Voltage unbalance degrades the performance and shortens the life of a three-phase motor. Voltage unbalance at the motor stator terminals causes phase current unbalance far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses, and motor overheating, which results in a shorter winding insulation life. It is recommended that the voltage unbalances at the motor terminals not exceed 1%. Unbalances over 1% require derating of the motor and will void most manufacturers’ warranties.

Common causes of voltage unbalance include:
• Faulty operation of power factor correction equipment
• Unbalanced or unstable utility supply
• Unbalanced transformer bank supplying a three-phase load that is too large for the bank
• Unevenly distributed single-phase loads on the same power system
• Unidentified single-phase to ground faults
• An open circuit on the distribution system primary

Suggested Actions
Regularly monitor voltages at the motor terminals to verify that voltage unbalance is maintained below 1%. Check your electrical system single-line diagrams to verify that single-phase loads are uniformly distributed. Install ground fault indicators as required and perform annual thermo-graphic inspections. Another indicator that voltage unbalance may be a problem is 120Hz vibration. A finding of 120 Hz vibration should prompt an immediate check of voltage balance.