The following information about the
inspection and maintenance of dc generator systems is general in nature
because of the large number of differing aircraft generator systems. These
procedures are for familiarization only. Always follow the applicable manufacturer's
instructions for a given generator system.
In general, the inspection of the generator installed in the aircraft should include the following items:
1. Security of generator mounting.
2. Condition of electrical connections.
3. Dirt and oil in the generator. If oil is present, check engine oil seal. Blow out dirt with compressed air.
4. Condition of generator brushes.
5. Generator operation.
6. Voltage regulator operation.
A detailed discussion of items 4, 5, and 6 is presented in the following paragraphs.
Condition of Generator Brushes
Sparking of brushes quickly reduces the effective brush area in contact with the commutator bars. The degree of such sparking should be determined. Excessive wear warrants a detailed inspection.
The following information pertains to brush seating, brush pressure, high mica condition, and brush wear.
Manufacturers usually recommend the following procedures to seat brushes which do not make good contact with slip rings or commutators.
The brush should be lifted sufficiently to permit the insertion of a strip of No. 000, or finer, sandpaper under the brush, rough side out (figure 9-30). Pull sandpaper in the direction of armature rotation, being careful to keep the ends of the sandpaper as close to the slip ring or commutator surface as possible in order to avoid rounding the edges of the brush. When pulling the sandpaper back to the starting point, the brush should be raised so it does not ride on the sandpaper. The brush should be sanded only in the direction of rotation.
After the generator has run for a short period, brushes should be inspected to make sure that pieces of sand have not become embedded in the brush and are collecting copper.
Under no circumstances should emery cloth or similar abrasives be used for seating brushes (or smoothing commutators), since they contain conductive materials which will cause arcing between brushes and commutator bars.
Excessive pressure will cause rapid wear of brushes. Too little pressure, however, will allow "bouncing" of the brushes, resulting in burned and pitted surfaces.
A carbon, graphite, or light metalized brush should exert a pressure of 1 1/2 to 2 1/2 psi on the commutator. The pressure recommended by the manufacturer should be checked by the use of a spring scale graduated in ounces. Brush spring tension is usually adjusted between 32 to 36 ounces; however, the tension may differ slightly for each specific generator.
When a spring scale is used, the measurement of the pressure which a brush exerts on the commutator is read directly on the scale. The scale is applied at the point of contact between the spring arm and the top of the brush, with the brush installed in the guide. The scale is drawn up until the arm just lifts off the brush surface. At this instant, the force on the scale should be read.
Flexible low resistance pigtails are provided on most heavy current carrying brushes, and their connections should be securely made and checked at frequent intervals. The pigtails should never be permitted to alter or restrict the free motion of the brush.
The purpose of the pigtail is to conduct the current, rather than subjecting the brush spring to currents which would alter its spring action by overheating. The pigtails also eliminate any possible sparking to the brush guides caused by the movement of the brushes within the holder, thus minimizing side wear of the brush.
Carbon dust resulting from brush sanding should be thoroughly cleaned from all parts of the generators after a sanding operation. Such carbon dust has been the cause of several serious fires as well as costly damage to the generator.
Operation over extended periods of time often results in the mica insulation between commutator bars protruding above the surface of the bars. This condition is called "high mica" and interferes with the contact of the brushes to the commutator.
Whenever this condition exists, or if the armature has been turned on a lathe, carefully undercut the mica insulation to a depth equal to the width of the mica, or approximately 0.020 inch.
Each brush should be a specified length to work properly. If a brush is too short, the contact it makes with the commutator will be faulty, which can also reduce the spring force holding the brush in place. Most manufacturers specify the amount of wear permissible from a new brush length. When a brush has worn to the minimum length permissible, it must be replaced.
Some special generator brushes should not be replaced because of a slight grooving on the face of the brush. These grooves are normal and will appear in ac and dc generator brushes which are installed in some models of aircraft generators. These brushes have two cores made of a harder material with a higher expansion rate than the material used in the main body of the brush. Usually, the main body of the brush face rides on the commutator. However, at certain temperatures, the cores extend and wear through any film on the commutator.
If there is no generator output, follow a systematic troubleshooting procedure to locate the malfunction. The following method is an example. Although this method may be acceptable for most 28 volt, twin engine or four engine dc generator systems using carbon pile voltage regulators, the applicable manufacturer's procedures should be followed in all cases.
If the generator is not producing voltage, remove the voltage regulator and, with the engine running at approximately 1,800 rpm, short circuit terminals A and B at the subbase of the regulator as shown in the diagram of figure 9-31. If this test shows excessive voltage, the generator is not at fault, but the trouble lies in the voltage regulator. If the test fails to produce voltage, the generator field may have lost its residual magnetism.
To restore residual magnetism, flash the generator field by removing the regulator and connect terminal A of the voltage regulator base to the battery at a junction box or a bus bar as indicated by the dotted line in the diagram of figure 9-32, while running the engine at cruising rpm. If there is still no voltage, check the leads for continuity shorts and grounds. If the generator is located where the brushes and commutator can be inspected, check each for proper condition as prescribed in the applicable manufacturer's procedures. If necessary, replace the brushes and clean the commutator. If the generator is located so that it cannot be serviced in the airplane, remove it and make the inspection.