We have installed a Low Voltage Switchgear at our Ammonia/Urea Plant. The switchgear was commissioned in 2019.
Two weeks ago, we encountered a significant issue with the failure of a DOL cubicle's magnetic contactor.
Motor Details:
Rating: 110kW
Current: 196A
Speed: 2980rpm
Voltage: 380VAC
Application: Centrifugal Pump

Contactor Details:
Make: ABB
Type: AF-265-30
Power: 132kW
Service: Continuous/24Hrs
Number of ON/OFF Operations: 4-5/Year max.

Sequence of Events:
-The motor was started after maintenance work on the pump.
-The motor-pump shaft attempted to rotate but tripped within a few seconds.
-Inside the motor cubicle, the Circuit Breaker was found tripped with a -Thermal trip indication on the Thytronic Relay. Recorded fault level at the time of thermal trip was L1=0A, L2= 6In (~1200A), L3= 5.99In (~1198A)
-Circuit Breaker SC Instantaneous trip setting, I3= 6.6x400 =2640A.
-Upon inspection, the contactor had one pole (L1) open while the other two poles (L2 & L3) remained closed.
Further investigation in the workshop revealed:
-Pole L1: The moving contact tip was wiped off and melted onto the fixed contact side, with signs of sparking evident. (See attached pictures)
-Poles L2 & L3: Welding of the fixed and moving contacts was observed.
-Attached are pictures of the faulty contactor.
This is a repeated incident; a similar AF-265 contactor failure occurred with its standby motor in October 2023.
Has anyone faced similar issues with ABB LV contactors?
What could be the possible causes of these failures?

 

My first suspicion here is one of rapid operation, i.e. something in your control system is "chattering", resulting in rapid cycling of the contactor, which very rapidly heats the contacts and welds them.

 

Log the motor current at 1 second intervals or less with a suitable logger, during start up with the highest mechanical load, this would be with all valves open to and from the centrifugal pump, unless you have controls to limit valve openings. Check the ABB short-time withstand current ratings/time of your contactor against the log. If you have a long start time for your motor you may be exceeding these ratings.