Designed with longevity in mind, electric motors are made to function safely, economically, and perpetually. In theory, proper care and use of these components will ensure many years of successful operation. However, the real world is a different story, where motors are subjected to numerous environmental and operational hazards that cause thermal stress on the windings. Thermal stresses degrade the insulation quality of the motor, thereby diminishing its life expectancy. That's why monitoring the operating condition of motors has become such an important aspect of predictive maintenance programs.
For many years, maintenance and reliability engineers were limited to static or off-line testing of motors — determining a motor's suitability for continued duty by using a handheld volt/ohm meter, clamp-on ammeter, or megohmmeter. But quite often these tools became forensic devices used mainly to diagnose “what happened” after the motor failed.
Although static testing remains an essential component in monitoring and trending motor health, the methods are limited and are mainly intended to define insulation quality. Modern online test equipment aids the technician in predicting imminent, insulation-related failures. The early detection of these potential problems enables him to schedule “downtime” and make scheduled repairs rather than perform emergency maintenance.
Online motor testing. Dynamic or online test equipment monitors motors in their natural environment. Dynamic testing provides details about power quality, the motor's operating condition, and a very large amount of information related to the load.
Modern test equipment collects data through three voltage probes and three corresponding current transformers, which are attached directly to the piece of equipment. More advanced equipment has the added ability to collect data through permanently mounted equipment located in the control cabinet of low-voltage motors or the low-voltage cabinet of medium- and high-voltage motors. Once recorded, the data is then calculated and interpreted by the test equipment and displayed for comparison, reporting, and trending.
The data collection process is non-intrusive, allowing the technician to collect a vast amount of information without interrupting operations or processes. Besides being nonintrusive, the dynamic tester is able to receive, store, diagnose, and trend information within seconds. Then, the technician can transfer the data to a desktop PC for further analysis and sharing with other maintenance personnel. Numerous areas of concern like high temperature readings or overvoltage situations, previously unavailable to most predictive maintenance personnel and reliability technicians, are now quickly and easily captured and displayed with electronics-based equipment.
Five key indicators. To fully analyze a motor's health, the technician must be able to see the complete picture, including all aspects that affect the normal operation of the motor. These areas of concern are grouped in five main categories: power condition, motor performance, motor condition, load and energy assessment, and operating efficiency.
Power condition. Incoming power quality is often over-looked by maintenance personnel, who usually only track and trend voltage levels. However, power condition has a real and significant effect on the proper and safe operation of motors.
Voltage unbalances are not uncommon, and voltage levels change throughout the day. Some plant electricians believe higher voltages cause no real damage to motors because the condition allows for lower current levels. They will actually raise voltages to a level that will bring the motor's current down to nameplate ratings, even though the motor is operating above rated horsepower. This myth is proven untrue because the resulting iron saturation causes excessive heat, which, in turn, degrades the insulation rapidly. As is well established and documented, lower voltages result in higher currents and, again, more heat buildup in the windings.
Power quality is also affected by harmonic distortion, much of which is created by variable-frequency drives. Modern plants and facilities use many drives that create some level of harmonic distortion. A small amount of voltage distortion results in a larger amount of current distortion, which, in turn, causes current levels to increase. This increase in current and distortion increases heat within the windings of a motor, which degrades its insulation and shortens its life. Plants can also be subjected to harmonic distortion from nearby sources outside their own perimeters. For example, other facilities in the vicinity can send harmonic problems upstream and, eventually onto the buss of its neighbors. More often than not, these distortions go undetected.
Motor performance. Tracking motor current levels and unbalances is a key component of a good online monitoring system. Sometimes it's found that current level is defined as the average current level for all phases. For predictive maintenance reasons, however, the highest line current level developed is of much greater importance. The heat in each motor winding is interdependent on the amount of current that flows through it. Hence, the weakest point of the motor's insulation with respect to current level is the phase with the largest current.
Motors typically see six to 10 times rated current levels during each start. “Hot” starts are even more detrimental to the windings because the excessive heat generated during this period causes rapid insulation degradation. Besides collecting and storing the normal operating data, modern dynamic test equipment is able to capture startup information, including voltage, current, and torque levels. This information displays the enormous stress to which a motor is subjected each time it's started. Many motors fail during this startup period due to these stresses.
Motor condition. You can think of motor condition as anything relating to a motor's health. Rotor bar problems are one issue that directly affects the motor's ability to handle its load. Rotor bar problems have become a major concern for reliability engineers, who spend a great deal of time investigating the recorded dynamic data associated with this issue in order to locate and define this problem. Rotors are subjected to massive stresses during startups, during fluctuating load conditions, and during power condition problems. Monitoring these stresses is of vital importance when determining a motor's ability to continue in operation. Broken or cracked rotor bars are responsible for losses in efficiency and dramatic increases in thermal stress.
Motor condition can also be defined as changes in the system that affects the operating condition of the motor. Operating condition is the complex signature of the motor reacting to load and power conditions. Tracking and trending these conditions provides evidence that the motor's environment has changed, and further scrutiny of data is needed.
Load and energy assessment. Load, percentage of load, horsepower demand, kilowatt usage, and power factor are also important factors relating to a motor's health. Significant load fluctuations might indicate a potential load-related problem. Many motors continue to operate within nameplate current ratings while being forced to carry load demands above their capacity. These excessive load demands cause the windings to run above a safe temperature level. Motors required to operate above nameplate horsepower ratings also suffer from greater torque demands, which inflict undo stress on the motor's rotor.
Dynamic test equipment can be set up to track torque data, including torque level and periodic fluctuations known as “torque ripple.” These torque signatures are visible pictures of the load's operational condition and offer insight into the stresses subjected on the motor's rotor. Monitoring and trending torque as a function of the everyday operating condition of a motor is essential when separating electrical concerns from mechanical issues. Bearing faults, variable-frequency-drive-related problems and many mechanical issues can be seen and diagnosed via the torque spectra (Fig. 1).
Operating efficiency. The never-ending efforts to reduce a plant's cost of operations dictates that facilities personnel carefully monitor power usage and motor repair/replacement costs. Often motors have been oversized or undersized due to incorrect engineering or subsequent changes to a connected machine. Oversized motors have higher initial costs and are typically more costly to repair and operate. Undersized motors perform poorly and suffer from higher losses, which forces them to fail much sooner than properly sized motors. In other words, improperly sized motors are less efficient, and therefore more costly to operate. Dynamic testing provides sufficient information about the motor's overall operating environment to allow a qualified technician to properly evaluate the sizing of all replacement motors.
Summary of benefits. Dynamic motor testing is quickly becoming a crucial part of all good predictive maintenance programs, as new innovations continue to become available in the form of both software and hardware upgrades. It can be performed much more often and without concern of interrupting any processes or plant operations. If not for the dynamic test, many problems that create insulation degradation would be overlooked.
Recent developments in dynamic test equipment capabilities have expanded their parameters, horizons, and expectations. Besides defining electrically related problem areas, many mechanical issues can now also be diagnosed, including bearing faults, imbalance, mechanical and physical looseness, and eccentricity.
All this capability provides support to the reliability engineer's approach to maintaining a safe, effective, and productive operation. But merely collecting data is not sufficient when managing a predictive maintenance program. As with any technical field, proper training and education is necessary for a reliability technician to successfully and properly diagnose and use the dynamic test data. In order to correctly manage a plant or facility motor reliability program, today's predictive maintenance personnel must be highly trained, use the newest equipment, and remain in the educational loop.
Thomas is an applications engineer at Baker Instrument Co., Fort Collins, Colo.