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Keeping "In Phase" With Induction Motors Background Unlike residential appliances that operate on a single phase of electrical supply, industrial loads normally operate using three-phase power sources. The use of three phases facilitates the efficient transmission of greater amounts of power and in the case of induction motors this allows for a simpler, more robust starting, and a more efficient motor. However, the characteristics of each electrical phase are important when considering motor operation and life. Phase Unbalance Because a three-phase electrical circuit has single-phase loads connected to each phase, and because it is impossible to perfectly balance the single-phase loads among the three phases, the voltages of the three phases will vary. From a technical standpoint, the amount of this voltage unbalance as defined by the National Electrical Manufacturers Association (NEMA) is given by the following equation. % unbalance = 100 x maximum deviation from average voltage For example, if the voltage (phase to phase) at the input terminals of a three-phase motor that is rated for 460 volts is measured, voltages of 465, 465, and 474 on the three phases might be found. For our example, the average voltage is (465 + 465 + 474) ¸ 3 = 468 volts. These voltages, while not very different from a mathematical standpoint, represent a voltage unbalance at the motor. % unbalance = 100 (474 - 468) ¸ 468 = 1.28% While this value may still seem small, its affect on motor operation can be detrimental. One percent voltage unbalance in a motor has been shown to cause a 5% current unbalance. As a result, NEMA recommends that motors be de-rated if operating at voltage unbalances above 1%. If the voltage unbalance is as high as 5%, NEMA recommends a de-rating factor of 0.75. That means a 10 horsepower motor operating under these conditions should not be loaded beyond 7.5 horsepower. To do so will significantly shorten motor life, which results from uneven heating of the motor windings due to the current unbalance created by the unbalanced voltage. Correcting Phase Unbalance Phase unbalance is not easily detected and will often increase gradually as single phase loads are added. The best way to detect phase unbalance is through the use of a high quality true - rms voltmeter. It is always a good idea to determine the voltage unbalance of three phase systems. If the unbalance is unacceptably high, work should be done to better balance the single-phase loads that are being place on each of the phases of the three-phase system. Single Phasing Single phasing occurs when one of a three-phase system is lost. This loss of a phase can be caused by a blown fuse, downed utility line, bad connection, etc. Single phasing can cause serious consequences for induction motors. If initially running, the motor will continue to run, but because it has lost some of its electrical power source, it can no longer support its rated three-phase load. As a result the motor will draw more current on the other phases and may be damaged due to overheating as it attempts to support its rated load, as shown in figure 1 below. A motor will not start in a single-phase condition, but it will draw excessive current, which can quickly damage a motor.
Figure 1: Damage caused by operating under single phase conditions on
(a) delta- and (b) wye-connected motors. Protecting Against Single Phasing The easiest way to protect a motor from potential damage caused by single phasing is to install overload protection sized for the particular application. To determine the appropriate size protection, consult the National Electrical Code (NEC) article 430. Another means of guarding against single phasing can be accomplished through the use of control circuitry. Devices are available that monitor the magnitude and phase of each line voltage. These devices can sense both phase unbalance and loss of phase and will trip the motor starter to take the motor offline. Typically, these devices can be adjusted to trip at different levels of unbalance, but should be set at 3% for motors operating at or near full-load. For more information about phase unbalance and single phasing, see the following references:
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