AC alternators produce a three-phase AC output. For each  revolution of the alternator, the unit produces three separate  voltages. The sine waves for these voltages are separated  by 120°.  [Figure 9-75]  This wave pattern is similar to those  produced internally by a DC alternator; however, in this case,  the AC alternator does not rectify the voltage and the output  of the unit is AC.  The modern AC alternator does not utilize brushes or slip  rings and is often referred to as a brushless AC alternator.  This brushless design is extremely reliable and requires  very little maintenance. In a brushless alternator, energy to  or from the alternator’s rotor is transferred using magnetic  energy. In other words, energy from the stator to the rotor  is transferred using magnetic flux energy and the process  of electromagnetic induction. A typical large aircraft AC  alternator is shown in  Figure 9-76 . As seen in  Figure 9-77 , the brushless alternator actually  contains three generators: the exciter generator (armature  and permanent magnet field), the pilot exciter generator  (armature and fields windings), and the main AC alternator  (armature winding and field windings). The need for  brushes is eliminated by using a combination of these three distinct generators.  The exciter is a small AC generator with a stationary field  made of a permanent magnet and two electromagnets. The  exciter armature is three phase and mounted on the rotor shaft.  The exciter armature output is rectified and sent to the pilot  exciter field and the main generator field.  The pilot exciter field is mounted on the rotor shaft and is  connected in series with the main generator field. The pilot  exciter armature is mounted on the stationary part of the  assembly. The AC output of the pilot exciter armature is  supplied to the generator control circuitry where it is rectified,  regulated, and then sent to the exciter field windings. The  current sent to the exciter field provides the voltage regulation  for the main AC alternator. If greater AC alternator output  is needed, there is more current sent to the exciter field and  vice versa.  In short, the exciter permanent magnet and armature starts the  generation process, and the output of the exciter armature is  rectified and sent to the pilot exciter field. The pilot exciter  field creates a magnetic field and induces power in the  pilot exciter armature through electromagnetic induction.  The output of the pilot exciter armature is sent to the main  alternator control unit and then sent back to the exciter field.  As the rotor continues to turn, the main AC alternator field  generates power into the main AC alternator armature, also  using electromagnetic induction. The output of the main AC  armature is three-phase AC and used to power the various  electrical loads.  Some alternators are cooled by circulating oil through the  internal components of the alternator. The oil used for  cooling is supplied from the constant speed drive assembly  and often cooled by an external oil cooler assembly. Located  in the flange connecting the generator and drive assemblies,  ports make oil flow between the constant speed drive and  the generator possible. This oil level is critical and typically  checked on a routine basis.  The unit shown in  Figure 9-78  contains an alternator assembly  combined with an automatic drive mechanism. The automatic  drive controls the alternator’s rotational speed which allows  the alternator to maintain a constant 400-Hz AC output.  All AC alternators must rotate at a specific rpm to keep  the frequency of the AC voltage within limits. Aircraft AC  alternators should produce a frequency of approximately  400 Hz. If the frequency strays more than 10 percent from  this value, the electrical systems do not operate correctly. A  unit called a constant-speed drive (CSD) is used to ensure  the alternator rotates at the correct speed to ensure a 400- Hz frequency. The CSD can be an independent unit or  Alternator Drive  Figure 9-75.  AC alternator sine waves. A B C 90° 180° 270° 360° 120° 120° 120° Phase C Phase A Phase B One full rotation of the AC alternator Figure 9-76.  Large aircraft AC alternator. 9-42