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parallel characteristics described earlier. The output of a compound wound generator is relatively constant, even with changes in the load. 

## Generator Ratings 

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. 

## DC Generator Maintenance 

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 seals. Blow out any dirt with compressed air. 

4. Condition of generator brushes. 

5. Generator operation. 

6. Voltage regulator operation. 

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 

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