Here is some reasons for reference:
1.After the diesel generator is stopped, the residual magnetism is often lost, because the material used for the magnetic pole of the exciter is close to the soft steel, and the residual magnetism is small. After the shutdown, when there is no current in the excitation winding, the magnetic field will disappear.
2.The magnetic pole of the diesel generator loses its magnetic properties
3.Damage of excitation circuit components or circuit breakage, short circuit or grounding phenomena.
4.The exciter brush is not in good contact with the commutator and pressure of brush holder is insufficient.
5.Excitation winding wiring error, opposite polarity.
6.Stator winding and rotor winding of generator set open circuit.
7.Poor contact between diesel generator brush and slip ring, or insufficient brush pressure.
8.Diesel generator lead wire wiring is loose or switch contact is poor.
9.The fuse box is fused.
10.The "auto-manual-de-excitation" switch on the control panel is in the de-excitation position (automatic generator set has "auto-manual-de-excitation" switch).
11.The voltmeter is damaged.
12.Rotary rectifier module is damaged.