3ph motor tripping

Hi, I'd appreciate a bit of guidance from the industrial sparks here, I'm out of comfort zone!

The setup is a wood pellet conveyer system with 3 motors driving an Archimedes screw, a vertical lift and another screw.
I've been told the whole lot has run for 3 years without issue
Last delivery the overload for one motor tripped regularly, virtually every two minutes.
This delivery I was called in to watch along with a guy from the company that installed it and it tripped a couple of times in the first 10 minutes and then ran for over an hour without issue. The motor got warm but not so hot I couldn't leave my hand on it. The other guy wanted to increase the trip setting, but I dug in and said if it's worked for years we're not changing that without knowing why it tripped. Obviously I couldn't start pulling things apart during a delivery so I went back later to do some testing.

So far I've done the following: Isolated, got cover of motor wiring box (sounds easy, was ridiculous access!), removed the permanent star connection and did the following tests:
winding 1 = 0.95 ohms
winding 2 = 0.95 ohms
winding 3 = 0.95 ohms

IR between windings and IR of each winding to earth all >300Mohms
Then I tested the wiring between control panel and isolator, and no faults found there either. Today it ran with no load for 30 minutes without issue.
To the extent of my knowledge I'm not seeing an electrical reason it would trip and it seems most likely to me that it was a mechanical issue that was putting higher load on the motor (e.g. oversized wood pellet) and the issue eventually cleared itself.
Are there any more tests I could have done?

Some pictures:

Check connections are sound at the relay, these are thermal in operation so lose connections can cause issues.
Check the FLC of the motor compares to the rating plate and the phases are pretty much balanced.
Does the heat generated from this motor compare with the other two.
Does it sound like it is running okay.

check your supply voltage. check all contacts etc. bad connection and you'll get similar to running without a phase

try turning the motor manually. see if there is anything jamming it from turning. bearings etc either on the motor or whatever its connected to

Thanks for suggestions.

Fleeting said:

Does it sound like it is running okay.

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Yes it sounds like … a motor!
I’m not sure yet how easily I can get to the other two but will see if I can compare heat next time.
Every connection at motor, isolator and terminal block of panel seemed ok.
I didn’t check the connections on the relay but will do.

Check the fan on the back of the motor is intact and not blocked, i.e. it is generating enough cooling air for the motor.

Tong test (Clamp on amp meter) on the 3 phases one at a time going to the motor, they should be equal and not above the name plate rated current of the motor. If it is below the rated current of the motor and tripping the overloads then the overloads need looking at.
If it is drawing more than the rated current then the overloads are doing there job so you need to look at at why there is an overload.
I would normally split the coupling or remove belts at the motor, That way you will be able to spin the motor by hand, if it feels rough or tight then the bearing are knackered or fan rubbing, I have also had it where as similar to Dave mentions above the cooling fins were all completely choked with plastic dust and acted like a blanket on the motor causing it to overheat, although from your pic the cooling fins look not bad.
If the motor turns nice and smooth then you are looking at the mechanical side of things:
Operator trying to push too much product through it working it too hard.
Or mechanical issues like feed screw bearing knackered, conveyor belts rubbing, product jammed up against things it shouldn’t be against etc. etc.
Good luck let us know how you get on.

roys said:

I would normally split the coupling or remove belts at the motor, That way you will be able to spin the motor by hand, if it feels rough or tight then the bearing are knackered or fan rubbing, I have also had it where as similar to Dave mentions above the cooling fins were all completely choked with plastic dust and acted like a blanket on the motor causing it to overheat, although from your pic the cooling fins look not bad.
If the motor turns nice and smooth then you are looking at the mechanical side of things:

Click to expand...

roys said:

mechanical issues like feed screw bearing knackered, conveyor belts rubbing, product jammed up against things it shouldn’t be against etc. etc

Click to expand...

Agree , try the above first,

monitor one phase amperage logging unit meter i use a the mooshi !