×

Motors

Article

Continue Reading

Introduction

Electric 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 a commercial building-boilers, chillers, air handlers, pumps, and cooling towers, to name a few-relies on electric motors.

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

Sources of potential motor efficiency loss. 

Types of Motors

Induction (AC) Motors

Induction motors are the dominant type of motor used in alternating-current (AC) applications. They account 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 fields in the stator.

Direct Current (DC) Motors

DC motors use direct current (DC) rather than AC. One type of DC motor-the electronically commutated motor (ECM)-is sometimes found in variable-speed HVAC applications, such as in fans and chiller compressors. 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 is required, 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 it to provide mechanical output. The corresponding parts for a DC motor are the stationary field pole and the spinning armature. In addition, most DC motors contain a commutator, which regulates the electric current 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 magnetic fields. This perpetual effort at alignment causes the motor's rotor to spin. The strength of the fields and their degree of offset determines the amount of work, in the form of the torque of the shaft, the motor can provide.

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, and bearings to support the shaft and rotor. The electrically generated magnetic fields result from coils or windings, which are wound around steel cores.

Safety Issues

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

Best Practices for Efficient Operation

The 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 exhaust fans running when ventilation needs are met, and escalators operating after closing. You may need to reprogram 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 motors need to run. For example, improving the performance of a cooling tower can reduce the run time that the fans 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 starts increase the need for maintenance.

Best Practices for Maintenance

Properly 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 periodic upkeep, 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 will result in overheating and premature motor failure.

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

Keep motor couplings properly aligned. Correct shaft alignment ensures smooth, efficient transmission of power from the motor to the load. Incorrect alignment puts strain on bearings and shafts, shortening their 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 that can 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 to ensure 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. Correctly tensioned 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 belt tension 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 can mitigate shaft-current problems.

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

Avoid painting motor housings. Paint acts as insulation, increasing operating temperatures and shortening 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 only required for DC motors with brushes) will be seen as discolorations, flat spots, or burn marks. Color patterns 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 can begin with the largest and those with the longest run times. This inventory lets facility managers make informed choices about replacement, either before or after a motor fails. Field-testing motors before they fail can help ensure that replacements are properly sized. (See Smart Replacement Strategies below for more information.)

Keep maintenance logs. These logs should contain vital information such as the make, model, serial number, 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 replacements when needed. In addition, comparing recent test results to past values can provide early indications of reduced motor performance.

Consider a computerized maintenance program that incorporates inventories and logs. These programs can also notify plant personnel of required upcoming motor maintenance, and include analysis tools for determining how effectively maintenance is being carried out. One such example is MotorMaster+, a widely used program developed by the U.S. Department of Energy. The figure below shows 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 their applications, 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 lower operating costs, since the new motor should operate closer to its point of maximum efficiency (generally around 75% of the motor's rated horsepower). In these situations, verify that the new motor can still provide sufficient output under all operating conditions.

Plan ahead for replacing a failed motor with a new energy-efficient model. Stocking premium-efficiency replacements 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 decide which motors warrant such advance planning.

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

Maintenance Schedule for Motors

DescriptionCommentsMaintenance Frequency
Motor use/sequencingTurn off or sequence unnecessary motors.Weekly
Overall visual inspectionVerify equipment is operating and safety systems are in place.Weekly
Check bearings and drive beltsInspect for wear, and adjust, repair, or replace as necessary.Weekly
Motor alignmentLook for rubber or steel savings under couplings, or listen for odd noises, as these may indicate a problem).Weekly
Motor conditionCheck condition by analyzing temperature or vibration, and compare to baseline values.Quarterly (or as needed on weekly inspections)
CleaningRemove dust and dirt to facilitate cooling.Quarterly
Check lubricationEnsure bearings are lubricated as recommended by manufacturer.Annually (or based on run hours)
Check mountingsSecure any loose mountings.Annually
Check terminal tightnessTighten any loose connections.Annually
Check for balanced three-phase powerTroubleshoot unbalanced motor circuit and fix problems if the voltage imbalance exceeds 1%.Annually
Check for over- or under-voltage conditionsTroubleshoot motor circuit and fix problems if the supply voltage differs significantly from rated voltages.Annually

References

Electrical Construction & Maintenance Magazine. Resolving Voltage Problems with AC Induction Motors. March 1, 2001. (available as of August 29, 2007).

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

HPAC Engineering 2007. Preventing Damage to Motor Bearings (available as of August 28, 2007).

PG&E 1997. Efficiency Improvements for AC Electric Motors. Application Notes-An In-Depth Examination of Energy Efficiency Technology.

PG&E 1997. Efficiency Opportunities through Motor Maintenance. Application Notes-An In-Depth Examination of Energy Efficiency Technology.

PG&E 1997. Motor Maintenance Efficiency Opportunities-A Guide to Help You Minimize Motor Energy Use. Fact Sheet-Tips for Reducing Energy Costs.

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

Pump and Systems 2007. How to Prevent Electrical Erosion in Bearings. (available as of August 28, 2007)

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

Continue Reading