by Timothy Thomas, Baker Instrument Company
Predictive maintenance programs (PMPs) are becoming universally accepted as the best method for maintaining motor reliability within most modern plants and facilities. A complete PMP will include as many technologies as possible, with each technology providing vital pieces to the diagnostic puzzle. Periodic static testing and more aggressive dynamic testing of motors are essential parts of predicting the potential for a motor to continue a safe and successful operation. Tracking and trending the results of electric motor testing on a regular schedule is the most effective method of making intelligent predictions.
The need for motor testing
The steady, safe and efficient operation of electric motors is essential to the productivity of all plants and facilities. Some facilities have many critical and/or expensive motors. A motor failure could be catastrophic, causing lost production and costly emergency repairs. For example, a motor failure at a nuclear plant can cost up to $1 million a day for critical motors and may have a disastrous, long-lasting impact. Even failures at a wastewater treatment facility can have a huge, negative environmental effect and can be very costly.
Motors fail due to numerous operational circumstances, including power condition, mechanical influences and environmental hazards. According to IEEE1 and EPRI2 studies, at least 35 to 45 percent of motor failures are electrically related. Monitoring the motors “electrical health” is, unquestionably, an important and vital consideration.
Trending the historical operating condition of a motor makes early detection of any decline in the motors “health” possible. Planning downtime and having only minor reconditioning repairs instead of a major rewind or replacement is far less expensive in both repair costs and lost production. Since electric motors begin deteriorating the instant they are started, it is necessary to monitor their operating condition on a routine, periodic schedule. Periodic monitoring and trending of data collected and properly diagnosed provides the technician with evidence needed to prepare for downtime before a catastrophe occurs.
It is no longer practical to just “megger” a motor in order to determine its condition. Plants and facilities depend on a complete predictive maintenance program (PMP) to monitor their operations and plan their repair schedules. A good PMP requires both static (and off-line) and dynamic (or on-line) testing, with educated and trained technicians monitoring data routinely, with quality equipment.
Besides voltages and currents, on-line test equipment must be able to capture and trend torque ripple and torque signatures as well as rotor bar sidebands. Off-line testing with modern, high-voltage test equipment is essential to getting reliable data. The voltages required to properly test motor windings cannot be reached with impedance-based or low-voltage test equipment.
On-line testing
Effective dynamic test equipment must be able to collect and trend all essential data that affects the operation of electric motors. Power condition – including voltage level, voltage imbalance and harmonic distortions, current levels and current imbalances, load levels, torque signatures, rotor bar signatures, service factors and efficiencies – should be tracked and trended.
On-line testing is performed at the motor’s MCC, at the load side of a variable frequency drive or at an installed port, which allows for on-line testing without opening the MCC. Data is collected through a set of current transformers and corresponding voltage probes. The data collected, processed and analyzed provides the technician with an overall view of the normal operational environment to which the motor is subjected on a daily basis and of how the motor is responding within this environment.
Often, a motor is subjected to incoming power problems including low or high voltages, voltage imbalances and harmonic distortions. Lower voltages cause higher currents and, therefore, more heat. Higher voltages cause lower power factors and ultimately higher losses. A small amount of voltage imbalance creates an exponential amount of current imbalance which causes temperature increases. Harmonic distortion also causes thermal stress in motors. Any of these voltage problems can cause severe overheating in the motor even without factually reaching an over-load situation, and excessive heat is the insulation’s major enemy. Some motors are subjected to physical obstacles that cause undue stress. Over-greasing, misalignment and over-tightened belts all cause thermal stress.
Many motor failures can be traced to load related situations. Erratic torque signatures can be an indicator of load-related problems. Broken or cracked rotor bars also can cause premature motor failures. On-line testing identifies these problems, and routine trending will reveal the rate of decline. Of major importance to the overall health of a motor also is the “effective service factor.” Two elements affect the service factor number are: real operating power condition (voltage quality) and steady state load conditions. The effective service factor number represents the thermal stress caused by these two conditions on the motor. On-line testing can find and trend all of these motor conditions.
Dynamic testing schedules should be tailored individually according to operating time, criticality and any other important element of operation. Generally, an on-line test should be performed at least quarterly. Motors that begin to show obvious decline or thermal over-stressing should be monitored more closely until the motor can be statically tested or removed from operation and repaired. New and recently repaired motors should be tested as soon as they are returned to service in order to provide a historical record (or baseline) of their performance when the motor is at its “best.”
Off-line testing
In general, motors are quite reliable, and when properly maintained, you should expect at least 100,000 hours of continual operation. That is to say, a new motor operated within nameplate parameters should give us at least 11 years of steady use. Unfortunately, motors are almost always subjected to a variety of damaging elements, with the end result being a rise in operating temperature. Thermal aging of the insulation is the major cause of insulation failure. Years of testing and numerous studies have shown that, as a “rule of thumb,” “for every 10 degrees centigrade increase in temperature, the winding life is decreased in half.”3
Besides thermal problems, other causes of insulation failures include incoming line related problems. Spikes caused by lightning and surges created by switching and contactor closing contribute to insulation breakdown. Motors also are subjected to mechanical influences including bearing failure, environmental hazards and magnet wire damage caused during the manufacturing process. Even the physical movements of the windings during startups causes wear to the insulation system, especially the magnet-wire insulation, as D.E. Crawford has shown.4
Proper testing of all components of a motor requires a series of tests designed to emulate the conditions the motor will see in the field. It has been proved in numerous studies that low-voltage testing, including capacitance, inductance, impedance, etc., are not effective tools in verifying insulation problems. Quality off-line test equipment will be able to perform winding resistance tests, insulation-resistance tests, high-potential (HiPot) tests, polarization index, and surge tests at IEEE, NEMA and EASA accepted standards. Top-quality test equipment will automatically run a series of preprogrammed tests and provide a complete final report.
This automatic equipment will stop testing before any damage is done to the windings.
The resistance test verifies the existence of dead shorts within the turn-to-turn coils and shows any imbalances between phases due to turn count differences, along with locating poor wire connections or contacts and finds open parallel coils.
DC insulation resistance testing detects faults in ground wall insulation or motors that have already failed to ground. Weak ground wall insulation (prior to copper-to-ground failure) can only be found successfully with the HiPot tests. The ground wall insulation system consists of the magnet wires insulation, slot liner insulation, wedges, varnish and often phase paper. DC HiPot test should be performed at twice line voltage plus 1,000 volts since motors will see voltage spikes of at least that level during each startup. HiPot testing is necessary to verify winding suitability for continued service.
Surge testing detects faults in both inter-turn winding and phase-to-phase insulation systems. Turn-to-turn faults will not be seen by a megger or HiPot test. Potential faults can only be seen when the coils see more than 350 volts from turn-to-turn or coil-to-coil, as described by Paschen’s Law 5 (Figure 1). The typical mechanism of fault progression is a turn-to-turn short, causing excessive heat and progressing within seconds or minutes to copper to ground faults. Faults are much more likely to occur between turn-to-turn winding coils due to the added stress caused by bending and exaggerated during the winding process. The ground wall insulation is generally many times stronger and more capable of withstanding voltage spikes and other stresses.
Figure 1.Conclusions
Integrating on-line and off-line testing into a predictive maintenance program provides the technician with verification of his motor’s condition (see case studies below). Both technologies are necessary in order to have a complete picture of a motor’s health.
Collecting both on-line and off-line data on a routine schedule allows for early warning of impending failures and opens the opportunity window for planned downtime. Performing resistance, HiPot and surge testing along with dynamic testing provides the technician with a total picture of the motor’s condition and allows him or her to track its rate of decline.
Modern test equipment includes enhanced and detailed reporting. Reports are easily generated, providing a written hard copy of test results and making diagnosing and comparing of data clearer and more accurate. Setting up and managing a program to monitor the motors within any facility is essential to insure the safe and continued operation and production of the facility. In most cases, a properly managed and operated PMP will save a plant or facility much more than it will cost to implement, administer and manage.
Case studies
1) At a large wastewater treatment facility in Florida, 14 identical motors were scheduled for predictive maintenance. These motors were 40-horsepower aerators for a large treatment tank and operated continuously. Static tests were performed on all 14, and each received passing marks on all tests. When dynamic testing was complete, it was noted that 13 motors were acting very similar running within expected parameters at approximately 85 percent load, while the 14th motor was running at just over 30 percent load. Further inspection revealed a sheared coupling on the motor running at reduced load. The operators had no way of detecting the problem, and the location of these motors made visual inspection difficult. The dynamic testing found a problem that was costing the customer both in wasted kilowatt usage and production.
2) Twelve 60-horsepower pump/motors were tested at a large office building. Six were chilled water pump/motors and six were condenser water circulating pump/motors. All 12 were installed at the same time and ran continuously. Dynamic testing was performed one day on all 12, and all appeared to be operating within expected parameters. The motors were shut down for a scheduled annual routine building maintenance, and static testing was planned for the following morning. Resistance tests appeared normal on all, but two would not pass HiPot testing at the preset voltage. Three others failed the surge tests. The five motors were removed from service, disassembled and inspected. Two were found to be extremely dirty, while three had no visual damage. All five were reconditioned, retested and replaced in service. The off-line testing prevented five potential catastrophic failures and allowed the customer to dictate the downtime.
Source: reliableplant.com
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