Figure 9-51.  Compound wound generator.  Figure 9-52.  Wear areas of commutator and brushes. Generator Ratings DC Generator Maintenance parallel characteristics described earlier. The output of a  compound wound generator is relatively constant, even with  changes in the load. A DC generator is typically rated for its voltage and power  output. Each generator is designed to operate at a specified  voltage, approximately 14 or 28 volts. It should be noted that  aircraft electrical systems are designed to operate at one of  these two voltage values. The aircraft’s voltage depends on  which battery is selected for that aircraft. Batteries are either  12 or 24 volts when fully charged. The generator selected  must have a voltage output slightly higher than the battery  voltage. Hence, the 14-or 28-volt rating is required for aircraft  DC generators. The power output of any generator is given as the maximum  number of amperes the generator can safely supply. Generator  rating and performance data are stamped on the nameplate  attached to the generator. When replacing a generator, it is  important to choose one of the proper ratings.  The rotation of generators is termed either clockwise or  counterclockwise, as viewed from the driven end. The  direction of rotation may also be stamped on the data plate. It  is important that a generator with the correct rotation be used;  otherwise, the polarity of the output voltage is reversed. The  speed of an aircraft engine varies from idle rpm to takeoff  rpm; however, during the major portion of a flight, it is at a  constant cruising speed. The generator drive is usually geared  to turn the generator between 1 1 ⁄ 8 and 1 1 ⁄ 2 times the engine  crankshaft speed. Most aircraft generators have a speed at  which they begin to produce their normal voltage. Called the  “coming in” speed, it is usually about 1,500 rpm.  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:  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 and possible replacement of various  components.  [Figure 9-52] Manufacturers usually recommend the following procedures  to seat brushes that do not make good contact with slip  rings or commutators. Lift the brush sufficiently to permit  9-33 1.  Security of generator mounting. 2.  Condition of electrical connections. 3.  Dirt and oil in the generator. If oil is present, check  engine oil seals. Blow out any dirt with compressed air. 4.  Condition of generator brushes. 5.  Generator operation. 6.  Voltage regulator operation. A B N S + − Series field winding Parallel field winding Load Armature winding Series field coil To load Armature Shunt field coil Compound wound