Jumermaintenance of motors

Electric Motor Parts,Maintenance and Safety Procedures

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.

Parts of Electrical Motor

1. Centrifugal Switch 2. Bearing3. Terminal Box4. Power Supply5. Cooling Fan6. Drive Pulley7. Rotor8. Stator

CENTRIFUGAL SWITCHA type of switch that

operates using the centrifugal force created from the rotating shaft. The centrifugal switch activates and de-activates depending on the speed of the motor. 

BEARINGA device that

supports, guides, and reduces the friction of motion between fixed and moving machine parts.

TERMINAL BOX

On a piece of electric equipment (such as a motor), a box within which the leads from the piece of equipment are connected to the leads supplying the equipment with power; usually provided with a removable cover plate for access.

POWER SUPPLYA power

supply is a device that supplies electric power to an electrical load. 

COOLING FAN is a self cooling electric

motor. Fan cooled motors feature an axial fan attached to the rotor of the motor (usually on the opposite end as the output shaft) that spins with the motor, providing increased airflow to the motor's internal and external parts which aids in cooling.

DRIVE PULLEYThe fixed pulley in

a belt drive system that receives energy from the power source and transfers it to the driven pulley. 

ROTOR In an electric motor the moving part is the rotor which turns the shaft to deliver the mechanical power. The rotor usually has conductors laid into it which carry currents that interact with the magnetic field of the stator to generate the forces that turn the shaft.

STATORThe stationary

part is the stator, usually has either windings or permanent magnets.

Operation and

Maintenance of Motors

SAFETY FIRSTHigh voltage and rotating

parts can cause serious or fatal injury. Safe installation, operation, and maintenance must be performed by qualified personnel. Familiariza- tion with and adherence to NEMA MG2, the National Electrical Code (NEC) and local codes is required.

It is important to observe safety precautions to protect personnel from possible injury. Personnel should be instructed to:1. Be familiar with the equipment and read all instructions thoroughly before installing or working on equipment.

2. Avoid contact with energized circuits or rotating parts. 3. Disconnect all power sources before initiating any maintenance or repair.4. Act with care in accordance with prescribed procedures in handling and lifting this equipment.

5. Be sure unit is electrically grounded in accordance with code requirements.6. Be sure equipment is properly enclosed or protected to prevent access by children or other unauthorized personnel in order to prevent possible accidents.

7. Be sure shaft key is fully captive before unit is energized. 8. Avoid contact with capacitors until safe discharge procedures have been completed.

9. Provide proper guarding for personnel against rotating parts and applications involving high inertia loads which can cause over speed. 10. Avoid extended exposure to equipment with high noise levels.

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.

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.

Properly lubricate moving parts.

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.

Abnormal wear patterns on belts may indicate problems. Loose belts may squeal and will slip on the pulley, generating heat.

Properly align and tension belts and pulleys when they are installed, and inspect them regularly to ensure that alignment and tension stay within tolerances.

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.

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.

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.

Check for proper supply voltages.

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.

Avoid painting motor housings.

Potential problems with commutators (which are only required for DC motors with brushes) will be seen as discolorations, flat spots, or burn marks.

Periodically inspect commutators visually.

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

The inventory should include all substantial motors, but can begin with the largest and those with the longest run times.

Maintain an up-to-date motor inventory.

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.

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.

Maintenance Schedule for Motors Description Comments Maintenance Frequency Motor use/sequencing Turn off or sequence unnecessary motors. WeeklyOverall visual inspection Verify equipment is operating

and safety systems are in place. WeeklyCheck bearings Inspect for wear, and adjust, repairand drive belts or replace as necessary.

Weekly

Motor alignment Look for rubber or steel savings under couplings, or listen for odd noises,

as these may indicate a problem).

Weekly

Motor condition Check condition by analyzing temperatureor vibration, and compare to baseline values. on weekly inspections)

Quarterly (or as needed

Cleaning Remove dust and dirt to facilitate cooling. Quarterly Check lubrication Ensure bearings are lubricated as recommended by manufacturer.

Annually(or based on run hours)

Check mountings Secure any loose mountings.Annually Check terminal tightness Tighten any loose connections. Annually Check for balanced Troubleshoot unbalanced motor circuit and Annually three-phase power fix problems if the voltage imbalance exceeds 1%. Check for over- or Troubleshoot motor circuit and fix problems under- voltage conditions if the supply voltage differs significantly from rated voltages.

Annually

Working Procedures

•Wipe, brush, vacuum or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling air flow. Heat reduces insulation life and eventually causes motor failure.

Dirt and Corrosion

•Feel for air being discharged from the cooling air ports. If the flow is weak or unsteady, internal air passages are probably clogged. Remove the motor from service and clean.

•Check for signs of corrosion. Serious corrosion may indicate internal deterioration and/or a need for external repainting. Schedule the removal of the motor from service for complete inspection and possible rebuilding.

•In wet or corrosive environments, open the conduit box and check for deteriorating insulation or corroded terminals. Repair as needed.

Lubrication

Lubricate the bearings only when scheduled or if they are noisy or running hot. Do NOT over-lubricate. Excessive grease and oil creates dirt and can damage bearings.

Heat, Noise and Vibration

Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise or vibration.

Heat

Excessive heat is both a cause of motor failure and a sign of other motor problems.Overheating results from a variety of different motor problems.

They can be grouped as follows:•WRONG MOTOR: It may be too small or have the wrong starting torque characteristics for the load. This may be the result of poor initial selection or changes in the load requirements.

•POOR COOLING: Accumulated dirt or poor motor location may prevent the free flow of cooling air around the motor. In other cases, the motor may draw heated air from another source. Internal dirt or damage can prevent proper air flow through all sections of the motor.

Dirt on the frame may prevent transfer of internal heat to the cooler ambient air.

•OVERLOADED DRIVEN MACHINE: Excess loads or jams in the driven machine force the motor to supply higher torque, draw more current and overheat.

MotorHorsepower

LightDuty(1)

StandardDuty(2)

HeavyDuty(3)

SevereDuty(4)

Up to 7-1/210 to 40

50 to 150Over 150

10 years7 years4 years1 year

7 years4 years

1-1/2 years6 months

4 years1-1/2 years9 months3 months

9 months4 months3 months2 months

Motor Operating Conditions

•Light Duty: Motors operate infrequently (1 hour/day or less) as in portable floor sanders, valves, door openers.•Standard Duty: Motors operate in normal applications (1 or 2 work shifts).

Examples include air conditioning units, conveyors, refrigeration apparatus, laundry machinery, woodworking and textile machines, water pumps, machine tools, garage compressors.

•Heavy Duty: Motors subjected to above normal operation and vibration (running 24 hours/day, 365 days/year). Such operations as in steel mill service, coal and mining machinery, motor-generator sets, fans, pumps.•Severe Duty: Extremely harsh, dirty motor applications. Severe vibration and high ambient conditions often exist.

•EXCESSIVE FRICTION: Misalignment, poor bearings and other problems in the driven machine, power transmission system or motor increase the torque required to drive the loads, raising motor operating temperature.

•ELECTRICAL OVERLOADS: An electrical failure of a

winding or connection in the motor can cause other Windings or the entire motor to overheat.

Noise and VibrationNoise indicates motor

problems but ordinarily does not cause damage. Noise, however, is usually accompanied by vibration.Vibration can cause damage in several ways.

It tends to shake windings loose and mechanically damages insulation by cracking, flaking or abrading the material.

Winding InsulationWhen records indicate a

tendency toward periodic winding failures in the application, check the condition of the insulation with an insulation resistance test. Such testing is especially important for motors operated in wet or corrosive atmospheres or in high ambient temperatures.

Cleaning and Drying WindingsMotors which have been

flooded or which have low megger readings because of contamination by moisture, oil or conductive dust should be thoroughly cleaned and dried. The methods depend upon available equipment.

A hot water hose and detergents are commonly used to remove dirt, oil, dust or salt concentrations from rotors, stators and connection boxes. After cleaning, the windings must be dried, commonly in a forced-draft oven. Time to obtain acceptable megger readings varies from a couple hours to a few days.

Tools and Equipment in Electric

Motor Maintenance

Basic tools

Vibration measurement tools 

Electrical discharge detector

Thermal scanner

Thermal image

Lubricants

Jumer P. EspañaBSIT-ET 4

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