CONTACTOR GP-ALARM 1234635 - Caterpillar

of permanent magnet (PM) and Pilot Exciter Armature (L5). The pilot exciter operates independently from the generator output voltage. Constant excitation during a large load application is possible. Constant excitation is possible because the irregularities that occur in the generator output voltage are not fed back into the exciter. The irregularities that occur in the generator output voltage are caused by load conditions. The independent operation also allows the generator to sustain excessive currents for short periods of time. When the engine starts turning the Rotating Field Assembly (RFA), the permanent magnet (PM) induces an AC voltage in the pilot exciter armature (L5). The pilot exciter armature has three coils of wire. The pilot exciter armature generates three-phase alternating current (AC). The resulting AC flows through wires "11", "12", and "13" to the voltage regulator. Within the voltage regulator, the three-phase alternating current is rectified to direct current (DC). A controlled amount of DC is fed to exciter field (L1) through terminals "F1" and "F2".Direct current now flows to exciter field (L1) which creates a magnetic field. Exciter armature (L2) rotates in this magnetic field. The exciter field and the exciter armature generate three-phase alternating current. The AC is then rectified by a three-phase full-wave bridge rectifier circuit. This rectifier circuit is made of the following diodes: CR1, CR2, CR3, CR4, CR5, and CR6.The DC output from the bridge rectifier is carried to main field (L3) by conductors which are routed through a passage in the generator shaft. Current through the main field creates the magnetic field of the generator. As the main field rotates, the main field induces a three-phase AC voltage in main armature (L4). This voltage is sent to the following terminals: T0, T1, T2, and T3. These terminals are connections for the load.To keep the output voltage constant with changing loads, it is necessary to control the exciter current. This control is the function of the voltage regulator. The voltage regulator senses the generator output voltage at the following wires: "20", "22", and "24". The regulator sends current to the exciter through wires "F1" and "F2". The amount of current depends on the sensed voltage. The current is drawn from the pilot exciter and the armature (wires "11", "12", and "13"). Regardless of the generator's type (PMG generator or self-excited generator), changing the exciter current has the same effect on the generator's operation. See the topic Self-Excited Generators for a description of generator operation when the exciter current changes.Note: For more information on voltage regulation, see the appropriate voltage regulator service manual.PMG generators provide the magnetism for start-up of the generator. A Permanent Magnet (PM) supplies the initial magnetism that is required at start-up. Flashing the field is not required for start-up of the generator.Rectifier Circuits
Illustration 3 g00610163
Rectifier Circuit
(CR1 - CR6) Diodes
(CR7) Varistor
(L2) Exciter armature (rotor)
(L3) Main field (rotor)
(R5) ResistorThe following diodes form a bridge rectifier circuit: CR1, CR2, CR3, CR4, CR5, and CR6. The bridge rectifier circuit receives three-phase alternating current from exciter armature