Three-phase generators are a type of generator, commonly used in large workshops and factories. Over time, the residual magnetism in a three-phase generator diminishes, causing the generator to lose magnetism and unable to build voltage, thereby not producing electricity. For smaller generators, they can carry a temporary high-power load to build voltage, essentially creating a closed circuit between the generator coil and the load, using the generator's remaining faint residual magnetism to establish voltage.

1. Initially start the water turbine (or diesel engine) to reach a certain speed, then use a 12V battery to magnetize and build voltage in the generator. This method can only be done by trial and error, as the excitation voltage in the generator can reach 40~90V after building voltage. If the battery is not disconnected quickly, it may damage the battery.

2. Magnetize the generator rotor using a generator magnetizer, which can be done while the generator is at rest.

Generator demagnetization poses hazards to the power system.

1. When a generator experiences demagnetization, it will absorb reactive power from the system, causing the voltage of the power system to drop.

2. If the power system capacity is small or lacks sufficient reactive power reserve, it can cause the generator terminal voltage, the bus voltage on the high-voltage side of the step-up transformer, or other nearby voltage points to drop below the allowable levels. This can disrupt the stable operation between the load and the power source, and may even lead to voltage collapse in the power system.

3. When a generator experiences low excitation or demagnetization, the voltage drops. Under the automatic adjustment of the excitation device of other generators in the system, their reactive power output will increase, which may cause certain electrical components within the system.

4. When a generator experiences low excitation or demagnetization, fluctuations in active power and a drop in system voltage may cause out-of-step conditions between adjacent generators operating normally and the system, or between different parts of the power system, leading to system oscillations and significant load shedding.