oh well, they paid me well :p
 

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Chief Flunky
Field Service Engineer
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2,078 Posts
Discussion Starter · #13 ·
Back when I was automating water/sewage plants, I'd get called out because "our RTU" wasn't working. (Remote terminal unit, very much like a PLC)

Get there, they'd show me motor doesn't start .... SEE !!!

I'd short out the Digital Output, still wouldn't start. That's all the RTU would do, close that contact, so no way was it the problem. Then explain to the plant electrician how to troubleshoot HIS controls.

Oh well, they paid me well :p
Most water plant maintenance guys are good at plumbing, but not when those teeny electrons go through those solid copper pipes.
 

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Band Member
DIYer Extrodinaire
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7,475 Posts
Thank you for your vote of confidence :cry:
Lol

By far, some of the best electricians I've ever met were plant electricians.


edit* I became one for a long time too. I was headhunted after automating the local water plant ! :LOL:
 

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Registered
Ready Mix concrete plant electrician
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2,156 Posts
It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.
Where I work we treat water with Portland cement, sand and stone. We always get the same results though, really hard water. :p
 

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Chief Flunky
Field Service Engineer
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2,078 Posts
Discussion Starter · #19 ·
It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.
Think about it. What’s the difference between a water plant and an industrial plant? The biggest difference is the water plants are very good at paying their bills.
 
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