Motor Diagnostics: The Multi-Technology Approach

Introduction

There has been a persistent misconception that there is a ‘magic bullet,’ in the form of a Condition Based  Monitoring (CBM) instrument, that will provide all of the information that you need to evaluate the  health of your electric motor system. This  misconception is often brought about by the  commercial presentations of the manufacturers or  sales forces of these CBM instruments. It is the very job of the salesperson to focus on the area of strength for their particular instrument(s) and present it as ‘the  only solution you will ever need to solve your every  problem.’  

In reality, there is no one instrument that will provide you with every piece of information that you need.  No ‘Holy Grail’ of CBM and reliability. However, through an understanding of the electric motor system, and the capabilities of CBM technologies,  you can have a complete view of your system, its health and have confidence in estimating time to failure in order to make a good recommendation to management.  

The purpose of this paper is simple: Outline the components of an electric motor system; Discuss the modes of failure of each major component; Discuss how each of the major technologies address each  component; Discuss how the technologies can be integrated for a complete view of the system; and, Discuss the bottom-line impact of the Multi Technology approach. The types of CBM equipment to be reviewed are standard off-the-shelf technologies that are used for periodic testing.  

The Electric Motor System 

The electric motor system involves far more than just the electric motor. In fact, it is made up of six distinct sections, all with their different failure  modes. The sections are (Figure 1): 

• The facility power distribution system which includes wiring and transformers.  
• Starting systems.  
•  The electric motor – A three phase induction  motor for the purpose of this paper.  
• 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 waste-water 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 components are focused on depends upon several factors, which include: 

• 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 since when is training ever not an issue?

The perceived areas of failure provides 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 had to do with control and cable problems, improper repairs and power quality.  Bearing issues had to do with improper lubrication practices.  

In effect, when determining the best way to implement CBM on your electric motor system, you need to take a system, 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 at the end of this paper:  

De-Energized Testing:  

1 Motor Circuit Analysis: Theory, Application and  Energy Analysis, Howard W. Penrose, Ph.D., SBD  Publishing, ISBN: 0-9712450-0-2, 2002.  

• DC High Potential 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.
• 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 with no true  ability to trend.  
• 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 values of resistance,  impedance, inductance, phase angle,  current:frequency response, and insulation  testing can be used to troubleshoot, commission  and evaluate control, connection, cable, stator,  rotor, air gap and insulation to ground health.  Using a low voltage output, readings are read  through a series of bridges and evaluated. Non destructive and trendable readings often months  in advance of electrical failure.

2 Potentially Destructive: Any instrument that can  potentially change the operating condition of the  equipment through mis-application or finish off  weakened insulation conditions shall be considered  potentially destructive. 

Energized Testing:  

Vibration Analysis: Mechanical vibration is  measured through a transducer providing overall  vibration values and FFT analysis. These values  provide indicators of mechanical faults and  degree of faults, can be trended and will provide  information on some electrical and rotor  problems that vary based upon the loading of the  motor. Minimum load requirements for electric  motors to detect faults in the rotor. Requires a  working knowledge of the system being tested.  

Infrared analysis provides information on the  temperature difference between objects. Faults  are detected and trended based upon degree of  fault. Excellent for detecting loose connections  and other electrical faults with some ability to  detect mechanical faults. Readings will vary  with load. Requires a working knowledge of the  system being tested.  

Ultrasonic instruments measure low and high  frequency noise. Will detect a variety of  electrical and mechanical issues towards the late  stages of fault. Readings will vary with load.  Requires a working knowledge of the system being tested.  

Voltage and current measurements will provide limited information on the condition of the motor system. Readings will vary with load.  

Motor Current Signature Analysis (MCSA) uses the electric motor as a transducer to detect electrical and mechanical faults through a  significant portion of the motor system. Usually  used as a go/no go test, MCSA does have some  trending capabilities, but will normally only detect winding faults and mechanical problems  in their late stages. Sensitive to load variations  and readings will vary based upon the load.  Requires nameplate information and many  systems require the number of rotor bars, stator  slots and manual input of operating speed.  

Major Components and Failure Modes  

Some of the major issues from the various  components of the motor system shall be reviewed in  order to provide an understanding of the types of  faults found and the technologies used to detect them.  As an overview, 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:  

• Voltage and current harmonics: With voltage  limited to 5% THD (Total Harmonic Distortion)  and current limited to 3% THD. Current  harmonics carry the greatest potential for harm to  the electric motor system.  
• Over and under voltage conditions: Electric  motors are designed to operate no more than +/-  10% of the nameplate voltage.  
• Voltage unbalance: Is the difference between  phases. The relationship between voltage and  current unbalance varies from a few time to  many times current unbalance as related to  voltage unbalance based upon motor design (Can  be as high as 20 times).  
• Power factor: The lower the power factor from  unity, the more current the system must use to  perform work. Signs of poor power factor also  include dimming of lights when heavy  equipment starts.  
• Overloaded system: Based upon the capabilities  of the transformer, cabling and motor. Detected  with current measurements, normally, as well as  heat.  

The primary tools used to detect problems with  incoming power are power quality meters, MCSA  and voltage and current meters. Knowing the condition of your power quality can help 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 both in the electric motor as well as other systems.  

Common transformer problems include (oil filled or dry-type transformers):  

• Insulation to ground faults.  
• Shorted windings.  
• Loose connections, and,  
• Electrical vibration/mechanical looseness  

Test equipment used for monitoring the health of  transformers (within the selection of instruments within this paper) include: 

• MCA for grounds, loose/broken connections and shorts  
• MCSA for power quality and late stage faults 
• Infrared analysis for loose connections 
• Ultrasonics for looseness and severe faults 
• Insulation testers for insulation to ground faults.  

MCC’s, Controls and Disconnects 

The motor control or disconnect provides some of the  primary issues with electric motor systems. The most  common for both low and medium voltage systems  are:  

• Loose connections  
• Bad contacts including pitted, damaged, burned  or worn  
• Bad starter coils on the contactor  
• Bad power factor correction capacitors which normally results in a significant current unbalance.  

The test methods for evaluating the controls include infrared, ultrasonics, volt/amp meters, ohm meters and visual inspections. MCA, MCSA 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, provide some of the biggest headaches.  Common cable problems include:  

• Thermal breakdown due to overloads or age 
• Contamination which can be even more serious in cables that pass underground through conduit 
• Phase shorts can occur as well as grounds.  These can be caused by ‘treeing’ or physical damage.  
• Opens due to physical damage or other causes. 
• Physical damage is often a problem in combination with other cable problems.  

Test and trending is performed with MCA, infrared, insulation testing and MCSA.  

Motor Supply Side Summary 

On the supply side to the motor, the problems can be broken down as follows:  

• Poor power factor – 39%  
• Poor connections – 36%  
• Undersized conductors – 10%  
• Voltage unbalance – 7%  
• Under or over voltage conditions – 8%  

The most common equipment that covers these areas include MCA, infrared and MCSA.  

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:  

• Bearings – general wear, misapplication, loading  or contamination.  
• Bad or worn shaft or bearing housings 
• General mechanical unbalance and resonance  

Vibration analysis is the primary method for  detection of mechanical problems in electric motors.  MCSA will detect late stage mechanical problems as  will infrared and ultrasonics.  

Primary electrical problems:  

• Winding shorts between conductors or coils 
• Winding contamination  
• Insulation to ground faults  
• Air gap faults, including eccentric rotors 
• Rotor faults including casting voids and broken rotor bars.  

MCA will detect all of the faults early in  development. MCSA 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 testing 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.  

• Belt or direct drive misalignment  
• Belt or insert wear  
• Belt tension issues are more common than most  think and usually result in bearing failure 
• Sheave wear  

The most accurate system for coupling fault detection  is vibration analysis. MCSA 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 MCSA, vibration, infrared analysis and  ultrasonics.  

Common Approaches to Multi-Technology 

There are several common approaches 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 under 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 MCSA 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 MCSA systems that include voltage  and current demodulation.  

The newest, and most effective, approach has been  vibration, infrared and MCA and/or MCSA. The  strength of this approach is that there is a  combination of electrical and mechanical disciplines  involved in evaluation and troubleshooting. As  found in the Motor Diagnostic and Motor Health  Study, 3 38% of motor system testing involving only vibration and/or infrared see a significant return on 3 Motor Diagnostic and investment. This number jumped to 100% in systems that used a combination of MCA/MCSA 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 their tool box, the ROI increased to $307,000, ten times the original by using a combination of instruments.  

Application Opportunities 

There are three common opportunities for electric motor system testing. These include:  

• Commissioning components or the complete system as it is newly installed or repaired. This  can provide a very immediate payback for the technologies involved and will help you avoid  infant mortality disasters.  
• Troubleshooting the system through the application of multiple technologies will assist  you in identifying problems much more rapidly and with greater confidence.  
• Trending of test results for system reliability, again using the proper application of multiple technologies. Using tests such as MCA, vibration and infrared, potential faults can be  trended over the long term, detecting many faults  months in advance.  

Conclusion 

This paper provided a brief overview of how multiple technologies work together to provide a good view of  the electric motor system. Through an understanding  and application of this approach, you will realize fantastic returns on your maintenance program.  

About the Author 

Dr. Howard W. Penrose, Ph.D. received his Ph.D. in General Engineering focusing on industrial system  process improvements, waste stream and energy  analysis and equipment reliability. He has 15 years experience in the electric motor and service industry leading PdM and Root-Cause-Analysis initiatives in a large variety of commercial & industrial locations.  

Table 1: Motor System Diagnostic Technology Comparison  

Table 2: Management Considerations 

Table 3: Common Approaches  

Table 4: Additional Considerations