International projectionist (Jan-Dec 1935)

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* Maintaining Motor Generators * By A. C. SCHROEDER THE first consideration in the maintenance of motor generator sets is lubrication. There must always be plenty of clean oil. Only mineral oil is used in these machines; animal or vegetable oils are not satisfactory. Everyone knows what the lack of oil will do: overheated bearings, premature wear, scored journals, ruined bearings. Dirt in the oil has much the same effect. Dirt is a cutting agent, embedding itself in the softer metal and then scratching the other surface against which it rubs. Oil that is too heavy will not allow the oil rings to rotate when the temperature is very low, thus "starving" the bearings. A light oil is the proper one to use, so that it flows freely even in the coldest weather. Keep the oil even with the top of the cups that are on the sides for checking the level. Notice if the rings are revolving, and whether ample oil is being carried up by them. Oil should be changed about every six months. If it comes out black and dirty, fill the bearings with clean oil and then drain it out again after running the machine a little. Do this until the oil comes out clean. In the case of overheated bearings on one of the smaller machines, slow it down if possible, but do not let it stop, as it is likely to stick. On a large machine il is safer to keep it running than to take a chance of it "freezing up." Flood the hot bearing with clean oil. Do not use water under any circumstances. It takes time to cool a bearing, as there is a large amount of metal that is overheated. When a bearing overheats, something is radically wrong. In a new machine this condition is not surprising, but when it occurs after a machine has been running well for some time, it is usually due to neglect or carelessness. Some causes of overheating are: a scored shaft, bent shaft, bearings too tight, bearings im Mica will project after the copper wears a little y ^: properly fitted, although not tight; bearings out of line, oil ring stuck or broken, lack of oil, dirty oil, diluted oil (water, coal oil or some other substance in the oil) and overloaded bearings. An overloaded bearing is a rare occurrence, except in a belt-driven machine where the belt is too tight. Bearings out of line really cause an overloaded condition, but be fitted and adjusted, also freed up in the holders; brush tension can be adjusted; "shorts" in the commutator can be found, such as metal particles lodged between the bars; sometimes "shorts" and open circuits can even be located and repaired in the armature, but usually these will have to be sent out. When the commutator is out of round, or a TAPE Figure 2 V. \ \ I --M--I x***~***A**.*m*»f**AML*tA*AA»AAaAbl Figure 1 this is not thought of in that way. The remedy for any of these conditions is obvious. When a shaft has been scored, it is important that it be put in condition before being used again. A commutator that is in good condition will be dark, very smooth and shiny. By dark I do not mean black. A black commutator will not be smooth nor will it shine; something is wrong. When it appears as at first mentioned it is easy to take care of. Never touch it with sandpaper. A lubricant must be applied sparingly and not too often. There is much discussion as to what is the best lubricant. I have used both vaseline and oil, but I cannot see any great difference, although I favor oil. A small amount is put on a cloth and applied to the commutator while the machine is running. Spread the lubricant over the entire surface. This will be enough to lubricate, yet there will not be enough to cause trouble. A burned, dirty, or rough commutator should be sanded, unless it is out of round, in which event it must be turned down. Use fine sandpaper, and (we repeat what has been said hundreds of times before) never use emery cloth. Sanding is usually not the only thing to be done in this case; the cause of the trouble must be found. The possible causes are numerous: brush tension too heavy or too light, brushes sticking, improper brush contact, some of the brushes disconnected or not touching the commutator on machines having two or more brushes in parallel, thus overloading the remaining brushes; out of round commutator, high or low bar in commutator, high mica, grounded or shorted commutator bars, open or short-circuits in armature, and a grounded armature. Loose bearings may cause commutator trouble, as will an overloaded machine. Some of the aforementioned troubles may cause the solder to fly out of the commutator. Not all of these troubles can be repaired by the projectionist. Brushes can From I. P., Jan. and Feb., 1933 segment is high or low, it must be sent out to be repaired. Ridges and uneven wear can be corrected by the use of special stones. They are mounted in holders with a handle on them and are applied to the commutator while the machine is running. Stoning is continued until all ridges and uneven surfaces have been removed. The commutator can be put in good condition this way, if it is not out of round. The mica must be inspected after stoning the commutator. It must be well below the surface of the copper bars. It will probably be necessary to undercut the mica, which is done with a tool similar to that shown in Figure 1. A piece of hacksaw blade is used, one end being wrapped with tape, so it can be held without injuring the hand. It often is necessary to grind the sides of the teeth so they just fit the space between segments of the commutator. The tool is moved back and forth on top of the mica, keeping the blade between the segments. Work carefully, as the blade has a habit of leaving the groove and cutting a deep scratch across the commutator face. Figure 2 shows the right and the wrong way of doing this job. The cut shows two commutator bars and the space between them. At A is an improperly undercut mica, shown by the shaded portion. At B the mica has been cut down clean on both sides of the groove and the top of the mica is well below the surface of the copper. When a commutator is undercut as at A, it requires only a little wear of the copper before the mica at the two corners of the segments will be flush with the copper. The mica does not wear as rapidly as the copper and from then on it will protrude further and further, partially holding the brushes away from the commutator, thus causing sparking and more rapid wear and burning of the copper bars. . After undercutting, sand the commutator with fine paper, blow away the dust with bellows and oil the surface lightly. When commutator and brushes are in (Continued on page 23) [20]