by Timothy M. Thomas
Motors are a part of a "system" which includes the incoming power, the load and, of course, the motor itself. Maintaining the safe and profitable operation of plants and facilities, demands a high degree of motor reliability. The power generation industry ranks at the top of this requirement for uninterrupted operation and safe, continued production. This paper seeks to present the most current, effective and widely accepted methods of electrically testing and trending the operational health of electric motors. The benefits and features of various modern electrical test equipment and testing methodologies will also be discussed.
Predictive maintenance programs are only effective when all available means of measuring and trending the condition of electric motors, cables and switch gear are utilized. Modern test equipment exist to both simulate "real-world" situations in static testing and acquire safe and complete dynamic data in the motor's natural environment.
Static data helps define the motor's insulation integrity and modern equipment is capable of aiding technicians in predicting imminent failures before they become catastrophic. Effective static test equipment is capable of testing the components of motors at voltage levels similar to those the motor will see in its normal operation without destructive currents. Static testing should include the surge test which is the most effective method of insuring the integrity of the copper-to-copper insulation. Modern test equipment will provide trending logs and reports allowing technicians to track the decline in the motors health.
State-of-the-art dynamic test equipment can locate power related issues and load problems as well as motor condition problems. The most modern of equipment can calculate speed and torque, define rotor bar problems and measure distortion. Dynamic testing of electric motors is a relatively new field that has huge potential and is growing rapidly. Numerous mechanical issues are being identified including bearing problems, mechanical looseness and many other concerns. Dynamic testing will help separate mechanical from electrical issues and provides extensive information regarding the root-cause of motor failures.
An effective Predictive Maintenance Program must include both static and dynamic motor testing if the program is to be successful. Each defines specific areas of concern regarding the motor system, each has its limitations, and each lends support to the other.
What are we really after?
The goal of a predictive maintenance program is always the same. Reducing unscheduled downtime by predicting imminent failures and identifying problem areas, determining the root-cause problem of failures and, ultimately, to save money.
Electrically testing motors is a major part of any well organized and executed predictive maintenance program. On-line and off-line testing and trending provides valuable information technicians need to make accurate decisions regarding the motors health. These technologies provide very different information as each looks at very different areas of concern. Both technologies are required to have a complete picture of operational health of a motor.
Static or off-line testing is usually performed once a year or during outages with the motor shut down. Off-line testing is also used as a quality assurance tool when first receiving reconditioned or rewound motors from the motor shop before they are stored or returned to service. Testing these incoming motors provides proof the motor shop is doing its job properly and becomes the new base-line for future trending. Off-line equipment can also be used as a troubleshooting tool. Any time a problem has occurred the motor involved should be tested for insulation integrity. Overload situations, contaminate issues and voltage problems can compromise the insulation.
Off-line testing includes winding resistance, meg-ohm, polarization index, high potential and surge testing. The tests should be performed in that sequence with modern, state-of-the-art test equipment. Equipment is manufactured today that can adequately reproduce "real world" experiences with out causing damage to the motors insulation system. It is important to test motors at voltage levels and conditions they will see in their normal, day-to-day operation.
Winding resistance tests confirm the phases are balanced finds shorts and opens in the windings as well as high resistance connections.
The meg-ohm test can determine if the windings are grounded or contaminated. The meg-ohm meter is probably the most used test instrument in the field but it has its limitations. Meg-ohm testing is usually performed at voltages slightly above line voltage. The meg-ohm test can determine if a motor is bad but can not confirm the motor is good. Low meg-ohm results are an indication of impending failure but high meg-ohm values do not insure a good motor. Performing a polarization index test can further confirm poor insulation systems and will indicate when the insulation is old and brittle but, again it does not find potential copper-to-copper faults.
A high potential or dc step voltage test raises the entire winding to a potential voltage equal to that seen at start up and shut down and looks for weak ground wall insulation. Weak or damaged cable problems will also show up during this test and it may be necessary to separate the motor at its junction box in order to determine where a problem lies. High potential testing is usually performed at twice line voltage plus 1000 volts. HiPot testing is not destructive when applied properly.
The final test we would conduct, once the motor has passed all the other tests is the surge test. Surge testing is the only way to locate potential copper-to-copper faults. Copper-to-copper faults are the main cause of over 80% of all winding related failures and they will go undetected if not or the surge test. Most motors, when allowed to run to failure will "blow" to ground in a slot, because that is where they can get to steel, but most will have started as a copper-to-copper fault. Locating these potential faults before they become hard welded faults allows the technician time to plan the required repairs before a catastrophic failure causes unscheduled down time, expensive repairs and lost production. Once these copper-to-copper faults have become hard welded faults, the motor's life is less than 15 minutes.
Dynamic or on-line testing is performed while the motor is operating within its normal environment. The collection of data is safe, fast and non-intrusive. On-line testing can and should be performed more often than off-line testing with the usual frequency similar to vibration analysis. The concept is a new but rapidly growing technology and its capabilities are only limited by its age. Besides the apparent electrical issues the technology can monitor, many mechanical issues are also perceptible with the collected data. Torque and current spectra have proven useful in determining bearing faults, looseness and eccentricity. The motor is part of a machine system with three links; power condition, load and the motor. The on-line equipment available today provides information about all three. Many motor problems are created by the load or by poor supply power and many times the "root cause" of the failure goes undetected. The ability to acquire and define torque provides the on-line user to separate mechanical from electrical issues.
On-line testing provides information regarding power quality and conditions such as voltage levels, unbalances and distortion. A small amount of voltage unbalance coupled with minor harmonic voltage distortion may result in a NEMA de-rating that will not be seen with simple multi meters and amp probes.
Current levels and current unbalances also affect motor performance and monitoring them is essential when trending motor health.
Another major issue with electric motors is the condition of their rotors. Modern on-line testers will be able to predict rotor bar failures or potential failures if the load is relatively steady. A pump, fan or blower operating at a steady frequency will show very clear rotor bar signatures making diagnosis easy.
During normal operation a motor's rotor is stressed by its load. The "torque ripple" provides a picture of those stresses and is an indicator of many mechanical problems. Cavitations and belt flapping are easily seen in the torque ripple signature. Defining other mechanical issues earlier and with more certainty is constantly on going in research and development labs and new progress is being made continuously.
On-line testing provides efficiency information allowing the technician to make wise and practical decisions when it is time to repair or replace a motor. Improving efficiency by just 2% may results in thousands of dollars in energy cost every year.
Off-line testing measures the integrity of the motor's insulation system. On-line testing provides information about the power condition, the load and the motor. Together they present a picture of the motors health and provide technicians with sufficient information required to accurately diagnose and predict imminent failures. Electrical testing is an essential part of a complete predictive maintenance program.
Timothy M. Thomas,
Senior Applications Engineer
Baker Instrument Company
4812 McMurry Ave.
Fort Collins, CO 80525