Commercial
and Industrial Safety and Energy Saving Tips
Eliminate
Voltage Unbalance
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.
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