Unbalanced Line Conditions And Motor Protection

Unbalanced line voltage can be caused by at least four significant factors:

1. Shunted single-phase load.
2. Unbalanced primary voltage.
3. Use of Delta-Wye or Wye-Delta transformers.
4. Defective transformers.

Effect on the motor performance:

1. Can cause serious reduction in starting torque due to low voltage in one or more phases.
2. Can cause excessive and unbalanced full load current.
3. Can cause nuisance overload tripping.
4. Can cause premature failure of motor windings if proper overload protection is not used and known to be functional.

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.