Selasa, 01 Juni 2010

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.

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 Figure 20-2 of NEMA MG-1-1993, Rev 3, 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.

The efficiency of a rewound, 1800-RPM, 100-hp motor is given as a function of voltage unbalance and motor load in the table. The general trend of efficiency reduction with increased voltage unbalance is observed for all motors at all load conditions.

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.

Example

Assume that the motor tested as shown in the above table was fully loaded and operated for 8000 hours per year, with an unbalanced voltage of 2.5%. With energy priced at $0.05/kWh, the annual energy and dollar savings, after corrective actions are taken, are:

Annual Energy Savings = 100 hp x 0.746 kW/hp x 8000 hrs/yr x (100/93 - 100/94.4) = 9517 kWh

Annual Dollar Savings = 9517 kWh x $0.05/kWh = $476

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