Overcurrent in that particular phase.

burnt out; the 110 volts may go lower when pump starts, causing the contactor to chatter. perry

Loose connection somewhere down the line to the pump.
Should change all contacts at same time as well as any mounting hardware or springs if applicable.
Inspect wire condition for good clean connections.

For starters.

May be an idea to check the rating of the contactor,the AC3 rating.

Yes check the contactor rating, also if pump is starting and stopping a lot the rating should be higher than for one which runs continuously

It's the same leg all the time, right?

Weak coil, or slight warping in the contactor housing not allowing the contactor points to close all the way on that side.

It's likely to be the high (perhaps sutained for a while) start-current of a motor downstream of the contactor.

This happens frequently in high starting-inertia applications such as rock crushing in mines and gravel pits.

At the very least, put in a soft-starter with a long-enough "soft-start" period. We put in an Integra unit manufactured by Somars, which completely solved one of our customers' similar problems. Integra units have soft-start capabilities, but they're much more than soft starters ... they're specialized computers that can be configured to both protect motors and to increase energy efficiency.

http://www.somar.co.uk/energy-saving-technology/Integra-motor-control.php

The starter on a rock crusher kept burning out at weekly intervals, which meant shutting down a crusher, filing (shaving) the starter contacts, and running the crusher for another week. Now, the contactor doesn't burn out any more.

Cheers!
DZ

The pump is 9KW 380V 3 phase but your DOL starter is 110Vac supply voltage ????Are you sure???How can you start the motor???

Probably read contactor coil control voltage??

you are correct. 110Vac is the contactor coil voltage.

Possible causes

1) Loose connection

2) Incorrect contactor sizing

3) Incorrect coil voltage (Low Voltage)

4) If there is dirt on the face of the moving metal parts on the contactor. it will chatter... and possibly cause this..

this is also true if the eddy current ring on these metal plates is missing

Did you check all 3 supply legs to the motor? You may have a dead leg and the other 2 legs are drawing excessive current. edmund

For premature failures like that, usual cause is insects: earwigs, spiders, and such. Assuming open contactors, mind you. If open, try to replace with enclosed contactors unit.