DEAR MEMBERS
WE HAVE ONE 110 KW 415 V 1450 RPM 50 HZ INDUCATION MOTOR RUNNING ON ALMOST FULL LOAD . WE FOUND IT RUNNING WITH 150, 160,150 AMPS LOADING RESPECTIVELY R Y & B.
THE VOLTAGES FOR THE SAME FEEDER ( COMPLETE MCC) ARE 417 422 416 VOLTS.
WE HAVE TIGHTEN THE TERMINATION CONNECTIONS AT MCC AS WELL AS MOTOR END. STILL UBALANCE LOADING IS THERE. ALSO IN THERMOGRAPHY THIS 'Y' PHASE SHOWING BRIGHTER THAN REMAINING TWO PHASES.

CAN ANY ONE TELL FOR THIS UNBALANCED LOADING ?

WHAT IS THE MAX ALLOWABLE LIMIT FOR CURRENT UNBALANCE AS PER STANDARD AND HOW IT CAN BE CALCULATED IN PERCENTAGE? ( AS PER IEEE STANDARD OR ANY OTHER STANDARD)

PLEASE SHARE YOUR EXP AND KNOWLEDGE.

THANKS & REGARDS

In motors, an unbalance in the supply voltage is expected to create an unbalance in the current which is approx 5-10x as large. (the negative sequence impedance presented by the motor is approx 1/5 -1/10 of the postiive sequence impedance).

Unbalance is typically defined as (max deviation from average) divided by average.

For voltage, you have an an average of 418.3, a max deviation from average of 422-418.1=3.67 and an unbalance of 3.67/418.3 = 4.35%

For current, you have an an average of 153.3, a max deviation from average of 160-153.3=6.667 and an unbalance of 6.67/153.3 = 0.88%

Your current unbalance (4.35%)is approx 5x as large as your voltage unbalance (0.88%), indicating the voltage unbalance is the likely source of the current unbalance (not a problem with the motor).

NEMA recommends derating of motor full load when voltage unbalance exceeds 1%. Your are not quite there (0.88%) so no derating should be required and it should be ok to operate up to nameplate current.

If you exceed 1% voltage unbalance and want to derate, the NEMA derating curve (plus a little more background) is given on page 17 of the EASA handbook available here.
http://www.joliet-equipment.com/pdf/techinfo/EASAHandbook.pdf