Abstract:
This paper presents current methods of electrical tests and trend analysis of the operational health of electric motors in the context of successful predictive maintenance programs. It discusses the causes of winding insulation deterioration and how early detection can lead to fewer failures, less unscheduled downtime and better efficiency. The attempt of this paper is to explain, in layman’s language, the benefits of predictive maintenance in place of time based maintenance.
Over the years, numerous studies have been performed trying to establish guidelines for creating plant reliability and efficiency. Depending on whose research you adhere to, somewhere between sixty-five and seventy-five percent of all motor failures are mechanical in nature. For this reason, Vibration Analysis provides the “biggest bang for the buck” and is a vital cornerstone for any Predictive Maintenance Program. While an aggressive vibration analysis program may predict most of these mechanical issues with a great deal of accuracy, it cannot diagnose the twenty-five to thirty-five percent of motor failures that are due to electrical weaknesses and faults. The insulation system within the motor is its “heart” and no technology other than a series of electrical tests can evaluate its health and integrity. Electrical testing technologies include static testing and dynamic monitoring. This paper will discuss the need and the benefits of static testing. In order to understand the need we must first know why and how motors degrade.
Many factors affect the pace at which insulation degrades including its initial quality. Heat is the main enemy of all insulation materials so maintaining a cool dry environment will increase motor life. Many things contribute to excessive heat and most often the situation is acerbated by several issues that by themselves would otherwise not create a problem. High ambient temperature, numerous restarts, starting under heavy loads, slight misalignment, some unbalance with the supply voltage and contamination all impact the degree of heat the motor will see.
Another issue affecting the life of the insulation system is starting and stopping. There is an old saying “if you don’t want your motor to fail, don’t start it and if it’s running, don’t stop it.” Motor starting and to some degree, stopping, are generally the most stressful times in the motors life. Contactor and breaker “bounce” at start up can generate voltage spiking that reach four or five times greater than the operating voltage. The initial inrush of AC voltage pushed by up to eight or ten times the nameplate current “attacks” the insulation greatly affecting the early turns. This startup current causes the windings to flex or breathe and allows the copper magnet wire to rub and abrade. There is only about one and a half mils of insulation baked on the magnet wire and over time it will deteriorate resulting in arcing during starting and stopping. Once arcing starts the insulation is basically at the end of its life. The wearing away of that thin film of insulation is the beginning of the end for many motors. Once arcing starts it occurs on every startup and often during shut downs. Arcing may occur for weeks or even months before it creates a failure. Eventually, the arcing will create a carbon path and short out a portion of the windings. These shorted turns now act as the secondary side of a transformer with voltage and current being induced by the rest of the circuit. The ratio of good turns to shorted turns will dictate how severe and how quickly the failure will occur but once there are shorted turns the motor will fail within minutes. So it is most vital that we find that weak turn insulation before it becomes a hard welded fault. Once the turns are welded those few turns will carry an exponential amount of current that is induced by the transformer effect, and will quickly burn through the slot cell liner to ground in the laminations. The result is often a damaged core with a large hole that will always make the rewound motor less efficient and run hotter. Finding the weak turn insulation and taking the motor out of service before the short occurs has two very valuable affects. The major advantage is that we are in control. We dictate when the motor is removed from service, eliminating unscheduled down time, emergency repairs and lost production. The second benefit is now we have a good core and a reputable repair shop can rewind the motor using better materials than the original manufacturer, better bearings and a tighter balance spec. We will get back a “new” motor, not a patched up motor.
The day we start the motor the first time it begins to weaken and deteriorate. If it sees high ambient temperatures, some misalignment, some voltage imbalance and numerous starts under a heavy load its life will be short. Predictably this motor which should have lasted maybe twenty years will fail in two or three. Some issues could be corrected that would prolong its life. In any case, we need to know the condition of the motors insulation. A PdM strategy that employs all the tools is necessary to determine the motors health and allow us to make realistic predictions regarding its useful life expectancy.
In facilities where contamination is always an issue, Preventive Maintenance is necessary and a routine should be scheduled to reduce the effects on the motors life. [However, whenever possible, a Predictive Maintenance strategy should replace preventive activities and organized utilizing as many tools as possible.] If you want a complete diagnosis of your personal health your physician will perform numerous tests including blood analysis, urinalysis EKGs, and others before he/she makes a diagnosis. And you have to do the same with your motors in order to understand their weaknesses and issues that may cause premature failure and will affect its efficiency. A well monitored PdM program will define issues that can be corrected that would otherwise lead to a major failure if left unchecked. For example, slight misalignment will eventually lead to bearing problems and always stresses the rotor beyond its design. Fixing this minor issue will help the motor be more efficient, add years to its life and save operating costs.
Fortunately, we have state of the art detection equipment available that can identify early issues before they lead to a catastrophic failure. Routine testing and trending will detect weaknesses long before they can propagate into an insulation failure. In order to design a PdM program that works for a specific facility we need to understand what happens during operation of the facility. Knowing which motors are most critical and the ones that have shown to be problematic will dictate which tests we perform and how often.
Like the numerous set of tests your physician will undertake you too will have to employ a complete set of tests each designed to provide you with a piece of the puzzle. There are many independent testing organizations who have detailed, specific test parameters proven to provide sufficient data for the technician to evaluate the immediate health of the insulation. It is vitally important that those guidelines be strictly followed in order to insure accurate data. IEEE, IEC, EPRI and EASA all have similar testing parameters and requirements. A complete set of tests will include a winding resistance test, a megohm test, a high potential or step voltage test and a surge test. Low voltage testing comprised of capacitance, inductance and resistance tests provide some useful information but are not adequate to provide early warnings regarding the turn insulation.
A winding resistance test will verify that all three phases are similar and all internal connections are tight. The resistance test uses a Kelvin bridge and injects about twelve volts DC at approximately seven amps into the windings. One poor connection will lead to unbalanced current and uneven heating. After passing the winding resistance test, a megohm test is run using a low current DC Voltage that depends on the nameplate voltage of the motor. A polarization index test, (PI), which is an extended megohm test, may provide important information about the insulation if it is old, cracked or brittle. If the megohm and PI test are acceptable, a high potential or step voltage test may be performed. The HiPot test is usually performed at an elevated level at least twice the line voltage plus 1000 volts. EASA and other testing organizations recommend even higher test voltage levels. The hipot test may not be appropriate for in-service motors that display low megohms during the megohm test.
Once those DC test are satisfactorily finished, a surge test is performed. The surge test is the most important test and is achieved by applying a charge into a large capacitor at ever increasing voltage levels and discharging it into the windings while monitoring how it reacts. The purpose of the surge test is to recreate the spiking that occurs at startup. The capacitor “fires off” each pulse at a very fast rise time. The intention is to locate weak turn insulation before it has a chance to become a hard welded fault that will result in a quick failure.
Summary:
Predictive Maintenance Programs or strategies put the technician in control. Routine testing and trending provides sufficient data to make a diagnosis regarding the ability of an asset to remain in operation or dictate its need for attention. Being in control and dictating when a machine gets repaired is the basis of predictive maintenance.