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.
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Frequent causes of unbalanced voltages include:
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Unbalanced incoming utility suppl
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Unequal transformer tap settings
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A large single-phase distribution transformer on the system
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An open phase on the primary of a three-phase distribution transformer
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Faults or grounds in the power transformer
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Open delta-connected transformer banks
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A blown fuse on a bank of three-phase power factor improvement capacitors
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Unequal impedance in conductors of power supply wiring
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Unbalanced distribution of single-phase loads such as lighting
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Heavy reactive single-phase loads such as welders
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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.
Figure 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.
A word of caution: Not all voltage unbalances are created equal. The effect is more dramatic if the voltages of all three phases differ than if only one phase deviates from the other two. This is true even if the percent variation calculates to the same unbalance.
Figure 2 illustrates the typical percentage increases in motor losses and heating for various levels of voltage unbalance.
Figure 2. Motor heating and losses versus voltage unbalance.
A motor often will continue to operate on unbalanced voltages, although less efficiently. This reduced efficiency is caused by both increased current (I) and greater resistance (R) due to heating. 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.
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Single-Phasing
Single-phase operation of a three-phase motor deserves special attention. Electrical maintenance people often rely on a motor protection device to prevent loss of phase, only to find that it did not work and the motor failed. Single-phase operation of a three-phase motor can cause overheating due to excessive current and decreased output capability. If the motor is at or near full load when single-phasing occurs, it will not develop rated torque and may even stall-i.e., come to a stop. The stall condition generates tremendous amounts of locked-rotor current, resulting in an extremely rapid temperature rise.
An interesting example is what would happen if a pump motor lost a phase. Recall the formulas for AC power:
Single phase: P = V x A x pf
Three phase: P = V x A x pf x 1.73.
Where:
P = watts
V = volts
A = amperes
pf = power factor
If the three-phase motor supply becomes single-phased, the output power would drop to 1/1.73, or about 58 percent of rated, and rotor speed would decrease significantly from the reduced torque capability. Pump power varies by cube of the speed, so the power requirement would also drop. Since the motor current may not be significantly above rated, the overload protection devices would not trip. Still, reduced cooling at the slower speed could cause the motor to overheat and fail prematurely.
Without adequate motor protection, the stator winding may fail; the squirrel cage rotor also may be damaged or destroyed. A good reason not to rely on standard, three-overload starters to prevent single-phasing is that local, internal windings can overheat even when line currents do not exceed the setting of any one overload device. Effective protection against single-phasing requires special sensing devices such as negative-sequence voltage relays (see below).
A more complex scenario happens with several motors of different ratings on a single-phased circuit. Frequently in such cases, one of the larger motors will generate the missing third phase. (The same principle is used in commercial rotary single-phase to three-phase converters, except they use capacitors to start and adjust the voltage balance.)
If the motor is operating at less than rated load, its current may be too low to trip its over-current protection. In that case, smaller motors operating near rated load in the same circuit will be prone to rapid failure, because the generated phase will be approximately 10 to 15 percent undervoltage. (The explanation of how this undervoltage occurs is beyond the scope of this article.) The generated phase voltage will decrease further if the load on the large motor increases, thus worsening the situation for all the motors, both large and small.
Testing for Unbalanced Voltages
The first step in testing for unbalanced voltages is to measure line-to-line voltages at the motor terminals, following all applicable safety precautions. Likewise, measure the current in each supply line, because the current unbalance is often 6 to 10 times greater than the voltage unbalance. Suspect single-phasing when a motor fails to start. To check for this condition, simply measure the current in each phase of the motor circuit. One phase will carry zero current when a single-phasing condition exists.
Ways to Correct Unbalanced Voltages
Redistributing and reconnecting single-phase loads can reduce voltage unbalance caused by excessively unequal load distribution among phases. Most prevalent among heavy, single-phase loads are lighting equipment and occasionally welders. In addition, check for a blown fuse on a bank of three-phase power factor improvement capacitors.
Another corrective action, though generally undesirable, is to derate a motor. If voltage unbalance exceeds 1 percent, a motor must be derated to operate successfully. Figure 5 indicates that at the 5 percent voltage unbalance limit set by NEMA, a motor must be derated substantially, to about 75 percent of its nameplate horsepower rating.
Figure 5: Derating factor due to unbalanced voltage.
Automatic voltage regulators (AVRs) can be used to correct voltage unbalance, as well as undervoltage and overvoltage. These devices automatically compensate for all voltage fluctuations in real time if the input voltage is within their range of magnitude and adjustment speed. Although high-power AVRs are available, it is usually more practical to install a number of smaller units to protect the various circuits, as opposed to one large unit possibly at the plant service entrance.
Protective Relays
Special protective relays can detect voltage unbalance and shield equipment from its degrading effects. Typically, these unbalance relays are small, relatively inexpensive microprocessors with numerous features-e.g., automatic or manual reset, programmable trip time and unbalance limit settings. If voltage unbalance exceeds a predetermined limit, most of these devices can activate an alarm, trip a control, or both. They also can be retrofitted into a motor control circuit or any portion of a power distribution system.
Negative-sequence voltage relays can detect single-phasing, phase-voltage unbalance and reversal of supply phase rotation. These relays only sense anomalies upstream of their location in a circuit, so they cannot detect a problem in a motor or other load downstream.
Other relays that provide only limited protection in specific circumstances include phase-sequence undervoltage relays and phase-voltage relays. Phase-sequence undervoltage relays usually do not provide satisfactory phase-loss protection because, as mentioned previously, single-phased motors may generate enough voltage to make it appear that a relatively balanced condition exists. Phase-voltage relays provide only limited single-phasing protection by preventing the motor from starting if one phase of the system is open.
A Closing Point
Voltage unbalance and voltage variation are very different things. Voltage variation is the deviation of voltage from the rated voltage, and NEMA MG 1-2006, Part 12.68 allows a variation from rated voltage of ±10 percent. That rating assumes balanced voltages, and acknowledges that motor performance will not necessarily be the same as at rated voltage. The tolerance for voltage unbalance is only 1 percent, an order of magnitude less than the 10 percent tolerance for voltage variation.