International projectionist (Jan-Dec 1935)

BRUSHES: THEIR USE AND RELATION TO MOTOR TROUBLE L. L. Stoffel PARADOXICALLY enough, troubles which occur from time to time on the brushes of electric motors are in one way a positive advantage, for in most cases these annoying incidents give warning — often long in advance — of deeper-seated troubles in other parts of the machines. By watching brushes closely much trouble can be avoided. Periodical, systematic inspection is the only way to forestall equipment trouble. When a brush starts to spark, the first step is to examine the brush itself and its immediate surroundings. Are the brushes bedding properly on the commutator or slip-rings? Unless the contact is perfect all over, the brush naturally cannot function properly. If the brush has been recently replaced, has the right kind of brush been used? There are hundreds of possible combinations of resistance, hardness, abrasiveness, contact-drop qualities, and grades of material. These brush properties are really a part of the design of the machine. Even a much more costly brush may be less suitable than that recommended by the makers of the motor. And it by no means follows that because a brush happens to fit the brush-holder it will function correctly. Excessive Sparking If sparking persists after checking the foregoing, see if the brush is worn down too far for the tension spring to keep it in proper contact. Brushes which are too short (especially, if they are also somewhat too loose in the "box" of the holders) are apt to chatter and spark. In extreme cases, they may even break or slip from under the "box-holder" — generally with disastrous results to the motor itself. Next: is the brush free, but not too free, to feed forward in its holder? "Stuck" brushes are a frequent cause of sparking and of ultimate destructiveness to the surface on which they run. Dirt, corrosion due to chemical fumes, and brushes the sides of which are not parallel, are the principal causes of the jamming of brushes. Next try adjusting the tension spring on the hammer "which presses the brush against the commutator or ring. If the pressure is insufficient, the brush will spark and chatter. If too much pressure is used, the sparking may decrease, but the brush will get hot and wear quickly. Where the motor is exposed to much dirt or corrosion, it may not be possible to cure the sparking entirely by the methods previously cited, or by others detailed subsequently. In such a case, get in touch with the makers of the motor (or with a brush manufacturer) relative to using a type of brush with greater abrasiveness so as to keep the metal-collecting surface clean. Such changes should not be made haphazardly. Moreover, the new brushes should not be tested by running them together with the old brushes, for if the resistances of the two sets are different one set may "hog" the load, thus making the "test" worse than useless. Check Pigtail Connections Next, check up the pigtail connections which furnish a path for the current from holder to brush in most motors. Are they unbroken? Are they properly secured to the brush and holder? If not, the current must pass through the hammer and tension spring, and through the sliding contact between brush and metal holder surface. "Pitting," burning and arcing will then make short work of these. Are the connections between brush spindles and their cables tight? If not, one or more sets of brushes will have to carry more than their share of current, and will almost certainly spark excessively. The trouble will be on the set which does NOT spark, of course. Are the brush spindles loose in their bushings? Are they parallel with the commutator or ring surface? Are they also parallel with the slots in the commutator (if the machine be a commutator type) ? Are the brushes properly spaced around the commutator or rings? This is much more important in the case of a commutator machine than in one with slip rings. If the brush rocker is adjustable, does it hold the brush system as a whole in its right position as to the proper collecting points? Modern interpole machines are extremely sensitive to wrong position of even single brushes. Is the brush rocker clamped tight so that it cannot shift around? Testing for Overheating Does the sparking still persist? If the machinery driven by the motor has recently been rearranged or added to, test the load. TEST — don't guess. If this is the trouble, the motor armature will be hot all over, concurrently with the sparking. Test for the temperature rise above that of the surrounding air. Most motors are rated to operate on full load with a rise of 50 degrees Centigrade; though some are rated for 40 degrees only. Pack the thermometer directly in contact with the overheated part, covering it with tightly packed rags, so that the cooling effect of the surrounding air will not falsify the reading. Allow a few minutes for the thermometer to reach its maximum reading. If no thermometer is handy, and you use the rough test of noting if the armature is too hot to bear the hand on it for a few seconds, remember to feel the windings — not the metal. Metal always feels much hotter than it actually is. If the measured load is not excessive, Simple representation of the sound picture process — studio to theatre — used in popular magazine stories on the art t s.w. THE TALKIE CYCLE OF SOUND L.W.-LICHT WAVES E.W.-EIECTRIC WAVES S.W.SOUND WAVES 10U> SPEAKtfc [11]