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Overload relay keeps failing

14K views 27 replies 16 participants last post by  Zog 
#1 ·
i have an air compressor with a 300hp main motor and a 10hp fan motor, both running on 460v with 120v control voltage. it's a new machine that's been in service since june.

back in early september the fan's ol relay started failing. it would trip out, i would reset it and the unit would run for awhile, then trip again. over the course of a week it got so bad it would trip as soon as i started the machine. i checked the fan motor amp draw was between 12.5 -13.5 amps, the motor's nameplate says 13.3 fl amps. i replaced the ol relay and the problem went away.

this week the problem came back. sunday the compressor went down for motor overload. i reset the fan ol relay and the unit ran again. the motor amp draw is still 12.5-13.5 amps. from my past experiences dealing with motors, i don't think the motor is bad (yet). on the compressors i've delt with when the motor is shorted out i blow fusses and i haven't blown any fusses.

when i changed the ol relay last time i checked all the connections and they were tight. besides the motor what else would cause the ol relay to fail. it is a seimens esp 200.
 
#5 ·
The machine has a phase monitor so it's not losing a phase.

I have it set up the modulate at 100 psi so it's not running fully loaded to reduce amp draw on the main motor.

If there is a current imbalance would swapping the leads on the starter help. Ex: swap legs 1 and 3, so I keep the same rotation?
 
#7 ·
Wye delta soft start. I swapped phases a to b, b to c, and c to a and it still tripped out within 5 min. My current reading were 12.5, 12.5 and 13.7 across the legs before I changed them. Afterwards it read 13.8, 12.5, 12.6. The voltage is 470 when measuring 1-2,2-3 and 1-3. It's being feed from a nearby transformer.
 
#13 ·
I put another esp 200 in to see what happens.

The voltage on the contactor while stopped is all 470v. But when I start the machine it drops down to 459-460v.

I cut and retrimmed the leads coming from the motor. I also checked the connections at the motor. My current is now reading 13.1a, 13a and 12.4a.

I have the esp200 set for current unbalance off, but so was the other one I removed.
 
#16 ·
The motor runs 24/7. The "new" ol relay came out of another identical compressor and was preset from the compressor factory for around 16-17a...
What protection class is it set for?

You said phase unbalance was off, but what about "phase loss" and "ground fault"?

What do the relays do when you hit the "test" button?
 
#17 · (Edited)
i have an air compressor with a 300hp main motor and a 10hp fan motor, ...

back in early september the fan's ol relay started failing.... i replaced the ol relay and the problem went away.

this week the problem came back. sunday the compressor went down for motor overload. i reset the fan ol relay and the unit ran again....
Just want to get something clear. First you talk about the FAN tripping on OL, then you say the COMPRESSOR tripped on OL, after which you apparently reset the FAN OL relay again. So when you said the compressor tripped on motor OL, did you mean the compressor tripped because the FAN tripped on OL? That's how I interpreted it, but I just want to make sure.

Assuming that's correct, the situation sure is odd. No frequent starts, unbalance disabled (and always was it appears), not running over FLA setting, voltage seems reasonable. Running out of options, time to clutch at straws?.

The ESP200 also has GF protection in it. It's Residual GF protection, factory set for 50% of the motor FLC setting, so that means if all of the current going out doesn't come back, AND it is under 50% of the current going out, then it will trip on GF. An intermittent leakage to ground inside of a conduit can cause that, or maybe a motor winding fault to ground.

I had a large compressor once where we eventually found that the conduit from the starter to the motor was getting water in it, but only after it ran for a while. Turned out someone ran the compressor water drain line down under the slab using the hole cut in it for the conduit run. The drain line didn't really go anywhere, it just dumped the water into the dirt under the slab, but the soil was so compacted it just sat there for a while, built up and eventually worked it's way into the conduit fittings. So things only went haywire after the compressor had been running for a long enough time for the water to build up. But once the breaker tripped, by the time I got there the water had drained and everything tested fine. The only way we found it was by parking a guy there to sit around and wait for the compressor to trip again, then he immediately checked everything, thinking it was heat related. He meggered the leads and saw a short to ground where there was none before, and when I got there two hours later, it was gone again.
 
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