In testing for a grounded brush holder or rigging, touch one test lamp prod of the armature test set to the motor housing. With the other test prod, touch each brush holder individually. If the lamp lights, there is a ground in the brush holder.
CAUTION Remove all leads to the brush holders and brushes before you attempt this test.
The color of the commutator and slip rings will indicate the type of trouble. An even chocolate-brown color indicates a normal condition and a black color indicates brush arcing. You can remove slight burns on the commutator segments by polishing the commutator as the armature rotates. Use a canvas pad, as shown in figure 7-61. To remove the deeper burns, use fine sandpaper instead of the canvas pad. When a commutator is deeply scored, it must be reconditioned in a lathe or with a special tool.
CAUTION Never use emery cloth to polish commutators because the emery particles can lodge between the segments and cause the commutator circuits to short.
Slip rings used on rotors are usually made of bronze or other nonferrous metals. Under normal conditions, the wearing surface should be bright and smooth. When the rings are pitted, they should be polished. When excessively worn and eccentric, they should be trued with a special tool.
REASSEMBLY. - After you have inspected all parts and repaired or replaced the faulty ones, you are ready for reassembly. To assemble motors, follow in reverse order the procedures of their disassembly. Be sure to check any available literature you may find. Be sure to oil or grease the bearings as required. Remove the relief plug in the bottom of the housing while you apply grease.
Figure 7-61. - Fabricated cleaning pad.
The most important rule to remember when you are making repairs or inspecting motor controllers is as follows:
CAUTION Be sure the controller is disconnected from the power source before touching any of the operating parts.
Control equipment should be inspected and serviced on the same maintenance schedule as motors. Motor starters can normally be repaired on the job site at the time of inspection. After the power has been secured, the first thing you should do to keep controllers operating at maximum efficiency is to keep them free of dirt, dust, grease, and oil, both inside and out. Clean the operating mechanism and contacts with a clean, dry, tintless cloth, or vacuum cleaner. Small and delicate mechanical parts may be cleaned with a small, stiff bristle brush and a Navy-approved solvent.
Check the contacts to ensure proper electrical connections. When contacts open and close, the rolling and rubbing action keeps the contacts bright and clean. Infrequently operated contacts or contacts under heavy loads can overheat and create oxidation on the contacts.
Copper contacts are used for most heavy-duty power circuits, and, in many cases, in relay and interlock circuits. They should be inspected regularly. If projections extend beyond the contact surfaces or if the contacts are pitted or coated with copper oxide, they should be sanded down with fine sandpaper.
Welding of contacts sometimes occurs, in spite of all precautions. Low voltage is the most common cause. Welding may also result from overloads, low-contact pressure resulting from wear or weak springs, loose connections, or excessive vibrations. If welding occurs, it is an indication of trouble in the electrical system. The contacts must be replaced, but it is useless to replace them unless the cause of the welding is found and corrected.
Carbon contacts are used when a contactor is frequently opened and closed. It is essential that the contactor be open when it is de-energized. Since carbonContinue Reading