We had faced similar problems on few of our motors. I would suggest:
1. Checking the percentage voltage unbalance. should be between 1-1.5 %. Limit is perhaps 2 %. At startup, it could be more pronounced with some inherent phase resistance unbalance.
2. Check the motors phase resistances with...
I am sorry to jump in this discussion but could not resist.
I am also working on a proposal for installation of VFD on centrifugal pump with throtlling valve. The application is a standard boiler feed water pump 13 stages providing water to steam drum through a control valve.
Motor is 2600...
