desirable. Most machines require a complete overhauling and thorough cleaning out once a year.
Essential for satisfactory operation of brushes is free movement of the brushes in their holders. Uniform brush pressure is necessary to assure equal current distribution. Adjustment of brush holders should be set so that the face of the holder is approximately one eighth of an inch up from the commutator; any distance greater than this will cause brushes to wedge, resulting in chattering and excessive sparking.
Check the brushes to make sure that they will not wear down too far before the next inspection. Keep an extra set of brushes available so that replacements can be made when needed. Sand in new brushes, and run the motor without a load to seat the brushes.
Make sure that each brush surface in contact with the commutator has the polished finish that indicates good contact and that the polish covers all contact surfaces of the brush. Check the freedom of motion of each brush in the brush holder. When replacing a brush, be sure to put it in the same brush holder and in its original position. It will be easier for you to replace the brush properly if you scratch a mark on one side of the brush before you remove it.
Check the springs that hold the brushes against the commutator. Improper spring pressure may lead to commutator wear and excessive sparking. Excessive heating may have annealed the springs, in which case you should replace them and correct the cause of overheating.
Inspect the commutator for color and condition. The part where the brushes ride should be clean and smooth and should be a polished brown color. A bluish color indicates overheating of the commutator.
You should remove any roughness on the commutator by sandpapering or stoning. Never use an emery cloth or an emery stone. For this operation, run the motor without load. If you use sandpaper, wrap it partly around a wooden block. The stone is essentially a piece of grindstone, known to the trade as a com- mutator stone. With the motor running without load, press the stone or sandpaper against the commutator with moderate pressure and move it back and forth across the commutator surface. If the armature is very rough, it should be taken out and the commutator turned down in a lathe.
Use care not to come into contact with moving parts.
The electrical shop should have a record card for every motor. As a minimum, the information on the card should include inspections, repair work, age, and replacement stock number.
About once a year or more often if conditions warrant, motors should be cleaned thoroughly. Smaller motors, the windings of which are not easily accessible, should be taken apart.
First, remove the heavy dirt and grease with a heavy, stiff brush; wooden or fiber scrapers; and cloths. You can use rifle-cleaning bristle brushes in the air ducts. You can blow-dry dust and dirt off, using dry-compressed air at a moderate pressure, perhaps 25 to 50-psi pressure at the point of application, taking care to blow the dirt out and away from the windings. If the dirt and dust are metallic? conducting, or abrasive, using air pressure is not as satisfactory as using a suction system.
When cleaning motors with compressed air, wear safety goggles and hearing protection. Dispose of lubricants and contaminated materials in an environmentally responsible manner.
You can easily remove grease, oil, and sticky dirt by applying cleaning liquids specifically designed for the purpose. These liquids evaporate quickly and, if not applied too generously, will not soak or injure the insulation. If you do use one of these liquids, be sure to follow the manufacturer's direction for use.
After new motors and controls are installed, they should be checked for operation under load for an initial period of at least 1 hour. During this time, the electrician can observe if any unusual noise or hot spots develop. The operating current must be checked against the nameplate ampere rating. This check requires skill in the proper connection, setting, and reading of a clamp-on ammeter. The nameplate ampere reading multiplied by the service factor (if any)Continue Reading