By Han Tran, application engineer, Fluke Corp.
Industrial and manufacturing processes depend upon functional and efficient electrical motors. They’re critical machines that consume about 70% of a plant’s total energy, and the efficient usage of electricity can make all the difference between financial gains and losses. Uptime and profitability often are hit hard by failed motors, sending maintenance technicians and engineers scrambling for a solution. Consequently, looking toward improved and sustained motor efficiency is an obvious focus area for saving money.
Common power quality problems are classified into two wide-ranging areas: voltage anomalies and harmonic distortion issues. Voltage anomalies can cause numerous problems that can be corrected easily by spotting the symptoms before the motor stops working. Symptoms include anomalies such as dips and swells. Dips can cause intermittent loss of power, while swells can cause physical damage to instruments.
After securing a set of up-to-date electrical drawings to determine where best to place voltage probes and current clamps, you can then use a power quality analyzer to measure and record the specific parameters associated with power quality. A power quality and motor analyzer (see photo) helps identify and troubleshoot in three-phase and single-phase power distribution systems, and companies can see the benefits of consistent monitoring reflected in their bottom line.
Your troubleshooting efforts can be aided by using other tools, such as a data logger, thermal imager, infrared thermometer and recording digital multimeter. For a longer-term monitoring, semi-permanent- or permanent-install condition monitoring instruments are available, although, the initial capital investment sometimes is tough to justify the purchase. That’s why condition monitoring is an investment that larger facilities deploy, because the benefit of avoiding route-based maintenance outweighs the cost.
Advanced power quality analyzers also employ consistent firmware updates for continuous improvements. For example, there are analyzers that now can support variable-speed drives (VFDs) and voltage-source inverter drives with specific output frequencies, while staying in sync with motor design types reinforced by the International Electrotechnical Commission (IEC) and the National Electrical Manufacturers Association (NEMA).
Firmware updates to these tools make iterative changes to features, from improved user interface to default measurement settings and other software performance issues. These slight adjustments can make a notable difference in measurement accuracy when getting to the root cause of a power quality problem.
So, what are the 6 most common problems affecting power quality, and what can you do to mitigate them?
These are responsible 80% of all power quality issues. A dip or sag occurs when the system voltage drops to 90% or less of nominal system voltage for a half-cycle to one minute. The most common symptoms of dips include:
Tips for troubleshooting: Monitor at the load where the dip symptoms first occur. From there, you can compare the time of the equipment’s failure to when the voltage dip happened. If you can’t find a correlation, then a voltage dip likely isn’t the problem plaguing your machine. Simply proceed to troubleshoot by monitoring power farther upstream.
To deepen your learnings, make sure to reference plant one-line drawings to find out if large motors starting up are producing the dip, or if it’s an unexpected increase in current demand.
2. Voltage Swells or Surges
Occurring about half as often as dips, voltage swells or surges are increases in system voltage for short periods up to a cycle or more. These inevitably lead to bigger problems, and as with all power quality problems, you must monitor for a period of time, then observe and interpret the results.
Common symptoms include the immediate failure of equipment, typically the power supply section of electronics. Some failures might not be so instantaneous because of voltage swells that occur over a period of time and prematurely break down components.
Tips for troubleshooting: If analysis of electronic equipment reveals faulty power supplies, monitor voltage trends on the feeders and branch circuits feeding the equipment. Where possible, compare failure rates of similar equipment operating on portions of systems known not to be experiencing swells.
Look for any sudden line-to-ground faults on a single-phase line when analyzing power quality survey results. Voltage swells also can be caused by large plant loads abruptly dropping offline and power factor correction capacitor switching.
3. Voltage Transients
Transients, sometimes referred to as spikes, are significant surges in voltage that last for only microseconds. Common causes include mechanical switching and lightning strikes; the latter often don’t require power quality monitoring, because the source of equipment failure occurs during a storm.
Other causes of transients include:
Tips for troubleshooting: For these transient conditions, monitor at the load and correlate the equipment’s operational problems or failure with distribution system events. Normal arcing across contacts by interrupting large loads can be a cause of transients. Use the facility one-line to move the monitoring farther upstream in the distribution system until you find the source.
4. Voltage Interruptions
These will last anywhere from two to five seconds, or even longer. Interruptions that endure beyond the 5 sec. mark typically are referred to as sustained interruptions. Equipment stops operating, and most motor control circuits won’t reboot after a brief power interruption.
Tips for troubleshooting: First, the bad news. If a voltage interruption occurs when equipment isn’t being monitored or watched, it will be very difficult to properly identify the cause of the equipment shutdown. However, with a power quality analyzer, you can continuously monitor, then correlate the time of any power interruptions to the time of equipment issues in identifying voltage interruptions.
5. Voltage Unbalance
Three-phase systems face voltage unbalance regularly, but it’s often unnoticed and can result in irreversible equipment damage. Unbalance can occur at any point throughout the distribution system.
To go further, loads should be equally divided across each phase of a panelboard. Should one phase become burdened by higher when compared to the other phases, voltage will inevitably be lower on that phase. Transformers and three-phase motors fed from that panel may run hotter, be atypically noisy, vibrate excessively, and even undergo untimely failure.
Tips for troubleshooting: A voltage unbalance of 2.3% on a 230V motor results in a current unbalance of almost 18%, causing a temperature rise of 30° C. While a digital multimeter (DMM) and some quick calculations can be used for averaging voltage readings, a power quality analyzer provides the most accurate information about unbalance.
Monitoring over time is the key to capturing unbalance. In a three-phase system, the maximum variation in voltage between phases should be no more than 2% (the Vneg % value on the analyzer), or significant equipment damage can occur.
Harmonics are voltages and currents whose frequency is said to be an integer multiple of the fundamental frequency. These unwanted frequencies cause many symptoms, including overheating in neutral conductors and the transformers supplying these circuits. Reverse torque creates heat and efficiency losses in motors.
The most severe symptoms created by harmonics typically are the result of the harmonics distorting the fundamental 60 Hz sine wave found in facilities. This sine wave distortion results in improper operation of electronic equipment, false alarms, data losses, and what often are reported as “mysterious” problems that confound engineers and maintenance technicians.
Tips for troubleshooting: When symptoms of harmonics occur, troubleshoot by observing total harmonic distortion (THD). Significant increase in THD under varying load conditions warrants a percentage comparison of each individual harmonic current level as compared to the total fundamental current flow in the system. Knowing the effects created by each harmonic current and comparing them to identified symptoms will aid in troubleshooting. The source of the harmonic must then be isolated and corrected.
Persistent monitoring with a power quality analyzer helps keep maintenance professionals informed about their motor’s health and efficiency, while providing opportunities to catch symptoms before they manifest as unplanned downtime. Knowing the common power quality problems, how to identify their potential causes, and then troubleshoot pushes your facility to operate from a more ideal state of sustained energy efficiency and financial security.
Fluke Corp. is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™. Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.