MAINTAINING ELECTRIC MOTORS USED FOR IRRIGATION

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1 MAINTAINING ELECTRIC MOTORS USED FOR IRRIGATION F. Richar Bear, Agricultural Equipment, Structures an Electricity Robert W. Hill, Biological & Irrigation Engineering August 2000 ENGR/BIE/WM/06 Electricity is a clean, economical an epenable source of power for irrigation. Electric motors can provie years of service when properly selecte, operate an maintaine. This fact sheet escribes factors that affect electric motor performance an service life an escribes proceures for controlling internal motor heat. PROPER MOTOR SELECTION Electric motors are size (rate) to operate uner a stanar set of conitions. Motors must be selecte for ifferent applications base on nameplate ratings. The nameplate escribes the operating parameters for an electric motor an communicates this information to the user. If a 40 horsepower (HP) motor is overloae (accientally use to rive a loa larger than 40 HP or operate at less than rate voltage), the motor will raw excessive amperage in an attempt to provie the necessary power to rive the loa. When an overloa excees the nameplate rating, the motor will run hotter than its esign operating temperature. This increase in temperature eteriorates motor wining insulation an shortens motor life. Motor conuctors an insulation are not esigne to power loas larger than the nameplate ratings. FACTORS AFFECTING ELECTRIC MOTOR LIFE The life expectancy of electric motors are base on several factors. The following operating conitions affect motor life: ambient or surrouning air temperature elevation above sea level ventilation service factor (inicates amount of overloa the motor can tolerate) operating voltage 1

2 Other factors affecting motor life inclue: moisture, corrosive environment (salt water), motor enclosure, maintenance, type of bearings, single or three phase power, motor type an uty rating. Uner normal operating conitions the expecte life of a motor may be 5 to 10 years. The operating life of a motor may be extene to 20 years when it is use for 6-month irrigation seasons. Operation of the motor at internal temperatures above the nameplate rating will reuce motor life by amaging the motor wining insulation. Insulation materials eteriorate ue to oxiation. The rate of oxiation increases very rapily at temperatures above that liste on the motor nameplate. When wining insulation eteriorates sufficiently the motor will short internally, reucing the power output, which further overloas the motor causing it to fail, commonly known as burning-up. Another cause of motor overheating is the lack of air circulation through or aroun the motor. Small roents or birs may buil nests aroun the motor or in the motor air vents causing ecrease air circulation an higher internal temperatures. Roents will chew on the electrical insulation of the motor winings resulting in an electrical short which can estroy the motor. MOTOR OPERATING TEMPERATURE There are several factors that contribute to an increase in electric motor operating temperatures an heat. Some of these factors are ambient temperature, elevation above sea level, sunlight, overloaing an low voltage operation. Ambient Temperature Motors are normally rate for sea level operation at a temperature of 104ºF. Temperatures above 104ºF cause the internal motor temperature to excee esign limitations. Elevation Above Sea Level Motors will operate within rate temperature limits at elevations up to 3,300 feet. Internal motor temperature increases approximately 1ºF for each 330 feet increase in elevation above sea level an beyon 3,300 feet, motor winings begin to overheat. Motors with a service factor greater than 1 (such as 1.15) can be operate at higher elevations (up to 9,000 feet) without overheating. Sunlight - Direct raiation from the sun can easily increase internal motor temperature by 10 to 20ºF. Overloaing When horsepower eman is greater than the nameplate rating a motor will attempt to prouce the aitional horsepower; however, the result is higher current (amperage) flow an overheating of the motor winings. Low Voltage Operation When an electric motor operates at voltages less than rate values (unersize power lines, improper connections, istribution line overloas) the motor will raw aitional current in an attempt to prouce the rate horsepower. This increase current flow creates a temperature buil-up of 1 to 2ºF for each percentage point the operating voltage is below rate voltage. 2

3 PREVENTING THE OVERHEATING OF MOTOR WINDINGS Overheating is the main cause of reuce motor life. A proucer who uses an electric motor for irrigation has little control over ambient air temperature or elevation. The amage cause by irect sunlight can be lessene if the motor is painte a reflective white or light color, an/or the motor is shae from irect sunlight. Circulating air aroun the motor (builtin fan) is a practice that reuces internal heat buil-up in the motor. Locating a motor in a small, well ventilate an screene builing will prevent small animals an birs from builing nests in or aroun the motor an protect the motor from amage by sunlight. A motor may be purchase with an overloa protective evice, calle a thermal protector, as an integral part of the motor or this evice may be installe in the circuit with a motor. Such evices etect the temperature of the motor winings an/or high current situations an turn off the motor when overheating/overloaing occurs. Although these protective evices will exten motor life, they may be inappropriate for pumping systems where irrigation is more critical than reuction of motor life. PROPER MAINTENANCE OF MOTOR BEARINGS Remember to follow the motor manufacturers recommene maintenance scheule. In general, motor bearings shoul be lubricate once each year. Each spring before the irrigation season begins is the best time. The following proceures shoul be use when lubricating the bearings of a soli shaft motor. Open the rain plug in the bearing housing. Forcing grease into the bearing housing without opening the plug can amage the bearing shiels an force ol grease, moisture an irt into the motor housing. Pump grease into the bearing housing until the new grease begins to come out of the rain. Replace the rain plug. In a close-couple centrifugal pump, where bearings are not seale, it is important to keep the pump packing glan properly ajuste an maintaine. Water spraying from the pump can enter the motor bearing causing it to prematurely fail. For vertical hollow shaft motors, the bearing oil shoul be change each year. When replacing the oil, follow motor manufacturers recommenations or use 32 W clear turbine oil. The bearing oil level shoul be maintaine at the proper level. Overfilling the oil reservoir can cause oil to overflow when a motor heats up uring operation. The excess oil aheres to the motor an ventilation screens, collecting irt an ebris, an ecreasing the motor s ability to issipate heat. Excessive motor or pump noise can be the result of motor bearing failure. When failing, motor bearings generate excessive heat that may also serve as a warning to the operator. Taking care of these problems before failure occurs can save money an prevent owntime. 3

4 GENERAL MAINTENANCE OF MOTORS Keep ventilation screens clean an in place Have motor winings ippe an bake every three to five years. This process as insulation to the motor winings, provies an opportunity to inspect an clean the motor bearings an extens the life of the motor. The motor winings can be ippe an bake for a fraction of the cost of rewining the motor. Replacing bearings before they begin to fail can save the cost of shaft repair, replacement of bearing housings an repairing amage pumps. Scheule maintenance of electric motors can also help avoi costly owntime uring the irrigation season. At the beginning of each irrigation season, before the motor is use, inspect the electrical wiring an tighten all electrical connections. Loose connections cause heat buil-up at electrical terminals an eterioration of motor wining insulation resulting in electrical shorts an/or motor failures. The voltage rop across loose connections cause the motor to operate at less than rate voltage, increasing internal motor temperature. A loose or broken connection can unbalance the phases of three-phase power an amage the motor winings. Use common sense; keep water away from the motor, keep the area aroun the motor ry an properly support the electrical panel to prevent motor vibrations from loosening electrical connections. ELECTRICAL PROTECTION OF THE MOTOR A motor s electrical panel shoul inclue circuit breaker(s) for overloa currents, a lightning arrester, surge protector an phase failure relay (to protect motor from phase reversal or failure, an low voltage). These evices protect the motor from external electrical problems, but they are not necessarily esigne to protect the motor from overloaing. Other safety evices such as a pressure switch (isconnects the motor if pumping pressure rops to unesirable levels), can be installe in the electrical panel for special situations. MAINTENANCE OF AREA AROUND PUMP AND MOTOR A well ventilate an screene she or shelter will provie shae for the pump an motor an protect the equipment from unesirable natural elements. Of special concern is the growth of plants an brush commonly foun aroun irrigation pumps an motors. Wees, grasses, an other plants shoul be kept away from electric motors. These plants provie a place for roents an insects to live an such pests can crawl insie motors, chew insulation, an nest in the motor enclosure. Such pests can cause faulty connections an electrical shorts within the motor enclosure an motor electrical panel. WINTER MAINTENANCE OF PUMPS AND MOTORS Ieally pumps an motors shoul be protecte from the elements year roun, an this is especially important in Utah uring the winter months. Temperature extremes, snow an moisture can amage electrical irrigation equipment. It is also critical that all water be raine from irrigation pumps prior to freezing weather. 4

5 SUMMARY The factors that affect an electric motor s service life inclue supply voltage, ambient temperature, overloaing, an ventilation. These factors are not easily controlle; however, selecting the proper motor type an size to match the loa, protecting the motor from irect sunlight, keeping the motor service an clean, keeping pests away from the motor, proviing goo ventilation an preventing overloas, will help assure maximum motor life. The following items summarize electric motor maintenance. 1. Operate motor at proper voltage (It is the responsibility of the power company to provie the proper voltage an your responsibility to perioically check the operating voltage). 2. Select the appropriate size an type of motor to prevent overloaing. 3. Have proper electrical motor protection installe in service panel an/or motor enclosure. 4. Regularly lubricate bearings, replace as neee. 5. Dip an bake winings at regular intervals, instea of rewining. 6. Keep electrical connections tight. 7. Keep ventilation screens clean an in place. 8. Keep motors an area aroun motors clean. 9. Shae the motor to keep it cooler. 10. Protect the motor from extreme weather conitions. 11. Use common sense. Maintain an service instea of repair. WHERE CAN YOU GET HELP? Utah State University - Extension Service Agriculture Systems Technology Biological an Irrigation Engineering 1498 North 800 East 4105 Ol Main Hill Logan, UT Logan, UT rbear@cc.usu.eu bobh@ext.usu.eu (435) (435) Information in this fact sheet was erive from Sorenson, R.B., L.N. Allen an R.W. Hill Extening Electric Motor Life, Colorao River Salinity Control (CRSC) Information Bulletin #14. (Aapte from Pacific Northwest Extension Publication 292, January 1986.) an from Allen, L.N. an R. B. Sorensen Maintaining Electric Motors Use for Irrigation, CRSC Information Bulletin #15. Utah State University Extension is an affirmative action/equal employment opportunity employer an eucational organization. We offer our programs to persons regarless of race, color, national origin, sex, religion, age or isability. Issue in furtherance of Cooperative Extension work, Acts of May 8 an June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert L. Gillilan, Vice-Presient an Director, Cooperative Extension Service, Utah State University, Logan, Utah.(EP/ /DF) 5