Part I
Types of Maintenance Programs
There are three basic trains of thought when it comes to maintaining motors and machinery in any facility or factory; corrective, preventative and predictive. Each has some benefits and some disadvantages and the correct choice requires consideration of many vital factors in each individual situation.
Corrective Maintenance is basically “run-to-failure”. Fix it when it breaks. This concept requires no pre-planning other than some spares and is usually only employed in very small operations, where the motors are cheap and easy to replace and where “down-time” is not a consideration.
As a maintenance plan, corrective maintenance is, by far, the most expensive way to operate. The disadvantages to a corrective maintenance program include the costs to inventory spare motors and parts, the interruption of processes and the expense of emergency repairs. Motors will almost always choose a very inopportune time to fail, and seldom provide any predictors to their deterioration.
A major aspect that should be considered before choosing to let a machine system run to failure is the worker safety issue. In many cases, machinery failure can result in a hazardous situation for the operator or others.
Another consideration to employing corrective maintenance concepts is the environmental impact. Unplanned motor failures can lead to harmful gases and liquids being “leaked” into the air or ground having a disastrous ecological impact.
Before choosing a Corrective Maintenance Plan, it is absolutely necessary to consider all the possible effects and consequences.
Preventative Maintenance is a great deal more practical than corrective but has numerous costly drawbacks. This is a “time-based” plan where repairs or replacements are based entirely on time in service without any consideration to asset condition. Preventative maintenance is often referred to as “fix it before it breaks” or “fix it even if it isn’t broken”. It is commonly known as “periodic maintenance”, “historical maintenance” and “calendar based maintenance”. It assumes that all machines will fail in time which is realistic, but when will it fail and why?
John Moubray, in his book, “Reliability Centered Maintenance II”, states “there is often little or no relationship between how long an asset has been in service and how likely it is to fail”.
One major disadvantage to a preventative plan includes the known fact that some machines will fail before the planned shutdown. Other drawbacks include: perfectly good machines being removed from service and over-repaired, the reconditioning process itself may introduce new faults and the procedure will require unnecessary down-time and unnecessary repair costs.
Follow-up studies in 1973 and 1982 produced similar findings to the original United Airlines study about age related equipment failures. As many as 92% of component failures are random and not time based. Preventative maintenance tasks are most often performed before the asset has operated for its entire life expectancy or too late and the asset has already failed.
Some industries continue to operate on a Preventative Maintenance Plan including, for the most part, the Marine industry. Cargo ships, fuel oil carriers, freighters of all kinds and even Cruise Liners, are bound by numerous governing associations including the Coast Guard, the American Bureau of Shipping and various insurance agencies. These organizations have stringent rules mostly centered on safety issues. Depending of their size, their cargo and their destinations, ships may be required to have “all” of its assets reconditioned periodically without concern of current condition. In their case, it may not make economic sense to do any condition based testing or verification.
Some industries utilize a combination of time-based and condition-based maintenance. Disney, Sea World and most tourist attractions operate in this manner. They cannot risk unplanned failures and the alternative leads them to over maintaining their assets. Their maintenance costs are considerable higher than most other operations which are, unfortunately, revealed in the cost of admissions.
Predictive Maintenance is defined as the routine monitoring and trending of data related to the overall health of an asset. Utilizing as many “tools” as practical will allow the technician to accurately “predict” imminent failures before they become catastrophic. A well designed and managed Predictive Maintenance Program will more than pay for itself by limiting unplanned down-time and allowing for minor repairs in place of the cost associated with emergency shutdowns.
The idea of predicting failures has gained world-wide acceptance and the practice is being employed in more and more situations, replacing most preventative maintenance activities. New technologies and better diagnostic equipment is being revealed constantly and the tools with which to make accurate predictions are rapidly becoming “smarter”. For this reason alone, it is absolutely necessary to keep up with advancements in all available diagnostic tools and equipment.
John Moubray defines predictive maintenance as “The process of monitoring the condition of an asset and taking an action to avoid the consequences of a failure”. Basically, “if it ain’t broke, don’t fix it”.
Predictive maintenance is most often referred to as “condition based maintenance”. The concept, when applied properly, allows operators to plan their down time at some convenient time eliminating the costs incurred when emergency repairs are required. It is proven that some motors will fail very quickly, known as “infant mortality” but once a motor has survived that period, its life expectancy can be many, many years. Those motors subject to infant mortality will fail before a time-based maintenance program would reveal their problems making preventative maintenance a costly failure from the start.
Routine monitoring provides information that will lead to indicators of imminent failure. The more tools a technician has the better his chances of locating problem areas and defining weaknesses that lead to asset failure. Properly managed predictive maintenance programs will insure the safe and continued operation of all assets, increased productivity and lower overall operating costs. The outcome will benefit both the level of production and the “bottom line”.
Where do We Start?
There are some essential facts that you must consider and evaluate when establishing a predictive maintenance platform. The first and maybe the most important issue is who is going to do the testing. Do you have the personnel and do they have the time. When setting up a new program, available man-hours can be a huge issue and often mandates the hiring of several new technicians. In some cases it may be more realistic to consider out-sourcing some or all of your predictive maintenance technologies. New technologies may require extensive and varied training. Are you willing to hire new people and spend the time and money necessary to train them? Under-trained technicians are a major problem throughout industry partially due to the “hidden” cost associated with state-of-the-art technologies. For Instance, vibration analysis training requires years of practical hands-on use and several levels of competency before a technician can be fully capable of making accurate calls.
Besides the personnel concern another important step is to define the issues that are important to your operation. Identify problem areas by reviewing past problems with emphasis on “root cause” analysis. Research all technologies and determine which ones would best help resolve those issues.
Establish criticality of equipment and classify equipment by level of importance to the operation. Again, worker safety along with production goals must be considered when establishing priorities.
Define your long range goals and needs. Purchasing several new technologies all at once is usually not a good idea. Besides the cost associated with purchasing, there are many other contributing factors to consider including the acquisition of personnel, training and integrating the diagnostic data into a useful and beneficial format. Purchase one or two technologies and allow time for them to become incorporated into your facilities operation and to function properly. Add new technologies as your awareness level increases and the aptitude of the technical staff becomes capable of effectively using the new “tools”.
When considering long range goals, determine how many motors you are concerned about. Are there other issues such as mechanical problem areas, including gear boxes, conveyor belts and other pieces of machinery? Their numbers may affect which technologies you acquire first. Which technologies do you need? Vibration analysis is an old standard and should be a part of any predictive maintenance program.
Once you have defined a plan to “build” your predictive maintenance program, begin acquiring equipment. Again, research all available equipment within each technology. For each technology, there are many manufacturers and the prices range from cheap to extravagant. Learn what the differences are between manufacturers and within manufacturer’s product lines. Cheaper equipment may identify the faults you want to find but they will probably not have the features you want. Less expensive equipment usually has no saving or reporting capability and those features are essential to a predictive maintenance program. Trending depends on historical data so it is vital that the equipment you purchase has the ability to save and compare data.
Top of the line and state of the art equipment may have more bells and whistles than you need but this level of equipment will almost always make your life easier. Generally state of the art equipment is computer controlled allowing collected data to be easily transferred to a desk top computer for evaluation and reporting. Safely testing of assets is also a major concern. Cheaper, manually operated equipment may allow an unskilled user to raise test voltages to an unsafe level which can introduce faults. Generally, top of the line equipment will have safeguards built in which prohibit unsafe levels of testing.
One major pitfall with predictive maintenance programs is the complacency of managerial level personnel and their emphasis on production. Once a good predictive maintenance program is established and operating efficiently, upper level managers tend to place less importance of keeping up with the very predictive technologies that made their facilities more productive and less prone to expensive unplanned down-time. And the vicious circle begins. The best way to insure that a predictive maintenance program, that has been established and performing properly, continues to receive the funding it requires is to document findings. When your program identifies and rectifies a potential problem, document the findings and make sure management is aware of your successes.
Another important and often overlooked issue is motor responsibility. In many situations, mechanical and electrical departments are separated and although each may be taking good data, there is no communication between them. A good predictive maintenance program requires a “motor owner”; someone who reviews all the collected data and who will be responsible to make decisions regarding the evaluation of each asset.
Requirements for a Successful PdM Program
- Regular routine maintenance activities are completed on a scheduled basis.
- Predictive maintenance activities have been established based on a process to identify critical equipment, common modes of failure and primary indicators of failure.
- Actual repair findings are compared with predicted condition to validate and evaluate the diagnostic applications.
- Predictive maintenance results are continually tracked and evaluated to ensure proper application.
- PM activities are replaced with PdM activities.
- The frequency and type of predictive and PM are adjusted based on operating experience, results of reliability analyses, changes in operating conditions and environment, and vendor recommendations.
- Maintenance history is analyzed to determine equipment failure trends attributed to aging.
- Corrective actions are taken before condition specifications are exceeded.
- Systems and methodologies for inspection are developed by a group of experts to ensure the inspection plan is comprehensive.
- New baseline performance measurements are documented whenever equipment has been repaired.
- Maximum use of see-through panels, inspection ports, and other visual indicators is used.
- Regular, routine, PdM activities occur on a scheduled basis. PdM monitoring is given priority for scheduling purposes and is executed on a timely basis.
- Establish a “motor owner” who has all the data collected and is responsible for making decisions regarding each asset.
Remember, research all technologies, analyze your specific needs, consider the pros and cons of equipment, be willing to invest and keep up with enhancements and new developments.
Part II
Adding Static and Dynamic Electric Motor Testing
A motor is one part of a “machine system” and there are three areas of concern we must address; the power quality supplying the motor, the motor itself and the driven load. Static electric motor testing addresses the motor’s insulation system and dynamic electric motor testing diagnoses issues regarding power quality and load issues. Both technologies are needed in order to have a complete report on the motor’s health.
When evaluating test equipment it is important to make sure the equipment is capable of meeting all industry test standards. There are established engineering standards that dictate which tests must be performed, how often and to what voltage level. Adhering to these standards will insure better diagnostic capabilities and provide proof of potential weaknesses within a machine system in time to remedy the issue before a catastrophic failure. Once a motor has failed it will have core damage which in many cases renders the motor unrepairable or at best less efficient and less reliable even when rewound.
Static Motor Testing
Static or off-line testing is performed with the motor off and is usually done once or twice a year or during scheduled outages. Static testing measures and evaluates the integrity of the insulation system. A motor insulation system has two basic areas of concern; copper-to-ground or ground wall and copper-to-copper or turn insulation. A good off-line test will consist of 1) winding resistance, 2) meg-ohm, 3) polarization index, 4) step-voltage or high potential test, and 5) surge test. This combination of tests will measure the integrity of the entire insulation system, identifying problem areas when they arise.
Consider a low voltage motor, (under 600 volts). The magnet wire that makes up the motor’s insulation system must be capable of withstanding a 6000volt AC spike. This voltage level is dictated and every supplier of low voltage magnet wire must maintain this standard. Modern off-line test equipment performs it’s test using on controlled DC voltage so we can multiply that AC voltage by the square root of two resulting in a spike resistant level of 8,400 volts DC. The slot-cell liner is rated at over 20,000 volts DC providing a combined total of up to 28,400 volts DC insulation to ground.
The weakest part of the insulation system is the copper-to-copper or turn insulation. It is proven that over 80% of winding failures begin as a turn-to-turn fault. If allowed to run to failure, the fault will cause turns to fuse and result in a failure to ground within the slot. The fused shorted turns will result in very high current that can be as high as twice locked-rotor current being circulated with the shorted turns. The vast majority of turn faults begin at the “early” turns. That is, the coils where the “T” leads are connected. This is a result of the spiking that occurs upon start up and shutdown. The spiking is a result of contactor bounce. Motor start up can result in spikes as high as five times the incoming voltage and with as many as six to ten repetitions and shut down spiking can be worse.
It is well documented that motor insulation begins deteriorating immediately and the rate of deterioration is dependent on many operating and environmental hazards. Starts and stops, load situations, mechanical issues such as misalignment, and localized environment issues all affect the longevity of the insulation systems. The question is not if a motor will fail, but when?
So what can you do? The answer is periodic off-line testing and trending of the motor’s insulation systems. To have an overall picture of the motor insulation health, a complete set of tests must be performed on a routine basis. How often you should test again calls for the consideration of many issues and defines what voltage levels you should attain. Criticality, past history, size, repair and emergency costs, availability of spares, daily starts and stops, ease of testing, available technicians and probably dozens of other issues play a part in determining how often and when you should test.
A complete diagnosis of the insulation system requires winding resistance test, meg-ohm test, polarization test, step-voltage or high potential test and a surge test. Each test provides a piece of the health puzzle and should be performed in that order.
All collected values should be compared to previous tests. In order to accurately compare current and past test, it is mandatory to temperature correct all our collected data. Temperature effects both resistance and meg-ohm values. The rule of thumb is that for every ten degrees Celsius increase in temperature, the resistance value halves.
The winding resistance test can find high resistance connections that lead to current unbalances and eventual failures. The meg-ohm test can determine if the motor is grounded or heavily contaminated. The Polarization test finds old brittle insulation and deteriorated ground wall insulation. The Step-Voltage or High Potential test further stresses the ground wall insulation and when performed at the motor control cabinet, tests the cables as well. Cables should be able to with stand the spiking and voltage levels the motor sees and bad cables can be a major nightmare.
Once all of these tests have been performed it is time or the surge test. This is the only test that can locate weakness within the copper-to-copper insulation, where over 80% of all winding failures begin. The surge test sends pulses at ever increasing voltage levels down one phase at a time looking for weak turn insulation. The pulses are generated in a fashion to simulate the spiking seen at start up and shut down. We know that the spikes can be up to five times the line voltage so we surge test at twice the line voltage plus 1000 volts. Weak turn insulation can easily be determined by the generated wave patterns and the digitized pulse-pulse comparison.
Dynamic Motor Testing
Dynamic or on-line motor testing is performed with the motor running in its normal environment and load condition. Equipment connects directly to the load side of low voltage motors or in the low voltage cabinet of medium and high voltage motors collecting voltage and current information. This data is analyzed and processed through numerous algorithms, displayed in categorized layouts and diagnosed with preset pass/ fail criteria. Red, yellow and green “flags” quickly and clearly indicate conditions of concern.
Today, we can test motors safely and quickly with the new state of the art equipment available. Besides the usual portable cable, a newer, safer and quicker method makes data collection easier than any other technology. A device that mounts inside the motor control cabinet with a collection port mounted through the door insures the safety of the technician and prevents costly shut downs for equipment connection and removal.
Dynamic testing provides information regarding power quality, the motor, including many mechanical issues and the load. State of the art equipment diagnosis torque, not only in a numerical value but in a spectrum and in an instantaneous view known as “torque ripple”.
Power quality issues including voltage levels, voltage unbalances, harmonic distortion, current levels and current unbalances are all gathered, displayed and “graded” against pre-set values.
Many mechanical conditions are definable including rotor bar issues, cavitations, bearing faults and others are defined by the current and torque spectra.
Summary
Predictive maintenance programs are by far the most cost effective method of operating. The vast array of tools makes it easier to identify potential problem areas and to take steps to remedy the problem before it manifests itself into a major catastrophe. The cost savings from predicting imminent failures in advance more than offsets the cost of administrating and employing the predictive maintenance program.
A successful program requires state of the art equipment, qualified technicians, routine monitoring, vigilance and continuous adherence to details. A good program requires motor ownership where one person or small group of people see all the collected data and are allowed or obligated to make decisions regarding the future of an asset.