A recent measurement shows that voltage unbalance exceeds 1% for this motor. An indicator of power quality, voltage unbalance measures the deviation between incoming line voltages.
Even small voltage unbalances degrade motor performance and shorten motor life. Per NEMA MG-1-2011, voltage unbalance exceeding 1% requires derating the motor and voids most motor manufacturers’ warranties.
Voltage unbalance causes motors to draw unbalanced currents, sometimes far out of proportion to the voltage unbalance. 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. For these reasons, NEMA recommends that users halt all motor operations once voltage unbalance reaches 5%.
Voltage unbalance at the motor indicates uneven loading of the three-phase power system within your facility or unbalanced voltages coming from your utility. To troubleshoot, take voltage readings at your utility meter; contact your utility if voltage unbalance at the meter exceeds 1%. If your utility is supplying you unbalanced voltages, you may be eligible to receive a rebate on your electricity bills or to be reimbursed costs related to failed motors and motor-driven equipment. If the voltage unbalance originates within your facility, investigate the following causes of unbalanced voltages:
- Faulty operation of power factor correction equipment
- 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.
- 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:
- Use near-real-time power monitoring to monitor voltage at motor terminals and verify that voltage unbalance stays 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.