Operation and Maintenance of Motors

8/9/2019 Operation and Maintenance of Motors

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Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance

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Operation and Maintenance of Motors

Topics:

Introduction

Types of Motors

Key Components of Motors Safety Issues

Best Practices for Efficient Operation

Best Practices for Maintenance

Maintenance Schedule for Motors

References

IntroductionElectric motors convert electrical energy into useful mechanical energy by running electrical current

through a coil, resulting in the torque needed to turn a shaft. Almost every major piece of equipment in acommercial buildingboilers, chillers, air handlers, pumps, and cooling towers, to name a fewrelies onelectric motors.

As they run, motors can become less efficient because of wear, breakdown of lubricants, andmisalignment. Good motor-maintenance practice helps avoid or postpone these problems. A lack ofmaintenance can reduce a motors energy efficiency and increase unplanned downtime. Scheduledmaintenance is the best way to keep the motors operating efficiently and reliably.

Sources of potential motor efficiency loss


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Types of Motors

Induction (AC) Motors

Induction motors are the dominant type of motor used in alternating-current (AC) applications. Theyaccount for more than 90% of all installed horsepower, largely because they are rugged, simple, reliable,and cheap. They are so named because the power in the rotor is induced through moving magnetic fieldsin the stator.

Direct Current (DC) MotorsDC motors use direct current (DC) rather than AC. One type of DC motorthe electronically commutatedmotor (ECM)is sometimes found in variable-speed HVAC applications, such as in fans and chillercompressors. They can be more efficient and easier to control than induction motors.

Synchronous Motors

Synchronous motors are generally used for very large AC applications where more than 100 hp isrequired, and are rare in commercial facilities.

Key Components of Motors

AC motors have a fixed outer portion called the stator, and a rotor attached to a shaft that spins inside itto provide mechanical output. The corresponding parts for a DC motor are the stationary field pole andthe spinning armature. In addition, most DC motors contain a commutator, which regulates the electriccurrent to the armature. Virtually all motors use rotating magnetic fields to spin their rotors. Inside a motor,the magnetic fields try to align, just as two magnets close to one another will try to align their magneticfields. This perpetual effort at alignment causes the motor's rotor to spin. The strength of the fields andtheir degree of offset determines the amount of work, in the form of the torque of the shaft, the motor canprovide.


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Parts of a typical induction motor

Other major parts of a motor include a frame to protect its components and anchor them to a base, andbearings to support the shaft and rotor. The electrically generated magnetic fields result from coils orwindings, which are wound around steel cores.

Safety IssuesBefore servicing motors and motor-operated equipment, disconnect the power supply to the motor andaccessories. An electrical lockout/tagout procedure is recommended, where every piece of equipmentserviced is logged, and electrical disconnects are physically disabled.

Best Practices for Efficient OperationThe following best practices will reduce the cost of operation and maintenance:

Turn Off Unneeded Motors

Identify motors that run unnecessarily, and turn them off when appropriate. Examples include exhaustfans running when ventilation needs are met, and escalators operating after closing. You may need toreprogram the building control systems to accomplish this.

Reduce the Use of the Motor System

Increasing the efficiency of mechanical systems can reduce the amount of time that associated motorsneed to run. For example, improving the performance of a cooling tower can reduce the run time that thefans need to reject the same amount of heat. Eliminating excessive starts and stops is also worthwhile.Starting and stopping a motor stresses its parts and degrades its performance. Frequent stops and startsincrease the need for maintenance.


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Best Practices for MaintenanceProperly selected and installed motors can operate for many years with minimal maintenance.Nonetheless, regular care will extend their life and maximize their energy efficiency. In addition to periodicupkeep, good recordkeeping and smart replacement planning are key elements of a good motor-maintenance program.

Regular Upkeep

Clean motor surfaces and ventilation openings periodically. Heavy accumulations of dust and lint willresult in overheating and premature motor failure.

Properly lubricate moving parts. Some motors have sealed bearings that require no servicing. Forothers, regular lubrication will avoid unnecessary wear. Be sure to apply appropriate types and quantitiesof lubricant. Applying too little or too much can harm motor components.

Keep motor couplings properly aligned. Correct shaft alignment ensures smooth, efficient transmissionof power from the motor to the load. Incorrect alignment puts strain on bearings and shafts, shorteningtheir lives and reducing system efficiency. Shafts should be parallel and directly in line with each other.Shaft alignment should be checked and adjusted regularly. Many couplings have hard rubber inserts thatcan degrade, so rubber dust on the equipment base may indicate problems.

Properly align and tension belts and pulleys when they are installed, and inspect them regularly toensure that alignment and tension stay within tolerances. Abnormal wear patterns on belts may

indicate problems. Loose belts may squeal and will slip on the pulley, generating heat. Correctlytensioned pulleys run cool. Excessive tension strains bearings and shafts, and shortens their lives.

Maintain bearings by keeping them clean, lubricated, and loaded within tolerances. Proper belttension or shaft alignment minimizes strain on the bearings and helps them achieve their expected life.Pay particular attention to bearings on motors equipped with VFDs. These can be prone to shaft currents,which can cause serious damage to the bearings. Fortunately, there are several technologies that canmitigate shaft-current problems.

Check for proper supply voltages. Unbalanced powerthat is, three-phase motors where the supplyvoltage to the phases varies by more than 1%can lead to overheating and reduced motor life. So toocan situations where the supply voltage is much higher or lower than the motors rated voltage.

Avoid painting motor housings. Paint acts as insulation, increasing operating temperatures andshortening motor life. One coat of paint has little effect, but years of paint buildup can have a significant

effect.

Periodically inspect commutators visually. Potential problems with commutators (which are onlyrequired for DC motors with brushes) will be seen as discolorations, flat spots, or burn marks. Colorpatterns can be normal as long as they appear around the entire commutator. If you notice problems,remove and repair the commutator, or replace key components.

Good Recordkeeping

Maintain an up-to-date motor inventory. The inventory should include all substantial motors, but canbegin with the largest and those with the longest run times. This inventory lets facility managers makeinformed choices about replacement, either before or after a motor fails. Field-testing motors before theyfail can help ensure that replacements are properly sized. (See Smart Replacement Strategies below formore information.)

Keep maintenance logs. These logs should contain vital information such as the make, model, serialnumber, type, and specifications of each motor; the locations and specifications for belts, pulleys, etc.;and a historical record of maintenance activities. This helps the maintenance staff remember when tests,inspections, or servicing are due. It also allows the staff to quickly identify spare parts or replacementswhen needed. In addition, comparing recent test results to past values can provide early indications ofreduced motor performance.

Consider a computerized maintenance program that incorporates inventories and logs. Theseprograms can also notify plant personnel of required upcoming motor maintenance, and include analysis


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tools for determining how effectively maintenance is being carried out. One such example isMotorMaster+, a widely used program developed by the U.S. Department of Energy. The figure belowshows examples of the Motor Inventory Forms in MotorMaster+.

Example motor inventory forms (from Motormaster+)

Smart Replacement Strategies

When a motor fails, use an appropriately sized replacement. Many motors are oversized for theirapplications, resulting in poor motor efficiency and excessive energy use. If an oversized motor fails,replace it with a smaller, energy-efficient model. Doing so will reduce the replacement cost and will loweroperating costs, since the new motor should operate closer to its point of maximum efficiency (generallyaround 75% of the motors rated horsepower). In these situations, verify that the new motor can stillprovide sufficient output under all operating conditions.

Plan ahead for replacing a failed motor with a new energy-efficient model. Stocking premium-efficiencyreplacements for critical motors can help avoid the hasty replacement of a failed motor with a standard-

efficiency model that happens to be the only one available on short notice. Maintenance staff can decidewhich motors warrant such advance planning.

Replace, rather than rewind, motors when appropriate. Many motors have been repaired more thanonce, with a typical loss of nearly 1% in efficiency at each rewind. These motors may be much lessefficient than their nominal ratings, making them good candidates for replacement when they next fail. It ismore common to rewind larger motors due to their high capital cost. But these motors usually operate atvery high duty, and even a modest efficiency improvement may make it worthwhile to replace them withnew, premium-efficiency motors rather than repair them.


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Maintenance Schedule for Motors

Description Comments Maintenance Frequency

Motor use/sequencing Turn off or sequence unnecessary motors. Weekly

Overall visual inspection Verify equipment is operating and safetysystems are in place.

Weekly

Check bearings and drivebelts

Inspect for wear, and adjust, repair, or replaceas necessary.

Weekly

Motor alignment Look for rubber or steel savings under couplings,or listen for odd noises, as these may indicate aproblem).

Weekly

Motor condition Check condition by analyzing temperature orvibration, and compare to baseline values.

Quarterly (or as neededon weekly inspections)

Cleaning Remove dust and dirt to facilitate cooling. Quarterly

Check lubrication Ensure bearings are lubricated as recommendedby manufacturer.

Annually (or based on runhours)

Check mountings Secure any loose mountings. Annually

Check terminal tightness Tighten any loose connections. Annually

Check for balanced

three-phase power

Troubleshoot unbalanced motor circuit and fix

problems if the voltage imbalance exceeds 1%.

Annually

Check for over- or under-voltage conditions

Troubleshoot motor circuit and fix problems if thesupply voltage differs significantly from ratedvoltages.

Annually

ReferencesElectrical Construction & Maintenance Magazine. Resolving Voltage Problems with AC Induction Motors.

March 1, 2001. http://powerquality.com/mag/power_resolving_voltage_problems/ (available as ofAugust 29, 2007).

E-Source, Inc. 1996. E Source Technology Atlas. Drivepower

HPAC Engineering 2007. Preventing Damage to Motor Bearings. http://www.hpac.com/Technologies/MotorsDrives/Article/47488/Preventing_Damage_to_Motor_Bearings (available as of August 28,2007).

PG&E 1997. Efficiency Improvements for AC Electric Motors. Application NotesAn In-DepthExamination of Energy Efficiency Technology.

PG&E 1997. Efficiency Opportunities through Motor Maintenance. Application NotesAn In-DepthExamination of Energy Efficiency Technology.

PG&E 1997. Motor Maintenance Efficiency OpportunitiesA Guide to Help You Minimize Motor EnergyUse. Fact SheetTips for Reducing Energy Costs.

PNNL 2004. O&M Best Practices - A Guide to Achieving Operational Efficiency, Release 2.0. Preparedfor Federal Energy Management Program (FEMP)

Pump and Systems 2007. How to Prevent Electrical Erosion in Bearings. http://www.pump-zone.com/article.php?articleid=322 (available as of August 28, 2007)

U.S. Department of Energy. MotorMaster+, Version 4.00.06 (released 3/1/07).

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Operation and Maintenance of Motors