What consumes more than half of the world's electricity and supports nearly every facet of industrial production? The answer is the AC induction motor. Although these motors typically tolerate variations in utilization voltage, power quality professionals continue to spend a great deal of time answering questions about proper utilization voltage for a given motor. While plant personnel can operate the motor with variations in the nominal voltage, they must understand all potential impacts on the motor and the supported process.
The voltage quality factors that create the most serious problems and confusion in the field include 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 imbalance. With these factors in mind, we can formulate a systematic approach to investigate and resolve potential power quality problems.
Matching the Motor Nameplate
You can find the United States standard for motor nameplate information in the NEMA Motors and Generators Standard MG-1-1993. Motors meeting the criteria contained in the NEMA standard will operate safely within 510% of the rated voltage. For example, if the voltage rating on the motor nameplate is 460V, that motor should operate safely when the utilization voltage is between 414V and 506V. As the voltage changes (even within the NEMA range), so will the torque, temperature, current, motor speed, and other motor characteristics. 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 a rise in steady-state operating temperature of 10°C can reduce insulation life by 50% or more.
Table 1 shows some common motor voltages and the range in which you may safely operate them. Table 2, on page 42, shows the effects of voltage variations on 3-phase motors.
Imbalanced motor voltages may cause a current imbalance that increases the operating temperature and energy losses of the motor. In fact, these voltages can magnify the current imbalance in the stator windings of a motor as much as 20 times. When the voltage imbalance is more than 1%, derating the motor will help mitigate its effects. If the imbalance exceeds 5%, it is not advisable to operate the motor at all — even if you have derated it. When a voltage imbalance exceeds 3%, you should identify and remedy its root cause.
You must treat voltage imbalance separately from unusually low- or high-voltage conditions for 3-phase motors. Although the presence of these conditions simultaneously is a worst-case scenario for any motor, you can conduct several checks to alleviate concerns. If the motor nameplate current is not exceeded on any of the phase conductors and the actual motor speed is greater than or equal to the nameplate rpm, you can assume detrimental effects on the motor are minimal. The preceding description holds true with a lightly loaded (less than 50%) motor.
Motor problems related to voltage imbalance or voltages not matching the nameplate rating are not always easy to diagnose. This is because the utility and facility distribution voltages vary at the same time the system load and other system characteristics vary. Measuring the steady-state voltage at accessible points in the motor circuit is a good way to determine whether a potential for voltage problems exists. Symptoms 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, or
Nuisance tripping of a motor-protective device.
Problem Solving Investigation
When you suspect a voltage quality problem with a motor, investigate it with this proven procedure:
Step 1. Determine if the problem is isolated to one motor circuit or common to the entire facility. This will help you decide where to measure and possibly whether the source is internal or external. Develop a worksheet, similar to the one shown in Table 3, to calculate imbalances and record circuit voltages, phase currents, motor nameplate voltages, currents, and rpms.
Step 2. Measure the voltage and current at accessible connection locations between the source transformer and the motor terminals. For 3-phase motors, record voltage and current measurements for all three phases. If possible, obtain the measurements two ways: with the motor not running and with it operating at its maximum steady-state loaded condition. Record these values on separate copies of the worksheet.
For loads such as a chiller motor, it may 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. Motor tripping problems for AC induction motors are commonly associated with sags and low voltages at the control relay and starter coils.
Step 3. If the motor is 3-phase, calculate the percent voltage imbalance 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. Divide that phase-to-phase voltage by the average. Repeat the calculation for percent current imbalance, and record both imbalances in the worksheet.
Step 4. If steps 1 through 3 reveal a motor current above the rated current, a voltage imbalance above 1% and not present when the motor is shut off, or a utilization voltage outside the appropriate voltage range in Table 1, do the following before continuing to Step 5:
Inspect all motor circuit elements downstream from the main disconnect, including contactors, connectors, and conductors.
Ensure all connectors have tight low-impedance connections, including those inside the motor connection box.
Ensure the connectors are compatible with the metallic conductor type used.
Ensure motor contactors are free from serious wear or the presence of high resistance.
Ensure motor circuit conductors are properly sized, made of the same material, and in similar condition.
If the voltage imbalance is greater than 3% while the motor is not running, contact your utility to determine the cause. If the above inspections reveal one or more problems, take steps to resolve them, then repeat steps 1 through 4 before moving to Step 5.
Step 5. If steps 1 through 4 show a low-voltage, high-voltage, or voltage imbalance greater than 1%, consider the following remedies:
If the steady-state voltage is too high or too low: There are times when the motor utilization voltage is higher or lower than the 510% specification, or you want the motor to operate closer to the nameplate nominal voltage. In these cases, there are several acceptable methods for increasing or decreasing the supply voltage.
If you decrease the utilization voltage, the susceptibility of motor starters and control circuits to voltage sags will increase. You can adjust utilization voltages via no-load tap changers on existing step-down service transformers, but changing these taps interrupts the power to all transformer loads. Therefore, shut down entire processes within a facility if you go this route. Tap changes will also affect terminal voltages throughout the plant, potentially altering equipment voltages that do not require a change.
Other methods of adjusting utilization voltages include step-up or step-down transformers. Other transformers, such as constant-voltage transformers or autotransformers, also can mitigate the effects of voltage sags on motor-control circuits. Fig. 1 shows how you can field connect a “buck-boost” autotransformer to increase (boost) or decrease (buck) a utilization voltage from 5% to 20%, depending on the way the primary and secondary windings are connected. A buck-boost transformer may be rated up to 10 times lower than a fully isolated two-winding transformer because only the secondary windings carry current. Although buck-boost transformers are single-phase, you can apply them to most 3-phase equipment by matching three single-phase transformers.
If the voltage imbalance is high: There are several possible causes for voltage imbalance, including imbalanced single-phase loads, high-impedance connections, and malfunctioning voltage regulators. In many cases, checking the list from Step 4 will uncover the root cause and lead to a fairly inexpensive solution. If you cannot trace the imbalance, ask the local utility for assistance. Its staff can evaluate the percent imbalance of the distribution system and the condition of the voltage regulation devices.
For voltage imbalances less than 1%, no remedial steps are necessary — unless nuisance tripping or trouble during start-up is associated with the imbalance. As the percent imbalance increases, the likelihood of problems increases. The NEMA design standard states that a motor will operate properly at its rated load with a voltage imbalance up to 1% at the motor terminals. The ANSI/IEEE C84.1 standard implies that an adequately designed system can withstand a 3% inherent voltage imbalance. However, if measurements at the motor terminals indicate more than a 1% voltage imbalance, derate the motor according to the factors indicated in Fig. 2.
To avoid overheating, apply the derating curve in Fig. 2 to small and medium motors. The curve assumes that the motor is already operating at its rated load. Many motors, however, do not operate at the rated load — they are, in effect, already derated. See the procedure in the next section to find out how to estimate the load on a motor.
Estimating Motor Load
Use the following procedure and Table 4 (on page 45) to estimate motor load:
Step 1. Determine the synchronous speed of the motor by using the equation:
ns = 120 f/p
Where ns is the synchronous speed of the motor in revolutions per minute (rpm), f is the frequency of the input line power (60 Hz in the U.S. and 50 Hz overseas), and p is the number of motor poles. The no-load motor will run very close to the synchronous speed. The rotor speed of a loaded motor, however, will be slower than the synchronous speed because then the motor tends to slip.
Step 2. Record the full-load speed of the motor from the motor nameplate.
Step 3. Calculate the full-load slip by subtracting the full-load speed in Step 2 from the synchronous speed in Step 1. Slip is the difference between the motor's synchronous speed and the actual operating speed in rpms.
Step 4. Measure the speed of the motor during normal operation. Many motor supply vendors have instruments to measure the motor speed easily. A couple of non-intrusive handheld examples include strobe meters and shaft tachometers. The strobe meter requires the calibration of the strobe frequency with a mark on the spinning motor shaft. The shaft tachometer is pressed directly against one end of the rotating shaft, which causes the tachometer to spin at the same speed as the shaft.
Step 5. Calculate the real slip of the motor by subtracting the measured speed in Step 4 from the synchronous speed in Step 1.
Step 6. Calculate the estimated motor load by dividing the real slip in Step 5 by the full-load slip in Step 3.
At 75% of the motor capacity, the motor is in effect derated for a 5% voltage imbalance. You should calculate percent loading to the derating curve in Fig. 2 (on page 44) to ensure the motor is sufficiently derated for the calculated voltage imbalance in the worksheet.
Having an understanding of voltage quality impacts on AC induction motors and a systematic investigative approach will help you solve most problems effectively and efficiently. The solutions are simply a matter of having the proper tools and the know-how to identify and isolate the cause. Of course, access to a device that generates voltage sags on demand certainly helps out, too!
Motor Failure Case Study No. 1
An industrial customer called its utility to report the plant's unexplained, excessive motor failure. There was no history of failures, so the utility dispatched a complaint investigator to look into the problem.
The customer's facility is fed from a 3-phase, 750kVA, 480Y/277V transformer. The investigator took the initial measurements at the main service panel because the failures were occurring on multiple circuits. By using the steps outlined in this article, the investigator determined that a voltage imbalance existed inside the facility. The measured voltages showed 469.5V for Phase A to B, 503.3V for Phase B to C, and 490.4V for Phase C to A.
The average voltage from these readings came out to 487.7V, with the maximum voltage deviation from this average equaling 18.2V (487.7 minus 469.5).
The investigator calculated the voltage imbalance at 3.7% [(i.e., the maximum voltage deviation from the average/average) 2100]. This imbalance 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 12.47kV side (i.e., the feed to the customer's pad-mounted 750kVA transformer). The measured currents were 14.4A for Phase A, 16.1A for Phase B, and 17.7A for Phase C.
Using the formula, (maximum current deviation from average/average) 2 100, the current imbalance for the facility figured at 10.6%.
After analyzing the measured results, the investigator's focus shifted to the utility. An examination of the circuit feeding indicated potential contributors to the imbalance included a line-voltage regulator (located 1.6 miles from the facility) or a set of power-factor correction capacitor banks farther away.
When the investigator read the settings on the line-voltage regulator, the problem quickly became apparent. The A-phase setting was at position 12 buck (lower), the B-phase at 4 boost (raise), and the C-phase at 8 boost (raise). The malfunctioning of the phase A and C regulators caused the voltage imbalance, and repairing the malfunctioning voltage regulators solved the problem. While this problem was fairly easy to resolve, the steps described in “Problem Solving Investigation” (page 42) proved useful in identifying the root cause.
Motor Failure Case Study No. 2
Electric problems are the prime suspects for an unacceptable number of process dropouts occurring at a polymer processing plant. Estimates indicate annual losses totalling $1 million, with an average of 15 process dropouts per year.
The plant is fed from a 12.6kV circuit, which is prone to numerous problems ranging from cars hitting poles to animals faulting the power lines. An investigation indicates the majority of monetary losses come from dumping kill agents into the chemical reactors to stop the exothermic (heat generating) reaction.
The plants use these kill agents only in emergencies — for example, when the facility loses cooling water because the motors for the cooling process' pumps and fans fail or trip offline. If this occurs, the plant produces approximately two weeks worth of reduced (out-of-spec) product while the residual kill agent gradually works its way out of each stage of the process.
After discussing the problem with plant personnel, investigators determined the kill agent would not have to be injected into the reactor if the staff maintained three critical cooling process components: the instrument control air compressors, the agitator motors for the reactor vessels, and the cooling tower fans and pumps.
This plant had adequate voltage balance and nominal operating voltage level at the equipment. Investigators suspected the root cause stemmed from voltage sags tripping the controls. Reviewing the utility's power quality data for voltage variations 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 ⅓ of a second (20 cycles). Based on this information, investigators knew simply holding the critical process elements in for a half second or so would solve this costly problem.
EPRI-PEAC's control circuit testing with a portable voltage sag generator confirmed the sensitivity of the control relay and motor starter coils to voltage sags. Plant personnel received an overview of the identified problem and a range of solutions including pneumatic relays, constant voltage transformers, and coil hold-in devices. Once they understood momentarily holding in these processes would have no unsafe effects, they were eager to solve the problem.
Another key criterion for the staff was a cost-effective solution that would fit inside its existing control cabinets without the need for an extensive redesign. The solution: a coil hold-in device they could mount in a standard relay socket next to the sensitive relays and starters. The device is connected between the AC source voltage and the coil of the relay or starter. During a voltage sag condition, it maintains a current flow through the coil while sufficiently holding the contacts.
Before making control modifications, the compressor required manufacturer approval. It was recommended that the plant supply the manufacturer with the range of options, along with an explanation of the ½-sec hold-in objective. The manufacturer could then propose the best solution for its specific brand, enabling the facility to meet its objective.