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Voltage Unbalance
Voltage unbalance degrades the performance and shortens the life of a three-phase motor. Voltage unbalance at the motor terminals can cause current unbalance that is far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses resulting in lower efficiency, and motor overheating, which reduces winding insulation life.
It is recommended that voltage unbalances at the motor terminals do not exceed 1%. Unbalances that exceed 1% require derating of the motor, per NEMA MG-1-2011, and will void most manufacturers’ warranties.
Motors@Work calculates your motor’s voltage unbalance when you enter measurements for the three different voltages between the terminals of the three-phase motor.
Some of the more common causes of unbalanced voltages are:
- 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.
- Unbalanced incoming utility supply
- Unequal transformer tap settings
- Large single phase distribution transformer on the system
- Open phase on the primary of a 3 phase transformer on the distribution system
- Faults or grounds in the power transformer
- Open delta connected transformer banks
- A blown fuse on a 3 phase bank of power factor improvement capacitors
- Unequal impedance in conductors of power supply wiring
- Unbalanced distribution of single phase loads such as lighting
- Heavy reactive single phase loads such as welders
Suggested actions:
- Near real time or periodically monitor voltage at motor terminals to verify that voltage unbalance is maintained below 1%. Consider installing sensors or sub-meters that send readings for Motors@Work analytics to identify unacceptable values or rates of change of values.
- Check electrical system single-line diagrams to verify that single-phase loads are uniformly distributed
- Install ground fault indicators as required and perform annual thermographic inspections
- Periodically monitor vibration and record in Motors@Work. Another indicator that voltage unbalance may be a problem is 120-hertz (Hz) vibration. A finding of 120-Hz vibration prompts an immediate check for voltage unbalance in Motors@Work.