Single-phase operation of a three-phase motor deserves special attention. These factors "stack up," producing an exponential increase in motor heating that sometimes leads to "thermal runaway" (uncontrollable heat rise), rapid deterioration of the insulation system and premature winding failure. This reduced efficiency is caused by both increased current (I) and greater resistance (R) due to heating. Motor heating and losses versus voltage unbalance.Ī motor often will continue to operate on unbalanced voltages, although less efficiently. This is true even if the percent variation calculates to the same unbalance.įigure 2 illustrates the typical percentage increases in motor losses and heating for various levels of voltage unbalance. The effect is more dramatic if the voltages of all three phases differ than if only one phase deviates from the other two. Noise and vibration levels also can increase due to voltage unbalance.Ī word of caution: Not all voltage unbalances are created equal. If the condition is severe enough, the motor may not produce enough torque to reach rated speed. 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. The percentage of harmonic current may increase significantly due to both third- and even-order harmonics in the circuit. Although beyond the scope of this article, these currents cause additional heating in motors and supply wiring (sometimes including the neutral). Unbalanced voltages also can introduce harmful harmonic currents. 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. If the current unbalance is 10 percent (1.10), the high-current phase will have at least 21 percent (1.10 2 = 1.21) more loss (loss = heat) than any other phase.įigure 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.Įach 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 additional heating (called "winding losses") is calculated by the formula I 2R, where I is current and R resistance. 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. Voltage unbalance produces even larger phase current unbalances that can damage electric motors, generators, transformers and power supply wiring. "Single-phasing" (the complete loss of a phase) is the ultimate voltage unbalance condition for a three-phase circuit. 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. Unbalanced voltages can exist anywhere in a three-phase power distribution system. (Note that the 1.96 percent unbalance above exceeds the NEMA standard. For reliable motor operation, be sure to keep this limiting value in mind. Therefore, the percent unbalance is 1.96 percent:Īcknowledging 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 example, with line-to-line voltages of 460, 467 and 450, the average voltage is 459, and the maximum deviation from average is 9.
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