by Thomas H. Bishop, P.E., EASA

Pumps & Systems, June 2008

It is impossible to balance line-to-line voltages perfectly in a three-phase circuit. In fact, line voltages typically differ by a few volts or more, but a difference that exceeds 1 percent can lead to serious trouble on the plant floor. To maintain peak energy efficiency and thwart premature failure of three-phase motors, install adequate protective devices and periodically check for voltage unbalance at the motor terminals. 

What It Is

Simply stated, voltage unbalance describes when not all line voltages in a three-phase circuit are equal. The effect on motors and other devices in the circuit depends directly upon the percent of unbalance present. The National Electrical Manufacturers Association (NEMA) defines percent voltage unbalance as follows in its standards publication MG 1-2006: Motors and Generators, Part 14.36:

% voltage unbalance  =  100   x    (maximum voltage deviation from average voltage)

                                                                      (average voltage)

For example, with line-to-line voltages of 460, 467 and 450, the average voltage is 459, and the maximum deviation from average is 9. Therefore, the percent unbalance is 1.96  percent:

100 x (9/459) = 1.96 %

Acknowledging possible differences in performance, NEMA MG 1-2006, Part 12.45 calls for three-phase motors to "operate successfully" at rated load if voltage unbalance at the motor terminals is 1 percent or less. For reliable motor operation, be sure to keep this limiting value in mind. (Note that the 1.96 percent unbalance above exceeds the NEMA standard.

Common Causes

Unbalanced voltages can exist anywhere in a three-phase power distribution system. Usually, the source of the problem is unequal line loads due to system voltage unbalance, different system impedances (voltage divided by current), the nature of the loads and the operating load on equipment, particularly motors. "Single-phasing" (the complete loss of a phase) is the ultimate voltage unbalance condition for a three-phase circuit.

  • Frequent causes of unbalanced voltages include:
  • Unbalanced incoming utility suppl
  • Unequal transformer tap settings
  • A large single-phase distribution transformer on the system
  • An open phase on the primary of a three-phase distribution transformer
  • Faults or grounds in the power transformer
  • Open delta-connected transformer banks
  • A blown fuse on a bank of three-phase 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
  • Large heater controls that cycle rapidly

Effects of Voltage Unbalance

Voltage unbalance produces even larger phase current unbalances that can damage electric motors, generators, transformers and power supply wiring. For example, voltage unbalance of 1 percent at the terminals of a fully loaded motor can result in phase current unbalance of 6 to 10 percent, which raises the operating temperature of the motor, reduces its energy efficiency and shortens its life.

The additional heating (called "winding losses") is calculated by the formula I2R, where I is current and R resistance. If the current unbalance is 10 percent (1.10), the high-current phase will have at least 21 percent (1.102 = 1.21) more loss (loss = heat) than any other phase.

2easa figure 1.jpgFigure 1 clearly shows how voltage unbalance affects the current and temperature rise of a typical three-phase electric motor rated 5-hp, 230/460V, 60-Hz, 1725-rpm and 1.0 service factor.

Each 10-deg C (18-deg F) above the rated temperature rise will shorten the life of winding insulation by about half, so even a small increase in the percent voltage unbalance could seriously damage a motor. The 5.4 percent voltage unbalance in Figure 1 adds 60-deg C (108-deg F) to the temperature rise, which means the life expectancy of the winding (and motor) would drop to about 1/64 of normal-a substantial and unacceptable reduction.

Unbalanced voltages also can introduce harmful harmonic currents. Although beyond the scope of this article, these currents cause additional heating in motors and supply wiring (sometimes including the neutral). The percentage of harmonic current may increase significantly due to both third- and even-order harmonics in the circuit.

Other effects of unbalanced voltages on motors are that the locked-rotor current of the stator winding (already relatively high) will be unbalanced in proportion to the voltage unbalance; full-load speed will drop slightly; and torque will decrease. If the condition is severe enough, the motor may not produce enough torque to reach rated speed. Noise and vibration levels also can increase due to voltage unbalance.