Unbalanced line voltage can be caused by at least four significant factors:
Effect on the motor performance:
How to determine source of unbalanced motor current:
In many cases only slight variance in supply voltage may be measured, but considerable variance in motor current may be observed. To determine cause of unbalance rotate line connection to motor.
For example, T2 shows high current when connected normally. If after line-to-motor rotation T2 no longer shows high current, unbalance is in supply line.