Current unbalance measures the difference in current a motor draws on each of its three legs as a percentage of the average current (note: In four-wire wye systems, any unbalanced current flows through the neutral conductor).
Motors present higher current unbalance at low loads (≤ 50% rated load) than at or near full-load, driven by differences between the phases’ magnetizing impedance. Current unbalance for three-phase motors should not exceed 10% when the motor is operating at or above 50% of its rated load. Excessive current unbalance shortens a motor’s useful life and decreases its efficiency. Prolonged current unbalance can lead to torque pulsations, increasing motor vibration and mechanical stresses; higher core losses, resulting in lower motor efficiency; and excess motor heating, degrading winding insulation. If current unbalance is due to supply and cannot be corrected, derating the motor’s rated load is recommended.
High current unbalance indicates that there may be loose connections on this motor’s leads; a blown fuse within a power-factor-correction device, starter, or VFD; significant levels of harmonic distortion; uneven loading of the three-phase power system within your facility, causing unbalanced voltages at the motor; unbalanced voltages coming in from your utility; or this motor is nearing failure.
Suggested actions
- Troubleshoot whether current unbalance comes from the motor or your power supply: Rotate phases at the output of the starter or VFD (i.e., move motor lead L1 from phase A to phase B, L2 from B to C, and L3 from C to A), remeasure current on all three phases, and recalculate current unbalance.
- If unbalance changes phases following the direction of your phase rotation (i.e., the same motor lead has highest/lowest current regardless of incoming phase), the motor is nearing failure; evaluate repair or replace and take the most economical action.
- If unbalance stays on the same phase regardless of phase rotation, it’s your supply. Distribute any single-phase loads more equally across the phases. If loads cannot be redistributed, derate motor load by five times the measured current unbalance; i.e.,
Derate (%) = 5 × current unbalance (%)
Effective derated size = [100% – Derate (%)] × Size (Hp)
E.g., If you’ve measured a current unbalance of 11.1% on a 100 Hp motor,
Derate (%) = 5 × 11.1% = 55.5%
Effective derated size = (100% – 55.5%) × 100 Hp = 45.5 Hp