Electric Motor Voltage Quality Problems
From the U.S. Department of Energy's Publication, Energy Matters
By Doug Dorr, EPRI Solutions Inc., and Philip Lim, Memphis Light Gas & Water

Voltage nameplate ratings found on many alternating current (AC) motors and drives can be a source of confusion for utilities and their industrial customers. The confusion stems from the voltage range in which a particular motor may be operated safely. Additionally, voltage unbalance is known to create premature failure of heavily loaded motors if they are not properly derated. This article, the second and final article in our series that began with the Winter 2005 issue, discusses standards associated with AC induction motors and their nameplates and details a range of voltage quality issues that may warrant a problem-solving investigation.

AC induction motors support nearly every facet of industrial production. These workhorses of industry have been estimated to be part of the utilization of over half of the world's electric power generation. The AC induction motor typically has an inherent amount of tolerance to variations in utilization voltage as specified by the National Electrical Manufacturers Association (NEMA), however, utility power quality engineers can spend a great deal of time simply answering customer questions regarding proper utilization voltage for a given motor. While the motor can be operated with variations in the nominal voltage, it is important to understand all of the potential impacts on the supported process as well as on the motor itself.

The voltage quality related factors that tend to create the most serious problems in the field (and the most confusion) are nominal utilization voltage that does not match the motor nameplate, proper voltage sag ride through protection for the motor control circuitry, and phase-to-phase voltage unbalance. With these factors in mind, a systematic approach to investigating and resolving potential problems can be formulated.

Nominal Utilization Voltage

The U.S. standard for motor nameplate information can be found in the NEMA Standards Publication MG 1-2003: Motors and Generators. Motors meeting the criteria contained in the NEMA standard will operate satisfactorily within plus-or- minus 10% of the rated voltage. For example, if the voltage rating on the motor nameplate is 460 volts, that particular motor should operate safely when the utilization voltage is between 414 and 506 volts. However, as the voltage changes-even within the NEMA range-so will the torque, temperature, current, motor speed, and other motor characteristics. Additionally, any increase in operating temperature may accelerate the deterioration of the motor's electrical insulation system. Studies of operating temperature and its effect on insulation life suggest that a rise in steady-state operating temperature of 10 degrees Celsius can reduce insulation life by 50% or more. Table 1 shows some common motor voltages and the range in which the motors may be operated. Table 2 shows the effects of voltage variations on three-phase motors.

Table 1. Utilization Voltage Ranges for Induction Motors (from NEMA Motors and Generators Standard MG-1-1993)
Rated Motor Voltage (V)Rated Motor Frequency (Hz)Minimum Motor Voltage (90%)Maximum Motor Voltage (110%)

* The plus-or-minus 10% voltage rating for AC induction motors assumes that the motor is operated at the nominal frequency. If the frequency is not the same as the nameplate frequency and in particular when 60-hertz motors are operated on 50-hertz systems, the sum of the percent of voltage difference and the percent of frequency difference from the nameplate ratings must not exceed10%. Values are approximate and voltages at or slightly above nominal are preferred for lower operating temperatures and higher starting torques.

Table 2. General Effect of Voltage Variations on Characteristics of Induction Motors (from IEEE Std 141-1993)
Motor CharacteristicVoltage Variation
90% of Nameplate110% of Nameplate
Starting and Maximum Running Torque-19%+21%
Percent Slip+22%-19%
Full-Load Slip-0.2% to -1.0%+2.0% to +1.0%
Starting Current-10%+10%
Full-Load Current+5% to +10%-5% to -10%
No-Load Current-10% to -30%+10% to +30%
Temperature Rise+10% to +15%-10% to -15%
Full-Load Efficiency-1% to -3%+1% to +3%
Full-Load Power Factor+3% to +7%-2% to -7%
Magnetic NoiseSlight DecreaseSlight Increase

Voltage Unbalance

The second voltage quality related issue that the NEMA standard addresses is voltage unbalance. Unbalanced motor voltages may cause a current unbalance that in turn increases the operating temperature and energy losses of the motor. A voltage unbalance can magnify the percent current unbalance in the stator windings of a motor by as much as 6-10 times the percent voltage unbalance. When the voltage unbalance is more than 1%, derating the motor will help to mitigate the effects of the voltage unbalance. If the voltage unbalance exceeds 5%, it is not advisable to operate the motor at all-even when the motor has been derated. When a voltage unbalance exceeds 3%, the root cause of the unbalance should be identified and remedied. In cases where motor failures are occurring repetitively and the unbalance is greater than 1%, it may be prudent to investigate and resolve the root cause of the unbalance.

Voltage unbalance must be treated separately from unusually low or high voltage conditions for three phase motors. As a matter of fact, both conditions in tandem would be a worst case condition for any motor, however there are a couple of sanity checks that can be performed to alleviate concerns (even when both voltage related problems are present). Provided that the motor nameplate current is not exceeded on any of the phase conductors and provided the actual motor speed is greater than or equal to the nameplate revolutions per minute (RPM), one can assume that detrimental affects on the motor are minimal. The condition under which the preceding statement would hold true would be that of a lightly (<50%) loaded motor. This is explained in more detail below, in the section on remedying voltage problems.

Voltage Related Symptoms

Symptoms of motor problems related to either voltage unbalance or to voltages not matching the nameplate rating are not always easy to diagnose because both the utility and facility distribution voltages vary as the system load and other system characteristics vary. Measuring the steady-state voltage at accessible points in the motor circuit is a very good way to determine whether a potential for voltage problems exists. A few symptoms that may trigger such an investigation include:

  • Unusually high numbers of motor failures
  • Not getting the expected motor life between rewinds
  • Unexplained motor trips
  • Motors that are more sensitive to voltage sags than other electrical process equipment
  • Difficulty getting a specific motor started
  • Nuisance tripping of a motor-protective device.

Additional possibilities beyond operating voltage and voltage balance can cause these symptoms. But the list provides a good starting point for deciding whether to conduct a voltage investigation.

Problem Solving Investigation

When a voltage quality problem with a motor is suspected, a proven procedure for investigating the problem is as follows:

Step 1.

Find out enough information about the problem to determine whether the problem is isolated to one motor circuit or is common to the entire facility. This will help determine where to measure and possibly whether the source is internal or external. Develop a worksheet similar to the one shown in Table 3 to record circuit voltages (phase to phase/line to line for all phases), phase currents (using a true-RMS meter to detect the contribution of harmonics, if present), calculate unbalances and to record motor nameplate voltage, current, and revolutions per minute (RPM).

Step 2.

Measure the voltage and current at accessible connection locations between the source transformer and the motor terminals. If the motor is three-phase, record voltage and current measurements for all three phases. If possible, obtain the measurements with the motor not running and also with the motor operating at its maximum steady-state loaded condition. Record the measured values in separate copies of the worksheet. For loads such as a chiller motor, it may also be useful to record steady-state voltages and currents at loading conditions other than full load. Don't forget to measure the coil voltages at the motor control circuit. It is very common to find that the motor tripping problems are associated with sags and low voltages at the control relay and starter coils for AC induction motors.

Table 3 presents a sample motor worksheet in two parts with a formula. In part 1, users enter voltage, current and revolutions per minute (RPM) information. In part 2, users enter measurements. These include voltage, phase A to B; voltage, phase B to C, and voltage, phase C to A. Also to be entered are measurements for current, including current from phase A to B, current from phase B to C, and current from phase C to A. In the formula part of the worksheet, voltage unbalance percent is equal to maximum deviation divided by average multiplied by 100. Current unbalance percent is equal to maximum deviation divided by average multiplied by 100.
Step 3.

If the motor is three-phase, calculate the percent voltage unbalance using the following method. First, average the three voltages (the sum of phase A to B, phase B to C, and phase C to A divided by three). Then, select the phase-to-phase voltage that deviates most from the average. Determine the difference between the average voltage and the maximum deviation from the average. To determine the percent voltage deviation, multiply the difference times 100, and divide that number by the average. For example, if the measured voltages are 462, 465 and 447 volts: 461 + 465 + 447 = 1373; 1373/3 = 458. The greatest variation is 11 volts (458 - 447 = 11). 100 x 11/458 = 2.4% voltage unbalance. Repeat the calculation for percent current unbalance. For every 1% voltage unbalance, expect 6-10% current unbalance. Record both unbalances in the worksheet.

Step 4.

If steps 1 through 3 reveal either 1) a motor current above the rated current, 2) a voltage unbalance above 1% that is not present when the motor is shut off, or 3) a utilization voltage outside the appropriate voltage range in Table 1, do the following before continuing:

  • Inspect all motor circuit elements downstream from the mains disconnect, including contactors, connectors, and conductors.
  • Ensure that all connectors have tight low-impedance connections, including those inside the motor connection box.
  • Ensure that the connectors are compatible with the metallic conductor type used.
  • Ensure that motor contactors are not seriously worn or deteriorated to a point where high resistance is present.
  • Ensure that the motor circuit conductors are properly sized and all of the same conductor material and in similar condition.

If the voltage unbalance is greater than 3% while the motor is not running, then contact your local utility to determine the cause of the unbalance. If one or more problems were found from the above inspections resolve the problems and then complete Steps 1 through 4 before continuing to Step 5.

Step 5.

If Steps 1 through 4 reveal a low voltage, high voltage, or voltage unbalance greater than 1%, consider the following remedies:

If the steady-state voltage is too high or too low:

If the motor utilization voltage is higher or lower than the plus-or-minus 10% specification, or if the user desires that the motor operate closer to the nameplate nominal voltage, several acceptable methods exist for increasing or decreasing the supply voltage. If you decrease the utilization voltage, remember that as the utilization voltage decreases, the susceptibility of motor starters and control circuits to voltage sags will increase.

Utilization voltages can be adjusted via no-load tap changers on existing step-down service transformers. However, changing these taps interrupts the power to all transformer loads. Therefore, entire processes within a facility must be shut down during tap changes. Additionally, changing the taps of the service transformer will affect terminal voltages throughout the plant, potentially changing voltages at equipment that do not require a different voltage.

Step-up or step-down transformers can also be used to adjust utilization voltages. Some transformers, such as the constant-voltage transformer, can also mitigate the effects of voltage sags on motor-control circuits. Another way to adjust a utilization voltage is to boost or buck the voltage with an autotransformer. The buck-boost transformer can be field-connected to increase (boost) or decrease (buck) a utilization voltage from 5-20%, depending on the way the primary and secondary windings are connected. Because only the secondary windings carry current in an autotransformer configuration, a buck-boost transformer may be rated as much as 10 times lower than a fully isolated two-winding transformer. And although buck-boost transformers are single-phase, they can be applied to most three-phase equipment by matching three single-phase transformers. Caution: the transformer impedances must all match when applying single-phase transformer in a 3-phase configuration.

If the voltage unbalance is high:

The root cause of the unbalance condition must be identified and the percent unbalance evaluated to determine what to do. There are a large number of possible causes for voltage unbalance, for example utility supply voltage unbalance, unbalanced single phase loads, high impedance connections, and malfunctioning voltage regulators. In many cases, the checklist from Step 4 above may uncover the root cause of the unbalance and lead to a fairly inexpensive solution. If the unbalance cannot be traced to an internal distribution element or to unbalanced single-phase loads in the facility, the local utility may need to assist by evaluating the percent unbalance of the distribution system, and the condition of the voltage regulation devices.

For a voltage unbalance of less than 1%, no remedial steps are necessary unless nuisance tripping or trouble during startup is associated with the unbalance. As the percent unbalance increases, the likelihood of problems increases. The NEMA standard for voltage unbalance states that a motor will operate satisfactorily at its rated load with a voltage unbalance up to one percent at the motor terminals. The American National Standards Institute (ANSI)/Institute of Electrical and Electronics Engineers (IEEE) C84.1 standard for nominal voltages implies that an adequately designed power system can have up to a 3% inherent voltage unbalance. However, if measurements at the motor terminals indicate more than a 1% voltage unbalance, the motor should be derated according to Figure 2.

 Figure 2 presents a derating graph for induction motors based upon percentage of voltage unbalance. For a 1 percent unbalance, the approximate derating is zero. For a 2 percent unbalance, the approximate derating is 95 percent. For a 3 percent unbalance, the approximate derating is 88 percent. For a 4 percent unbalance, the derating is 82 percent. And for a 5 percent unbalance, the approximate derating is 75 percent. 

The derating curve in the figure can be applied to small and medium motors to minimize overheating. The curve assumes that the motor is already operating at its rated load. However, many motors do not operate at the rated load and are thereby in effect already derated.

A Motor Failure Case Study

An industrial customer called the local utility to report that the plant was experiencing excessive motor failure for no apparent reason. There was no history of motor failures so a utility voltage complaint investigator was dispatched to look into the problem. The customer's power is fed from a three-phase 750 kilovolt-ampere (kVA), 480Y/277 volt transformer. Because the motor failures were occurring on multiple circuits, the initial measurements were taken at the main service panel. Using the steps in the investigation procedure, a definite voltage unbalance was discovered inside of the facility. The measured voltages were:

  • Phase A to B: 469.5 V
  • Phase B to C: 503.3 V
  • Phase C to A: 490.4 V

The average voltage from these readings was calculated to be 487.7 V, with the maximum voltage deviation from this average being 18.2 V (487.7-469.5)

The voltage unbalance at this facility was calculated to be 3.7% [(Maximum Voltage Deviation from the Average/Average) * 100]. This unbalance is above the level where we might expect internal loads and circuits to be the source of the problem.

Current measurements were then taken at the riser pole on the 12.47 kilovolt (kV) side (the feed to the customer's pad mounted 750 kVA transformer). The measured currents were:

  • Phase A = 14.4 A
  • Phase B = 16.1 A
  • Phase C = 17.7 A

Using the formula [(Maximum Current Deviation from Average/Average)*100] the current unbalance for the facility was calculated to be 10.6%.

With the measured results in hand, a decision was made to focus the investigation on the utility source. An investigation of the circuit feeding the facility indicated that potential contributors to the voltage unbalance could either be a line voltage regulator (located 1.6 miles from the facility) or a set of power factor correction capacitor banks farther away. The voltage unbalance problem was explained rather quickly when the investigator read the settings on the line voltage regulator. The setting for A-phase setting was at position 12 buck (lower), B-phase setting at position 4 boost (raise), and C-phase setting at position 8 boost (raise). The voltage unbalance was caused by the malfunctioning of phase A and C regulators. Repairing the malfunctioning voltage regulators solved the problem. While this problem was fairly easy to resolve, the steps described in the investigation section proved useful in identifying the root cause.

A Motor Failure at a Polymers Plant

A polymers processing plant was experiencing an unacceptable number of process dropouts that plant engineers felt were electric power-induced problems. Plant personnel estimated the losses to be greater than $1 million a year with an average of 15 process dropouts annually. The plant was fed electrically from a 12.6 kV circuit prone to numerous types of problems ranging from cars hitting poles to animals faulting the power lines.

An investigation of the critical components at this plant indicated the majority of dollar losses were experienced when kill agents were dumped into the chemical reactors to stop the exothermic (heat generating) reaction. These kill agents are only used in an emergency if facility cooling water is lost due to the motors for the pumps and fans for the cooling process either failing or tripping off line. The result is approximately two weeks' worth of reduced grade (or out of spec) product while the residual kill agent works its way out of each stage of the process.

After discussing the problem with plant personnel it was determined that the kill agent would not have to be injected into the reactor if three critical cooling process components were maintained. These were the instrument control air compressors, the agitator motors for the reactor vessels and the cooling tower fans and pumps. At this particular plant, the voltage balance and nominal operating voltage level at the equipment were adequate, and it was suspected that voltage sags tripping the controls were the source of the problem.

Reviewing the utility's power quality data for voltage variations experienced at the substation feeding the plant indicated that about 90% of the sags were less severe than 50% of nominal voltage and did not last longer than about one-third of a second (20 cycles). Based on this information, it was clear that simply holding the critical process elements in for a half second or so would solve this costly problem.

Control circuit testing with a portable voltage sag generator confirmed the sensitivity of the control relay and motor starter coils to voltage sags. The facility's electrical maintenance group was provided with an overview of the identified problem and given a range of solutions that included pneumatic relays, constant voltage transformers and coil hold-in devices. Once they understood that holding in these processes momentarily would have no detrimental impact on plant or personnel safety they were eager to get the problem solved. The solution was a coil hold-in device that could be mounted in a standard relay socket next to the sensitive relays and starters. The coil hold-in device is connected between the AC source voltage and the coil of the relay or starter to be protected and substantially improves voltage-sag and tolerance. During a voltage sag condition, the device maintains a current flow through the coil sufficient to hold in the contacts. These coil hold-in devices are designed to protect the circuit from voltage sags, but are also designed to drop the circuit out if power is interrupted or if an emergency stop signal is applied.

Because the compressor required manufacturer approval before making modifications to the controls, it was recommended that the manufacturer be supplied with the range of options along with an explanation of the half-second hold in objective. The compressor manufacturer could then propose the best solution for their specific brand that would enable the facility to meet the hold in objective.

Motor power quality is a topic of concern to industrial customers and utility personnel alike. With a proper understanding of the impacts voltage quality may have on AC induction motors and a systematic investigative approach, most problems can be effectively and efficiently solved. As with nearly all power quality related problems, the solutions are simply a matter of having the proper tools and the know how to identify and isolate the root cause of the problem.

Of course having access to a device that can generate voltage sags on demand instead of waiting months for the next event to occur certainly helps out too! - U.S. Department of Energy