Electric Motor Energy and Reliability Analysis

How DOE's MotorMaster+ computer program can be used to manage electric motors. Latest modifications accommodate reliability information.

Energy efficiency in electric motor systems presents significant opportunities within industry. In a 1998 U.S. Department of Energy (DOE) report provided by Xenergy, Burlington, MA, "In 1994, electric motor-driven systems used in industrial processes consumed 679 billion kWh23 percent of all electricity sold in the United States& . Implementation of all well-established motor system energy efficiency measures and practices that meet reasonable investment criteria will yield annual energy savings of 75-122 billion kWh, with a value of $3.6-$5.8 billion& ."

A number of organizations, including electric motor service centers, equipment manufacturers, and utilities, have been developing electric motor system maintenance and management programs since 1993. The concepts, in general, have been to include both energy and condition analysis to provide electric motor users with reliable and energy efficient motor systems. The planned result has been to provide a win-win solution for end-users to improve costs and cost avoidance, reduce power demands on utilities, and expand service capabilities for service companies.

In recognition of these efforts, and to support new efforts within industry, the DOE, the Electric Power Research Institute (EPRI), utilities, trade associations, and others have funded and supported a variety of informative materials, support lines, and software tools. The DOE Office of Industrial Technologies' Best Practices program offers a wide variety of information, tools, and support to assist industrial plants in identifying opportunities for energy efficiency in common systems such as compressed air, motor, steam and pumping systems; and in evaluating opportunities for application of new technologies.

The focus of this article is to outline the development of a motor maintenance and management program using the DOE's MotorMaster+ (MM+) free software and simple tools available within industry. We also shall discuss an industry-funded modification to MM+ designed to allow for a reliability assessment of electric motors combined with an economic analysis. The new version of MM+, which includes the recent changes for reliability assessment, is presently in use within industry on a number of projects implemented through companies and utilities such as Pacific Gas and Electric (PG&E); Dreisilker Electric Motors, Inc.; Nicor Gas; Fermi Lab; BJM Corp.; Pruftechnik, Inc.; and others. The new version of MM+ also is available to anyone for download at the OIT, Best Practices website.

Developing the program
The purpose of an energy and reliability program for electric motor systems is to decrease the cost of energy, production, and maintenance overheads associated with the production of a product—in effect, reducing the cost per production unit as effectively as possible. According to a PG&E application note, "Motor maintenance is more than making sure the motor itself is operating correctly. It also involves ensuring that power supplied to the motor is within acceptable tolerances, that the motor's output power is efficiently transmitted to the load, and that the load itself is properly maintained so as not to make the motor work harder than necessary."

The key components of a motor maintenance and management program include:

  • Control of the electric motor system inventory in software
  • Pre-made repair versus replace and retrofit decisions
  • Predictive and preventive maintenance program implementation with a continuous improvement component
  • Top management commitment
  • An in-house energy coordinator
  • Employee buy-in
  • Pre-set energy conservation goals
  • Partnerships between vendors and owners implemented with pre-planned decisions and shared information.

Such a program can result in improvements of 10-15 percent or more. These opportunities result from such simple improvements as replacing failed electric motors with energy efficient or premium efficient electric motors; scheduling proper greasing of electric motor bearings, reducing electric motor system friction losses; correcting impedance unbalance in motor windings and electrical systems; correcting belt tension and alignment; properly sizing electric motors to the load; testing questionable equipment before and after repair; and other measures that can be immediately implemented or implementation planned for outages. These examples and other related benefits can have energy, reliability, waste stream, and production financial impacts that more than justify the combined energy and reliability effort.

In all dynamic systems, the chance that the system will operate as designed decreases over time. Electric motors are made up of a number of dynamic systems in which each has a reliability function that decreases as the motor ages. The purpose of a reliability-based motor program is to optimize the costs of operating the electric motor and equipment. Measuring the reliability of electric motor systems by quantifying the costs associated with unreliability places the reliability portion of the motor management program in the arena of business impact.

The reliability of the system, as defined within this article, is the measure of the chance that the equipment will operate over a period of time. One of the keys to understanding reliability is knowing the mean time between failures (MTBF). For instance, if an electric motor has a failure rate of 1 in 40,000 hours, the MTBF would be 40,000 hours. The failure rate for that motor would be 1/MTBF, or 0.000025 (identified as l).

Knowing the failure rate, the information can be applied to the reliability function

(R = e-tl)

Therefore, the chance that the motor system will operate for 50,000 hours would be: R= e(50,000)(0.000025) = 0.287, or 28.7 percent. In a redundant (parallel) system, the overall system reliability increases. The result of a single parallel system is

R = Ra + Rb  (Ra)(Rb).

Using the previous example, the parallel system has a 49.2 percent chance of operating through 50,000 hours.

In an electric motor maintenance and management program, there are several points in which the system reliability can be influenced. These points include:

  • Acceptance of new electric motors
  • Acceptance of motor vendors
  • Acceptance of repaired electric motors
  • Acceptance of motor repair centers
  • Tracking and correction of minor defects during the life cycle of the system (predictive and preventive maintenance, root cause analysis, reliability based maintenance, etc.).

It is important to note that the reliability of a vendor should be measured over time and not based upon singular visits and measurements. In particular, a series of specifications should be provided and the vendor measured against that specification over time.

The reliability costs of a motor system can be calculated. A motor fails twice per 50,000 hr, it takes 6 hr to repair the system upon each failure, the system operates 8760 hr/yr, production costs are $10,000/hr and maintenance costs are $100/hr (energy, motor repair or replacement, and waste costs not considered).

Should a maintenance and reliability program (for this one system of many) reduce the failures by half, the impact would be a cost of $58,300 over 50,000 hr, a reduction of $62,760 (52 percent).

There are two basic energy costs that must be observed in an energy and reliability program: life cycle or annual energy costs, and energy costs due to motor condition. In the first instance, the annual operating costs are based upon motor load, energy usage and demand charges, operating hours, and motor size and efficiency. When viewing energy costs due to condition, the increased losses due to phase unbalances or increased friction and windage (bearing failure, for instance) are taken into account.

Equation 1. Energy demand

kW usage = percent load x 0.746 x (horsepower/efficiency)

Equation 2. Energy demand between electric motors

kW = 0.746 x hp x percent L x (100/lower eff.  100/higher eff.)

When considering the previous (reliability) example as an 1800 rpm, 50 hp electric motor, 75 percent loaded, 92 percent efficient, operating 8760 hr/yr, the operating demand would be 30.4 kW. The annual usage would be 266,304 kWh. If the energy charges are an average of $14/kW demand and 0.06 cent/kWh usage, the associated costs would be (30.4 kW x $14/kW x 12 months) $5,107.20 demand and $15,978.24 usage per year for an annual energy bill of $21,085.44 or $120,397.86 over the 50,000 hr life cycle (5.7 years).

If the 50 hp electric motor is compared with a new, 95 percent energy efficient electric motor with a purchase price of $2400 and installation cost of $600, the annual cost savings would be $161.32 demand and $756.86 usage per year, or $918.18 total per year. This would yield a simple payback of 3.3 years ($3000 cost + installation/$918.18 annual savings). In many cases, companies will set a two-year payback as the minimum before performing a motor retrofit (replacing a working motor with a new energy efficient motor). However, when performing an economic (lifecycle) analysis, the before-tax benefit-to-cost ratio would be 1.62 and the after-tax return on investment would be 32.6 percent, which is normally an acceptable rate for a retrofit.

Should the 50 hp electric motor fail in operation, a repair versus replace scenario may be performed. The difference between the new motor cost and the repair cost is used to determine the simple payback. In this case, the repair costs $1250, resulting in a difference of $1150. The simple payback is 1.25 years ($1,150 cost/$918.18 energy savings) with a 5.53 after-tax benefit-to-cost ratio and 212.7 percent after-tax return-on-investment. Thus the motor should be replaced versus repaired.

The preceding examples assumed that only efficiency would be the appropriate evaluation. When considering condition, these numbers begin to change drastically. For the following example, a motor circuit analysis evaluation of impedance shall be reviewed. Impedance unbalance and voltage unbalance are similar as, per Ohm's Law: Current = Voltage/Impedance, resulting in the following examples being applicable to both voltage and impedance unbalance.

The purpose of an electric motor is to convert electrical energy to mechanical torque. It operates best when all three phases of a three-phase motor are 120 electrical degrees from each other and other stator, rotor, and friction losses are controlled. As the phases vary from 120 degrees from each other, the efficiency of the electric motor decreases because it becomes harder for the magnetic fields within the stator to turn the rotor, and, when far enough off, they interfere with each other. This effect is found in both voltage and impedance unbalances, including impacts to efficiency, reliability, and production.

A 50 hp electric motor, as shown in the previous examples, with a 3.5 percent impedance unbalance, would have a resulting efficiency of 89 percent (3 percent reduction due to heating). The resulting energy costs would be $5275.20 demand and $16,503.84 annual energy usage, totaling $21,779.00 per year, an increase of $689.64 per year.

Combined energy and reliability
When considering both energy and reliability, production losses can be incorporated as part of the costs. The following information is gathered for evaluation based upon the preceding examples:

  • Electric motor: 50 hp, 1800 rpm, 75 percent loaded, 8760 hr/yr, 92 percent efficient with a 3.5 percent impedance unbalance (89 percent resulting efficiency)
  • Electrical costs: $14/kW demand and 0.06 cent/kWh
  • Reliability: 2 failures every 50,000 hours
  • Lifecycle: For the purpose of this example, the lifecycle is 50,000 hours
  • Replacement motor: 50 hp, 1800 rpm, premium efficient motor (95 percent), balanced phases that will reduce the failures to 1 in 50,000 hours.

Selection of program tools
As part of each successful electric motor energy and reliability program, a series of tools and software has to be selected in order to monitor and maintain the program. Several considerations must be made when putting together an energy and reliability toolkit—initial cost, training requirements, ergonomics, accuracy, and least invasive to the process.

These concepts were incorporated in a recent PG&E study that focused on electric motor energy and condition issues only. The purpose was to assemble a "tool kit" based upon independent research into a number of datalogging, efficiency, and condition analysis tools to determine energy and condition opportunities and how they interrelate. The initial areas of study were software, dataloggers, motor circuit analysis, vibration analysis, and infrared analysis. The results were to be developed into an Electric Motor Performance Analysis Tool (PAT) that would be used as part of a market transformation strategy. The tools that resulted from this study included the DOE's MM+, the Fluke 41B, the Summit Technology PowerSight 3000 datalogger, the BJM ALL-TEST IV Pro motor circuit analyzer, and the Pruftechnik Vibrotip. Infrared analysis was determined not to play a part in the motor only analysis, but would be an effective tool in a motor system analysis.

MotorMaster+ is used as a motor management support tool for commercial and industrial sites. It is designed for auditors, industrial energy coordinators, and plant or consulting engineers to provide the most efficient and cost effective decisions for electric motor and system planning. MotorMaster+ is used to identify inefficient, undersized, and oversized electric motors, and then calculate the energy and demand savings associated with the selection of energy efficient or premium efficient replacements.

The software tool contains a hierarchy of each plant being analyzed, a field data module, a motor price and performance database on over 20,000 new motors, energy conservation analysis, life cycle analysis, energy accounting capabilities, and even an environmental conservation capability.

The field data module serves as a motor inventory and field measurement storage repository. The module houses motor nameplate information, identification, process, and location codes; load type, operating hours and working environment descriptions; and such measured data as voltage, amperage, power factor, and speed at the load point.

The user can choose from a variety of descriptor-based motor inventory sorts within the Field Data Module. Motors operating under abnormal power supply conditions also can be detected. Measured values are used to determine existing motor loads and efficiencies. Batch analyses can be conducted automatically for populations of motors, determining the costs and energy savings due to changing out all motors in a given facility or process, or only those motors with simple paybacks below a stated value.

MotorMaster+ Version 3.0 also includes the following features:

  • A database of performance and price information on more than 20,000 IEC (metric) and National Electric Manufacturers Association (NEMA) Design B, C, and D three-phase motors. The motors range from 1 to 4000 hp, with speeds of 900, 1200, 1800, and 3600 rpm, and open drip-proof (ODP), totally enclosed fan-cooled (TEFC), totally enclosed nonventilated (TENV), weather-protected (WP), totally enclosed air-over (TEAO), totally enclosed blower-cooled (TEBC), and explosion-proof (EXPL) enclosures. Motors rated to operate at 200, 208, 230, 460, 575, 220/440, 796, 2000, 4000, and 6600 V are included. Full- and part-load efficiency values are measured in accordance with the IEEE 112 protocol to guarantee consistency. Manufacturers supply the information, and the database is updated annually.
  • Technical data that can help optimize a drive system, such as data on motor part-load efficiency and power factor; full-load speed; locked-rotor, breakdown, and full-load torque; and idle and locked-rotor amperage.
  • Purchase information, including list price, warranty period, catalog number, motor weight, and manufacturer's address.
  • Analysis features that calculate the energy savings, dollar savings, simple payback, cash flows, and after-tax rate of return-on-investment from using a particular energy efficient motor in a new purchase or retrofit application. Variables such as motor efficiency, purchase price, energy costs, hours of operation, load factor, and utility rebates are taken into account.
  • Utility rate schedule and motor rebate program data, including minimum qualifying efficiency and rebate dollar values.
  • Energy accounting, conservation savings tracking, and greenhouse gas emissions reduction reporting capabilities.
  • Menus and extensive help screens that make MM+ easy to learn and use.

MotorMaster+ Version 3.0 contains many motor energy management features. An informed MM+ user can:

  • Create a list of available new motors that meet purchase specifications.
  • Determine both energy and dollar savings from selecting and operating an energy efficient motor model.
  • Compute annual cash flows and the after-tax rate of return on a motor systems investment.
  • Create a company motor inventory database and generate searches and reports based on motor and load descriptors.
  • Initiate motor repair or replacement analyses for populations of motors within a company.
  • Produce energy conservation summary, facility reduction in consumption, and greenhouse gas emissions reduction reports.

MM+ modification
A modification to the existing MM+ was necessary in order to perform the condition analysis portion of the PG&E market transformation project. The modification was to allow for the ability to enter and search phase balance data in resistance, impedance and inductance, insulation resistance, and vibration analysis data in velocity and shock pulse. BJM coordinated and led the effort to implement this "first ever" industry funded modification to the MM+ software. Other industry participants included Pruftechnik; Dreisilker; Washington State University; PG&E; Boeing; General Motors; Oak Ridge National Labs; the DOE; and many others. BJM, Dreisilker, Pruftechnik, and PG&E worked together to define and promote the MM+ modification. This group coordinated with Washington State University and the DOE to implement the change. The DOE and WSU welcomed the industry recommendations and financial support for the MM+ modification. The version of MM+ that includes this recent modification is available for anyone to download.

Electrical data collection, logging
There were two basic approaches selected for data collection. One was "snapshot" data collection for basic data entry into MM+ of voltage, current, power factor, and kW. The second was datalogging of these measurements over time.

The first instrument selected was the Fluke 41B which provided the snapshot measurements required for under $2K per instrument, was portable, and simple to learn. The datalogger selected was the PowerSight 3000 which provided the datalogging capabilities, ease of use, cost under $4K each, and was already on hand to the utility and its customers. FlowcareEngineering Inc., the primary contractor for the project, developed a special tool for consolidating the electrical data and providing it in a manner that data entry into MM+ was made much simpler.

Motor circuit analysis
A number of motor circuit analyzers were studied for implementation into the project. Both on-line and off-line instruments were reviewed and a number tested. On-line tests were found to have challenges when applied in certain electrical environments, including variable frequency drive outputs, and required a great deal of training and experience.

The All-Test IV Pro was selected because it was a static (off-line) impedance-based meter, which provided the necessary measurements of resistance, impedance, and inductance unbalance for the project. It was found to be the simplest to use, the most accurate, weighs less than 2 lb, was the least intrusive of the off-line tests (less than 4 min for a complete battery of tests), and cost under $8,000.

Vibration analysis
There was a much larger variety of vibration analyzers available for review. Based upon a survey of equipment users, ease of use, portability, and best cost (less than $10,000), the Pruftechnik Vibrotip was selected as the vibration analyzer of choice. It provided the necessary measurements of velocity, carpet shock pulse, and max shock pulse that allowed for a quicker determination of bearing condition. Shock pulse was selected because this measurement type was not proprietary to the equipment.

Equipment implementation costs
As part of the implementation phase of the utility study, a number of case studies are underway. The effectiveness of both a basic (electrical data only) and advanced (energy and condition data) industrial survey, reviewing best cost of training, personnel, equipment, and results, is being reviewed. A two-day training program covering data collection, data entry, equipment use and analysis, and report writing was developed, one of the benefits of the selected tools' ease of use. Equipment costs were as follows:

  • Basic analysis equipment—datalogger and snapshot instrument with MM+ was $6000
  • Advanced analysis equipment—datalogger, snapshot instrument, motor circuit analyzer, and vibration analyzer was $24,000

By using a variety of tools, more than one person may be collecting a variety of data at one time. Presently, systems to automate data entry are under development.

The first site selected was a paperboard plant where a study was performed by Newcomb Anderson and Associates. Forty electric motors ranging from 15-200 hp were found to yield annual savings of $15,000 per year based upon just the basic analysis and energy savings. The simple paybacks on all motors varied from 1-5 yr, the return on investment was well over 20 percent, and the benefit-to-cost ratio was over 2:1, with 16 motors found to be oversized, 2 overloaded, and 22 inefficient. This study provided a small sample of the electric motors within the selected plant and could be used to assist in the justification of a much larger survey.

Application of energy and condition analysis
In 1999, the University of Illinois at Chicago Energy Resources Center was contracted by Dreisilker to perform a combined energy and reliability assessment at a coal-fired power plant. The primary tool used for analysis was the MM+ software tool, Version 3.0. The project was a challenge as no listing or locations of electric motors existed for the plant. The survey was limited to support motors only.

The survey identified 366 motors for evaluation with 328 in-service and 38 spare electric motors. Of the in-service electric motors, 315 were Design B, 12 were Design C, and one was Design D. The Design B motors were primarily used with fans, pumps, and air compressors; the Design C motors were used for coal conveyors; and the Design D was a hopper motor. Of particular importance was the use of Design C motors for the incline coal conveyors. This is because of the particular torque requirements for the start-up and movement of the conveyors loaded with coal. The Design C motor is excellent for this type of application because of high start-up, pull-up, and breakdown torques. If a Design B motor were to be used in place of a Design C, as was the case at the plant prior to the survey, it most likely would stall during the pull-up torque portion of the torque curve.

Because of the age of the plant, a number of other considerations for retrofitting or repair versus replace decisions had to be observed:

  • As many of the larger electric motors are original frame or U-frame, base retrofits or modifications have to be considered as an additional cost.
  • Shaft couplings may have to be changed out to fit newer electric motors, due to different shaft sizes.
  • Heaters, fuses, starters, and wiring must be properly sized to work with appropriate electric motors.
  • Possibility of variable frequency drive applications for fans, pumps, and air compressors.
  • Operating speed differences between newer energy efficient and older electric motors.

Through the use of MM+, retrofit and repair versus replace decisions were analyzed from an energy standpoint. For the purposes of the study, the following information was used: Estimated energy costs, $0.025/kWh usage and $10/kW demand; a 35 percent discount factor for a particular brand of electric motors selected by the plant; and a maximum 5-year payback. As a result, 15 of the in-service electric motors were found to be excellent retrofit candidates, with a use reduction of 68,705 kWh and a demand reduction of 8.2 kW for a 37 percent after-tax return on investment and a 1.7 benefit-to-cost ratio. In addition, 51 electric motors were found to be excellent replace instead of repair candidates with a use reduction of 197,254 kWh and 23.5 kW demand ending with a 92.9 percent return on investment and a 3.2 benefit-to-cost ratio.

MotorMaster+ then was used to analyze the in-plant spare motors. Of the 38 electric motors in stock:

  • When comparing the existing in-use motors to the spares, it was found that 23 of the 38 electric motors did not match any motors in the plant.
  • Of the remaining electric motors, due to storage practices, not a single spare was ready for use. The majority were rusty with seized shafts and the remainder were failed motors.

Finally, a reliability, preventive, predictive, root cause analysis, and corrective maintenance program was recommended. The MM+ database and capabilities were implemented as part of the program. It was determined that program implementation, including equipment costs, would have an initial 3 month simple payback and a 0.5 month annual cost payback due to reduction in failures, downtime, and corrective action costs.

A combined energy and reliability program, using MM+ and selected logging and analysis tools, will have a tremendous payback in energy and industrial assessment programs. With the latest improvement within MM+, electric motors found in poor electrical or mechanical condition can be analyzed for repair versus replace using an energy-based financial assessment. The fact that the necessary modifications were fully funded by industrial users shows that industry recognizes the potential impact of this type of analysis. The combined energy, reliability, waste stream, and production cost avoidance impact in virtually any type of industrial or commercial facility is staggering, allowing for the improved competitiveness of U.S. industry.

Presently, energy and reliability assessments are under way with commercial buildings in Chicago, a national lab in association with a motor repair center and utility, a number of industrial sites including chemical and petroleum, and as case studies for at least one utility. It is expected that overall operating costs will be improved by at least 10 percent at each of the facilities. MT

This e-mail address is being protected from spambots. You need JavaScript enabled to view it , Ph.D., is the director of the BJM Corp. Electric Motor System Testing and R&D Division, Old Saybrook, CT (860) 399-5937. He is a past chair of the Chicago Section of the Institute of Electrical and Electronic Engineers (IEEE) and past chair of both the IEEE Dielectrics and Electrical Insulation Society and Power Electronics Society for IEEE Chicago.

Project Contributors
Howard W. Penrose, Ph.D, BJM Corp., All-Test Div.
Jim Hanna, Pacific Gas & Electric
Johnny Douglas, Washington State University
Chris Cockrull, U.S. Department of Energy
Greg Lee, Pruftechnik, Inc.
Dave Van Horn, Dreisilker Electric Motors, Inc.

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