Machinery's Handbook, 31st Edition
Electric Motor Maintenance 2657 In addition to these special-purpose motors, there are two types of explosion-proof mo- tors designed for hazardous locations. One type is for operation in hazardous dust loca tions (Class II, Group G of the National Electrical Code) and the other is for atmospheres containing explosive vapors and fumes classified as Class I, Group D (gasoline, naphtha, alcohols, acetone, lacquer-solvent vapors, natural gas). Electric Motor Maintenance Electric Motor Inspection Schedule.— Frequency and thoroughness of inspection de- pend upon such factors as 1) importance of the motor in the production scheme; 2) per centage of days the motor operates; 3) nature of service; and 4) winding conditions. The following schedules, recommended by the General Electric Company, and cover- ing both AC and DC motors are based on average conditions in so far as duty and dirt are concerned. Weekly Inspection.— 1) Surroundings. Check to see if the windings are exposed to any dripping water, acid or alcoholic fumes; also, check for any unusual amount of dust, chips, or lint on or about the motor. See if any boards, covers, canvas, etc., have been misplaced that might interfere with the motor ventilation or jam moving parts. 2) Lubrication of sleeve-bearing motors. In sleeve-bearing motors check oil level, if a gage is used, and fill to the specified line. If the journal diameter is less than 2 inches, the motor should be stopped before checking the oil level. For special lubricating systems, such as wool-packed, forced lubrication, flood and disk lubrication, follow instruction book. Oil should be added to bearing housing only when motor is at rest. A check should be made to see if oil is creeping along the shaft toward windings where it may harm the insulation. 3) Mechanical condition. Note any unusual noise that may be caused by metal-to-metal contact or any odor as from scorching insulation varnish. 4) Ball or roller bearings. Feel ball- or roller-bearing housings for evidence of vibration, and listen for any unusual noise. Inspect for creepage of grease on inside of motor. 5) Commutators and brushes. Check brushes and commutator for sparking. If the motor is on cyclic duty it should be observed through several cycles. Note color and surface con dition of the commutator. A stable copper oxide-carbon film (as distinguished from a pure copper surface) on the commutator is an essential requirement for good commutation. Such a film may vary in color all the way from copper to straw, chocolate to black. It should be clean and smooth and have a high polish. All brushes should be checked for wear and pigtail connections for looseness. The commutator surface may be cleaned by using a piece of dry canvas or other hard, nonlinting material that is wound around and securely fastened to a wooden stick, and held against the rotating commutator. 6) Rotors and armatures. The air gap on sleeve-bearing motors should be checked, especially if they have been recently overhauled. After installing new bearings, make sure that the average reading is within 10 percent, and reading should be less than 0.020 inch (0.51 mm). Check air passages through punchings and make sure they are free of foreign matter. 7) Windings. If necessary clean windings by suction or mild blowing. After making sure that the motor is dead, wipe off windings with dry cloth, note evidence of moisture, and see if any water has accumulated in the bottom of frame. Check if any oil or grease has worked its way up to the rotor or armature windings. Clean with carbon tetrachloride in a well-ventilated room. 8) General. This is a good time to check the belt, gears, flexible couplings, chain, and sprockets for excessive wear or improper location. The motor starting should be checked to make sure that it comes up to proper speed each time power is applied. Monthly or Bimonthly Inspection.— 1) Windings. Check shunt, series, and commutat ing field windings for tightness. Try to move field spools on the poles, as drying out may have caused some play. If this condition exists, a service shop should be consulted. Check motor cable connections for tightness.
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