Eliminate Voltage Unbalance

Are you finding that your three phase motors are failing at an alarming rate?  This building construction problem of 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.

Voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA) as 100 times the absolute value of the maximum deviation of the line voltage from the average voltage on a three-phase system, divided by the average voltage. For example, if the measured line voltages are 462, 463, and 455 volts, the average is 460 volts. The voltage unbalance is:

 (460 - 455) x 100 = 1.1%

               460

It is recommended that the voltage unbalances at the motor terminals not exceed 1%. Unbalances over 1% require derating of the motor per  NEMA MG-1, and will void most manufacturers’ warranties. Common causes of voltage unbalance include:

• 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.

 Results vary depending upon motor design, speed, full-load efficiency, and horsepower rating. Typically, electric motors have peak efficiency near 75% load, but the above motor tested in the lab showed otherwise.  Hal Finkelstein and his team have tested and researched solutions to such problems.

Voltage unbalance is probably the leading power quality problem that results in motor overheating and premature motor failure. If unbalanced voltages are detected, a thorough investigation should be undertaken to determine the cause. Energy and dollar savings occur when corrective actions are taken.

Further Considerations

Voltage unbalance causes extremely high current unbalance.  The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance.  For a 100 hp motor with a line voltage unbalance of 2.5% could have a current unbalance of 27.7% and cause it’s breakers to trip.

In addition a motor with a 2% voltage unbalance could have a temperature increase of 8 degrees C when operated under the 2% voltage unbalance.  Winding insulation life will be reduced by one half for each 10 degree C increase in operating temperature.  Learn more about voltage and motor problems here.

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More information on electric motor failures here