mounted within the alternator housing. When the CSD and  the alternator are contained within one unit, the assembly is  known as an integrated drive generator (IDG).  alternator. The speed control unit is made up of a wobble plate  that adjusts hydraulic pressure to control output speed.  Figure 9-80  shows a typical electrical circuit used to control  alternator speed. The circuit controls the hydraulic assembly  found in a typical CSD. As shown, the alternator input speed  is monitored by a tachometer (tach) generator. The tach  generator signal is rectified and sent to the valve assembly.  The valve assembly contains three electromagnetic coils that  operate the valve. The AC alternator output is sent through a  control circuit that also feeds the hydraulic valve assembly.  By balancing the force created by the three electromagnets,  the valve assembly controls the flow of fluid through  the automatic transmission and controls the speed of the  AC alternator.  The CSD is a hydraulic unit similar to an automatic  transmission found in a modern automobile. The engine of  the automobile can change rpm while the speed of the car  remains constant. This is the same process that occurs for an  aircraft AC alternator. If the aircraft engine changes speed,  the alternator speed remains constant. A typical hydraulic- type drive is shown in  Figure 9-79 . This unit can be controlled  either electrically or mechanically. Modern aircraft employ  an electronic system. The constant-speed drive enables the  alternator to produce the same frequency at slightly above  engine idle rpm as it does at maximum engine rpm.  The hydraulic transmission is mounted between the AC  alternator and the aircraft engine. Hydraulic oil or engine oil  is used to operate the hydraulic transmission, which creates a  constant output speed to drive the alternator. In some cases,  this same oil is used to cool the alternator as shown in the CSD  cutaway view of  Figure 9-79 . The input drive shaft is powered  by the aircraft engine gear case. The output drive shaft, on the  opposite end of the transmission, engages the drive shaft of  the alternator. The CSD employs a hydraulic pump assembly,  a mechanical speed control, and a hydraulic drive. Engine  rpm drives the hydraulic pump, the hydraulic drive turns the  It should be noted that an AC alternator also produces a  constant 400 Hz if that alternator is driven directly by an  engine that rotates at a constant speed. On many aircraft,  the auxiliary power unit operates at a constant rpm. AC  alternators driven by these APUs are typically driven  directly by the engine, and there is no CSD required. For  these units, the APU engine controls monitor the alternator  output frequency. If the alternator output frequency varies  from 400 Hz, the APU speed control adjusts the engine rpm  accordingly to keep the alternator output within limits.  9-43 Figure 9-77.  Schematic of an AC alternator.  Main AC alternator armature winding Exciter  electromagnet  field Pilot exciter armature 3-phase  bridge rectifier N S Main AC alternator field Exciter armature Exciter  permanent  magnet field