MANIFOLD GP-DEF 3892653 - Caterpillar

5 g03240236
Charging circuit with phase angle of phase "U" = 150°If another time instant is chosen (150° phase angle) where none of the voltages are zero, the current would flow as shown in illustration 5 Phase Relationships (150°).Equal currents flow from windings "U" and "V" to the respective positive diodes. They return to the neutral point through "W" winding negative diode and winding "W".Note: Not all the diodes belonging to the various phases are used at the instant of time examined in this example. This remains true for all the other instants of time which could be examined. Individual phase currents change in magnitude and polarity, while the output current to the battery or electrical accessories remains uniform.Excitation Circuit
The exciting current for generation of the magnetic field is taken from the stator winding. It is rectified by three exciter diodes and the three negative power diodes. The exciting current follows the path shown in the illustration 6.
Illustration 6 g03240256
Excitation circuit with phase angle of phase U = 120°Current flows from the output of winding "U" (positive at this instant of time), through its corresponding exciter diode and the alternator D+ terminal to the regulator D+ terminal. When the regulator is turned on, current then flows to the regulator DF terminal to the excitation winding by the way of terminal DF of the alternator. Current then flows to the alternator terminal D- and the negative power diode to winding "W" (negative at this instant of time) completing the circuit at the neutral point.Pre-excitation Circuit
When in operation at high speed, the alternator is self-excited. Excitation current for the magnetic field is obtained from the main current. However, when initially starting, the alternator depends on residual magnetism. When the current of an electromagnet is disconnected, the magnetic field does not disappear completely. The iron core remains slightly magnetic. This is called residual magnetism. There is enough residual magnetism in the core to induce a small voltage in the stator windings. This small voltage causes a weak current to flow through the closed circuit of the excitation winding. This current produces a weak magnetic field of its own. This is added to the residual magnetism of the core and slightly boosts the intensity of the exciter field. A boosted exciter field results in a higher voltage. The effect is cumulative, and the process repeats until the desired voltage, corresponding to the alternator speed, is reached.The exciter circuit in the alternator contains three positive diodes, three negative diodes, and three exciter diodes.Alternator self-excitation cannot start until the pre-excitation circuit produces the breakdown voltage required by two of the diodes (one negative diode and one positive diode per phase) connected in series.
Illustration 7 g03240276
Pre-excitation Circuit ( 7T-2876 Charging Alternator Gp Shown)The existing residual magnetic field of the rotor generates this breakdown voltage only at high speed.Therefore, the alternator has to be pre-excited during start-up. This is done by taking current from the battery through the ignition or key start switch, limiting resistor and on some alternators a