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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.
Thomas H. Bishop, P.E., is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO, 314-993-2220, Fax: 314-993-1269, www.easa.com. EASA is an international trade association of more than 2,100 firms in 50 countries that sell and service electrical, electronic and mechanical apparatus.
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