Tom Bishop, P.E.
EASA Technical Support Specialist

Unbalanced voltages are unequal line-to-line voltage values on 3-phase circuits that can exist anywhere in a power distribution system. Unbalanced voltages can cause serious problems, particularly for motors and other inductive devices. Perfectly voltage-balanced circuits are not possible in the real world. Typically, the circuit line-to-line voltages may differ by a few volts or more. It’s when voltages differ by more than 1% that problems tend to occur.

Variations in the load 
Unbalanced voltages usually occur because of variations in the line load. When the load on one or more of the phases is different than the other(s), un­balanced voltages will occur. This can be due to different impedances, or type and value of loading on each phase.

Essentially, the resulting current unbalance is caused not only by the system voltage unbalance but also by the system impedance (voltage divided by current), the nature of the loads causing the unbalance, and the operating load on equipment, particu­larly motors. Single-phasing, which is the complete loss of a phase, is the ultimate voltage unbalance condition for a three-phase circuit. 

Although a discussion of harmon­ics is beyond the scope of this article, they can be a result of unbalanced voltages. The percentage of harmonic current may increase significantly because of the introduction of not only third-order harmonics but even-order harmonics as well. When present, har­monic currents cause additional delete­rious heat in motors and supply wiring, sometimes including the neutral wire. 

Voltage unbalance defined 
Specific values illustrate the impact of unbalanced voltages and provide application guidance as well. The National Electrical Manufacturers As­sociation (NEMA), in its Motors and Generators Standards (MG1-2006) part 14.36, defines voltage unbalance as follows: 

The following example illustrates the formula. With line-to-line voltages of 460, 467, and 450, the average is 459, the maximum deviation from aver­age is 9, and the percent unbalance equals: 
100 x (9/459) = 1.96 %. 

MG1 part 12.45 states that “poly-phase motors shall operate success­fully under running conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1 percent. The performance will not neces­sarily be the same as when the motor is operating with a balanced voltage at the motor terminals.” Thus, for reli­able motor operation, it is significant to keep this limiting value in mind. Note that the percent unbalance calculated in the above formula, 1.96%, exceeds the NEMA standard. 

Common causes 
Some of the more common causes of unbalanced voltages are: 

• Unbalanced incoming utility supply 
• Unequal transformer tap settings 
• Large single phase distribution transformer on the system 
• Open phase on the primary of a 3-phase transformer on the distri­bution system 
• Faults or grounds in the power transformer 
• Open delta connected transformer banks 
• A blown fuse or faulty capacitors on a 3 phase bank of 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, which cycle rapidly 

Unbalanced voltages are harmful to electric motors. They damage power supply wiring, transformers, and gen­erators. Unbalanced voltages at motor terminals cause phase current unbalance typically ranging from 6 to 10 times the percent voltage unbalance for a fully loaded motor. As an example, if voltage unbalance is 1%, then current unbalance could be from 6% to 10%. 

This causes motor over current result­ing in excessive heat that shortens wind­ing and motor life. Winding losses are calculated by the formula I2R, with I be­ing current and R resistance. If the current unbalance is 10%, the high-current phase will have at least 21% (1.10 x 1.10) more loss (loss = heat) than any other phase. 

Other effects on motors are that locked rotor stator winding current (al­ready relatively high) will be unbalanced proportional to the voltage unbalance, full load speed will be slightly reduced, and torque will be reduced. If the voltage unbalance is great enough, the reduced torque capability might not be adequate for the application and the motor will not attain rated speed. Noise and vibra­tion levels can also increase as a result of voltage unbalance. Vibration can be particularly severe on 2-pole motors. 

Figure 1 illustrates voltage unbal­ance effects on a typical electric motor rated 5 hp, 3-phase, 230/460, 60 Hz, 1725 rpm, and 1.0 service factor. 

A most damaging effect is that winding insulation life is ap­proximately halved for every 10° C increase in winding temperature. The 5.4% unbalance shown in the third column would result in an 
the expected life of only 1/64 of normal due to the additional 60° C rise, a substantial and unacceptable reduction. A similar motor with a service factor of 1.15 could typically withstand an unbalance of about 4.5% provided it is not operated above its nameplate rated horsepower. In this case the 5.4% unbalance is excessive even for a 1.15 service factor motor. 

A word of caution: Not all volt­age unbalances are created equal. If the voltages of all three phases differ, the effect is more dramatic 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. 

Efficiency is reduced 
A motor often continues to operate with unbalanced voltages; however, its efficiency is reduced. This reduction of efficiency is caused both by increased current “I” and increased resistance “R” due to heating. The in­crease in resistance and current “stack up,” contributing to an exponential increase in motor heating. Essentially, this means that as the resulting losses increase, the heating intensifies rapidly. This may lead to a condition of un­controllable heat rise, called “thermal runaway,” which results in a rapid deterioration of the winding insulation concluding with failure of the winding. 

Motor may stall 
Single-phase operation of a motor deserves special attention because so often electrical maintenance people believe their motors have protection against single-phasing, only to find that their protection did not work and the motor has failed. Single-phase operation of a 3-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 therefore it may stall; that is, come to a stop. The stall condition generates tremendous amounts of current and heat resulting in an extremely rapid temperature rise. 

Effective protection 
If motor protection is not ade­quate, the stator winding can fail, and the squirrel cage rotor may be dam­aged or destroyed. The standard three overload starter should not be relied on to provide protection against single phasing. One reason for this is that local internal winding overheating can still occur even when line currents do not exceed the setting of any one overload. Effective protection against single-phas­ing requires special sensing devices such as negative sequence voltage relays, discussed later in this article. 

A particularly troublesome and complex scenario is the case of mul­tiple motors of different ratings on a circuit that has been single-phased. Frequently, one of the motors gener­ates the missing third phase by acting as a rotary converter. In fact, this form of generation is the principle that is used to make a commercial rotary single to three-phase converter. The key difference is that the commercial converter uses capacitors to start, and to adjust the balance of the intention­ally generated third phase for proper operation. Consider, for example, the case of a large motor operating in a single-phased mode but carrying less than rated load such that its current is low enough that it does not trip its overcurrent protection. If there are smaller motors operating near rated load in the same circuit, they will be prone to rapid failure because of the approximately 10% Undervoltage in the generated phase. The generated phase voltage will be further reduced if the load on the larger motor is increased, thus making the situation more severe for all the motors, both large and small. 

Tests for unbalanced voltage 
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 about 6 to 10 times greater than the voltage unbalance. Single-phasing should be suspected when a motor fails to start. This condition can be readily checked for by measuring the current in each phase of the circuit. One phase will carry zero current when a single-phasing condition exists. 

Voltage unbalance caused by excessively unequal load distribu­tion among phases can be reduced by reconnecting single-phase loads and redistributing them in as close to a balanced condition as possible. 

Most prevalent among heavy single phase loads are lighting equipment and occasionally welders. Also, check for a blown fuse on a 3-phase bank of power factor improvement capacitors. 

Although generally not desirable, another corrective action is to derate a motor. When voltage unbalance exceeds 1%, a motor must be derated for it to operate successfully. The de­rating curve of Figure 3 indicates that at the 5% limit established by NEMA for unbalance, a motor would be sub­stantially derated, to only about 75% of its nameplate horsepower rating. 

An automatic voltage regulator (AVR) can be used to correct undervolt­age and overvoltage, as well as voltage unbalance. As an active device, the AVR automatically compensates for all voltage fluctuations, provided that the input voltage to the AVR is within its range of magnitude and speed of adjustment. Although high power AVRs are available, it is usually more practical to install a number of smaller units for the various circuits to be protected, as opposed to one large unit possibly at the plant service entrance. 

Devices to detect unbalance 
Special protective relays can be used to detect voltage unbalance and protect equipment from the degrading effects of unbalance. Unbalance relays are usually of the micro­processor type and 
are available with numerous features.

Typically, these devices are small, relatively inexpensive, automatic or manual reset, and offer programmable trip time and unbalance limit settings. They also can be connected to acti­vate an alarm, trip a control circuit, or both when unbalance exceeds a predetermined limit. In addition, these versatile relays can be retrofitted into a motor control circuit or any portion of a power distribution system. 

Another type of protective relay, the negative sequence voltage relay, can detect single-phasing, phase-voltage unbalance, and reversal of supply phase rotation. These relays sense anomalies only upstream of their location in a circuit. Therefore this type of relay will not be able to detect an internal problem in a motor or other load downstream. Likewise, some other relay types provide only limited protection in specific circumstances. Phase se­quence Undervoltage relays, in most cases, do not provide satisfactory phase-loss protection. The reason is that a single-phased motor generates a voltage that is high enough such that a relatively balanced condi­tion appears to exist, thus inhibiting operation of the relay. Phase voltage relays provide only limited single-phasing protection by preventing the starting of a motor with one phase of the system open. 

A closing point
Voltage unbal­ance and voltage variation are very different terms. Voltage variation is the deviation of voltage from the rated voltage, and NEMA MG1­12.68 allows a plus or minus 10% variation from rated voltage. 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%, an order of magnitude less than the 10% voltage variation tolerance. 

Categories: Troubleshooting & failure analysis

Tags: Voltage unbalance

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