No magic bullets here. One instrument can't possibly provide all the information you need to evaluate the health of an electric motor system.
There is a persistent misconception that a "magic bullet," in the form of a condition-based monitoring (CBM) instrument, will provide all of the information one needs to evaluate the health of an electric motor system. Often, this misconception is reinforced through commercial presentations made by the manufacturers of such instruments or their sales representatives. In reality, though, there is no "Holy Grail" of CBM and reliability when it comes to electric motors. No single instrument will provide you with every piece of information that you need.
But, through a better understanding of your electric motor system(s) and the capabilities of CBM technologies, you can have a complete view of your system and its health, and gain confidence in estimating time to failure in order to make good recommendations to management.
Electric motor systems
An electric motor system involves far more than just the motor. In fact, it is made up of six distinct sections, all with their different failure modes. The sections are:
Most will view individual components of the system when troubleshooting, trending, commissioning or performing some other reliability-based function related to the system. What components are focused on depends upon several factors, which include:
The perceived areas of failure present an especially serious problem when viewing the history of your motor system. Often, when records are produced, the only summary might state something like, "fan failure, repaired," or "pump failure, repaired." The end result is that the perceived failure has to do with the pump or fan component of the motor system. This especially becomes more of an issue when relying upon memory to provide the answers to the most serious problems to be addressed in a plant, based upon history. For instance, when looking to determine what part of a plant has been causing the most problems, the answer might be, "Waste water pump 1." The immediate perception is that the pump has a consistent problem and, as a pump is a mechanical system, a mechanical monitoring solution might be selected for trending the pump's health. If a root-cause had been recorded on each failure, it might have been determined to be the motor winding, bearings, cable, controls, process or a combination of issues.
In a recent meeting, while discussing the selection of CBM equipment, the attendees were asked for modes of failure from their locations. The answers were fans, compressors and pumps. When discussed further, the fans were found to have bearing and motor winding faults being most common, pump seals and motor bearings for pumps, and, seals and motor windings for compressors. When viewed even closer, the winding faults were found to be asso-ciated with control and cable problems, improper re-pairs and power quality. The bearing issues had to do with improper lubrication practices.
In effect, when trying to determine the best way to implement CBM on your electric motor system, you need to take a system view, not a component view. The result is simple: improved reliability, fewer headaches and an improved bottom line.
Condition-based monitoring test instruments
Following are some of the more common CBM technologies in use. More detail on the technologies can be found in "Motor Circuit Analysis". Details as to the components of the system tested and capabilities can be found in Tables 1-4.
Major components and failure modesTo provide an understanding of the types of faults and technologies used to detect them, some of the major issues from the various components of the motor system are reviewed below. As an overview, however, this may not encompass all of the modes of failure that you may experience.
Incoming power. Starting from the incoming power to the load, the first area that would have to be addressed is the incoming power and distribution system. The first area of issue is power quality, then transformers.
Power quality issues associated with electric motor systems include:
The primary tools used to detect problems with incoming power are power quality meters, ESA and voltage and current meters. Knowing the condition of your power quality can help to identify a great many "phantom" problems.
Transformers are one of the first critical components of the motor system. In general, transformers have fewer issues than other components in the system. However, each transformer usually takes care of multiple systems-in the electric motor, as well as other systems.
Common transformer problems (oil-filled or dry-type models) include:
Test equipment used for monitoring the health of transformers (within the selection of instruments in this article) include:
MCCs, controls and disconnects. The motor control or disconnect is responsible for some of the primary issues with electric motor systems. The most common for both low- and medium-voltage systems are:
The test methods for evaluating controls include infrared, ultrasonics, volt/amp meters, ohm meters and visual inspections. MCA, ESA and infrared provide the most accurate systems for fault detection and trending.
Cables – Before and after the controls. Cabling problems are rarely considered and, as a result, they provide some of the biggest headaches. Common cable problems include:
Motor supply side summary. On the supply side to the motor, the problems can be broken down as follows:
The most common equipment that covers these areas includes MCA, infrared and ESA.
Electric motors. Electric motors include mechanical and electrical components. In fact, an electric motor is a converter of electrical energy to mechanical torque. Primary mechanical problems include:
Vibration analysis is the primary method for detection of mechanical problems in electric motors. ESA will detect late-stage mechanical problems as will infrared and ultrasonics. Primary electrical problems include:
MCA will detect all of the faults early in development. ESA will detect late-stage stator faults and early rotor faults. Vibration will detect late-stage faults, insulation to ground will only detect ground faults, which make up less than 1% of motor system faults. Surge testing will only detect shallow winding shorts and all other testiing will only detect late stage faults.
Coupling (direct and belted). The coupling between the motor and load provides opportunities for problems due to wear and the application.
The most accurate system for coupling fault detection is vibration analysis. ESA and infrared analysis will normally detect severe or late-stage faults. Load (fans, pumps, compressors, gearboxes, etc.) The load can have numerous types of faults depending on the type of load. The most common are worn parts, broken components and bearings.
Test instruments capable of detecting load problems include ESA, vibration, infrared analysis and ultrasonics.
There already are several common approaches to multi-technology within industry, as well as several new ones (See Table 3). The best use a combination of energized and de-energized testing. It is important to note that energized testing is usually best under constant load conditions and trended in the same operating conditions each time.
One of the most common approaches has been the use of insulation resistance and/or polarization index. These will only identify insulation to ground faults in both the motor and cable, which represents less than 1% of the overall motor system faults (÷5% of motor faults).
Infrared and vibration are normally used in conjunction with each other with great success. However, they miss a few common problems or will only detect them in the late stages of failure.
Surge testing and high potential testing will only detect some winding faults and insulation to ground faults, with the potential to take the motor out of action should any insulation contamination or weakness exist.
MCA and ESA support each other and detect virtually all of the problems in the motor system. This accuracy requires MCA systems that use resistance, impedance, phase angle, I/F and insulation to ground and ESA systems that include voltage and current demodulation.
The newest and most effective approach has been vibration, infrared and MCA and/or ESA. The strength of this approach is that there is a combination of electrical and mechanical disciplines involved in evaluation and troubleshooting.
As found in a recent "Motor Diagnostic and Motor Health Study,", 38% of motor system testing involving only vibration and/or infrared saw a significant return on investment (ROI). This number jumped to 100% in systems that used a combination of MCA/ESA along with vibration and/or infrared. In one case, a combined application of infrared and vibration saw an ROI of $30k. When the company added MCA to its toolbox, the ROI increased to $307,000-ten times the original-by using a combination of instruments.
There are three common opportunities for electric motor system testing. These include:
This article provides a brief overview of how multiple technologies can work together to provide you with a good view of your electric motor systems.
Through a good understanding of this approach, and proper application of it, you can realize significant returns in your maintenance program.