International projectionist (Jan-Dec 1951)

VOLUME XXVI JANUARY 1951 NUMBER 1 Maintenance and Servicing of Motors BEFORE attempting to diagnose motor troubles the projectionist should make certain of the type and specifications of the motor. This information is usually given on the nameplate of the motor. (1) Manufacturer's name. (2) Serial number. This number is necessary when ordering parts. (3) Type of motor, which may be D.C. (shunt, series, or compound) ; "universal A.C.-D.C. ; synchronous, or induction. Induction motors of the single-phase type are usually described as "split-phase," "capacitor," or "repulsion-induction" motors, depending upon the principle utilized for starting. (4) Phase. Most motors designed for operation on A.C. are single-phase and 3-phase. (5) Cycles: 25, 50, or 60 cycles. (6) Volts: line voltage should be within 10% of the rated voltage. (7) Horsepower. Motors rated at %, 1/6, 1/5, %, 1/3, V2, and % H.P. are known as "fractional horsepower motors." (8) R.P.M. refers to the speed in revolutions per minute. Explanation of Indicia In addition to these data, certain other information is sometimes given on the nameplate. The model and the order numbers of the motor, for example. Temperature-rise rating is a guarantee that the motor will not get hotter than room temperature plus the stated temperature rise, if correctly operated and not overloaded. The rise rating for most "open" motors is 40° C. (104° F.), or 50° C. (122° F.) Such a motor may feel decidedly warm to the hand, but not uncomfortably hot. Many "en By ROBERTA. MITCHELL III. Trouble-Shooting Charts closed" motors have a rating of 55° (131° F.) rise. The term "open" on a nameplate means that the end brackets have openings and that the rotor shaft has vanes affixed to draw cool air over the windings and expel the heat generated. Such a motor should not be housed in an airtight compartment, as this would lead to overheating of the motor. Obtain, if possible, the manufacturer's instructions for lubricating and servicing each type of motor used in the projection room, generator room, and backstage. The bearings are more subject to wear than any other part of a motor, and hence should be periodically inspected according to the schedule previously given. The condition of sleeve bearings is in a large measure indicated by the air gap between stator and rotor pole-pieces. Worn bearings will lower the rotor, and may even allow the rotor to strike against the stator. Ball bearings should be checked for heating and noisy operation. Commutators and brushes are involved in the servicing of D.C, universal, and repulsion-induction motors. Grooved and lop-sided commutators must be "trued" in a lathe, but minor irregularities may be removed by using a small commutator stone or 00 sandpaper wrapped on the end of a stick and applied to the commutator while the motor is running. (Never use emery paper on commutators!) The mica insulation between the commutator bars should be undercut about 1/16 inch, as copper wears faster than mica. A steel slotting tool is used for undercutting, and a V tool for giving the edges of the copper bars a very slight bevel. After the mica slots have been cleaned, the mica surfaces should be coated with glossy red enamel of the iron oxide or synthetic type. A commutator is "dressed" by cleaning with a dry rag, applying a small amount of vaseline to the copper bars with a clean rag, and then wiping off the vaseline. When in good condition, a commutator will have a chocolate-brown color due to an extremely thin film of copper oxide. The carbon of the brushes and the oxygen of the air help maintain this desirable brown coating. All brush rigging must be kept clean and in good working order. The tension of the brushes (about 1% pounds per square inch of brush surface) must be checked occasionally. Too much tension may cause chattering and rapid grooving of the commutator. The pigtail connections must be tight. Care in Brush Selection The importance of using the right type and grade of brush for each motor cannot be overemphasized. The brush material, whether graphite, electrographite, metal graphite, carbon graphite, or hard carbon, must have adefinite resistance in ohms per cubic inch, a certain current-carrying capacity in amperes per square inch of brush face, and a specified contact area. New brushes may be shaped to fit the curvature of the commutator by placing a strip of sandpaper on the commutator and working back and forth while the brush is pressed down upon it. Further servicing information is sup INTERNATIONAL PROJECTIONIST January 1951