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:

• The facility power distribution system, which includes wiring and transformers.
• The motor control, which may include starters, soft starts, variable frequency drives and other starting systems.
• The electric motor - a three phase induction motor for the purpose of this article.
• The mechanical coupling, which may be direct, gearbox, belts or some other coupling method. For the purpose of this paper, we will focus on direct coupling and belts.
• The load refers to the driven equipment such as a fan, pump, compressor or other driven equipment.
• The process, such as wastewater pumping, mixing, aeration, etc.

Most will view individual components of the system when troubleshooting, trending, commissioning or performing some other reliability-based function related to the system. What

• What is the experience and background of the personnel and managers involved. For instance, you will most often see a strong vibration program when the maintenance staff is primarily mechanical, or an infrared program when the staff is primarily electrical.
• Perceived areas of failure. This can be a serious issue depending upon how the motor system is perceived and will deserve more attention to follow.
• Understanding of the various CBM technologies.
• Training (but when is training ever NOT an issue?).

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"[1]. Details as to the components of the system tested and capabilities can be found in Tables 1-4.

De-energized testing:

• By applying a voltage of twice the motor rated voltage plus 1,000 volts for AC and an additional 1.7 times that value for DC high potential (usually with a multiplier to reduce the stress on the insulation system), the insulation system between the motor windings and ground (ground- wall insulation) is evaluated. The test is widely considered potentially destructive[2]. Surge comparison testing: Using pulses of voltage at values calculated the same as high potential testing, the impedance of each phase of a motor are compared graphically. The purpose of the test is to detect shorted turns within the first few turns of each phase. The test is normally performed in manufacturing and rewinding applications as it is best performed without a rotor in the stator. This test is widely considered potentially destructive, and is primarily used as a go/no-go test.
• Insulation tester: This test places a DC voltage between the windings and ground. Low current leakage is measured and converted to a measurement of meg, gig or tera-Ohms.
• Polarization Index testing: Using an insulation tester, the 10 minute to 1 minute values are viewed and a ratio produced. According to the IEEE 43-2000, insulation values over 5,000 MegOhms need not be evaluated using PI. The test is used to detect severe winding contamination or overheated insulation systems.
• Ohm, Milli-Ohm testing: Using an Ohm or Milli-Ohm meter, values are measured and compared between windings of an electric motor. These measurements are normally taken to detect loose connections, broken connections and very late stage winding faults.
• Motor Circuit Analysis (MCA) testing: Instruments using combinations of values for resistance,...(Read whole article)