Causes and Solutions for Diesel Generator Demagnetization
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Common Causes of Generator Demagnetization
Demagnetization in diesel generators is typically caused by various factors. Identifying these potential causes can help in taking preventative measures to avoid equipment failure or performance degradation. Here are some common reasons for generator demagnetization.
1. Rotor Winding Faults
The rotor windings in the generator are responsible for generating the magnetic field. If the windings break or short circuit, the magnetic field can no longer be sustained, leading to demagnetization.
2. Excitation System Failure
The excitation system is the key component in maintaining the generator's magnetic field. Issues such as exciter failure, automatic demagnetization switch tripping, or damage to components in the semiconductor excitation system can all cause the generator to lose its magnetic field. Line faults or operator errors can also affect the excitation system's performance.
3. Long Periods of Inactivity
If the generator set remains idle for an extended time, its internal magnets may gradually lose their magnetism, preventing the generator from starting properly. This is particularly common in standby generators, making it essential to run them periodically to maintain their functionality.
4. Low Inverter Carrier Frequency
A low carrier frequency in the inverter can cause abnormalities in the sine wave, leading to an increase in negative ID current, which hinders the formation of the magnetic field.
5. Extreme Temperature Fluctuations
Temperature is a crucial factor affecting magnetism. Magnets generate a magnetic field because the electron spin in the ferromagnetic materials aligns within magnetic domains. However, high temperatures can disrupt this alignment, weakening or even eliminating the magnetism. If the temperature exceeds the Curie point, the magnetism will be lost entirely, while temperatures exceeding the maximum operating range can cause instability and performance degradation.
6. Corrosion
Many magnets contain iron, which is prone to oxidation in moist environments, leading to the formation of iron oxide. This damages the magnet's internal structure, causing a loss of magnetism. Permanent magnets with iron content exceeding 60% are particularly vulnerable to corrosion, raising the risk of demagnetization.
7. Mechanical Impact
Although physical impacts can alter the alignment of magnetic domains in magnets, leading to loss of magnetism, the internal structure of diesel generators is typically well-protected by the housing. Therefore, this cause of demagnetization can largely be ignored.
Impact of Generator Demagnetization
When demagnetization occurs, the performance and safety of the generator are adversely affected. Demagnetization not only results in unstable power output but can also trigger a series of cascading effects, compromising the normal operation of the entire power system.
1. No Voltage or Current Output
Demagnetization causes a significant drop in voltage and current output, which in severe cases may result in complete power loss, affecting the operation of the electrical system.
2. Loss of Synchronization
After demagnetization, the rotor and stator lose synchronization, leading to differential currents in the rotor and stator windings. If these currents exceed allowable limits, the rotor can overheat or develop localized hotspots, threatening the equipment's safety.
3. Vibration of the Generator Set
Demagnetization can cause pulsating currents in the stator windings, producing alternating mechanical torques during operation, which induces vibrations in the generator set. This not only destabilizes the operation but also increases the likelihood of mechanical failures.
4. Excessive Reactive Power Consumption
Once demagnetized, the generator absorbs large amounts of reactive power to sustain the magnetic field, resulting in voltage drops. This is particularly problematic when reactive power reserves are insufficient, affecting system stability.
5. End Core Overheating
Under low excitation or demagnetized conditions, the leakage flux in the stator end increases, leading to overheating of the core ends and raising the risk of equipment overload and failure.
Solutions to Generator Demagnetization
Prompt and appropriate actions are essential to rectify demagnetization in generators. The solutions vary depending on the underlying cause, ensuring the generator can quickly return to normal operation. The following are some commonly used solutions for demagnetization.
1. Magnetization Using a Special Charger
A specialized magnetization device can be used to restore the magnetic field of the generator's rotor. This is the most direct and effective method.
2. Battery Magnetization
A 12V or 24V battery can be used to magnetize the generator's rotor. The steps are as follows:
Start the generator at slightly below the rated speed.
Connect the positive and negative terminals of the battery to the excitation terminals of the rotor, with positive to positive and negative to negative.
Touch the generator's brushes with the battery for 20-30 seconds. If the spark is minimal, the connection is correct. Then, disconnect the battery and start the generator.
3. Self-Excitation by Increasing Load
If the generator is not connected to the grid, adding load (the greater the better) can restore the magnetic field through self-excitation. This method does not require manual magnetization and is suitable for generators with higher power ratings.
4. Excitation Magnetization Button
Some generator sets are equipped with an excitation magnetization button, which restores the magnetic field with a simple press. If this button is not available, a battery can be used for magnetization.
Though demagnetization in generators is not common, it can significantly impact the generator's operation when it occurs. By regularly inspecting the excitation system, maintaining the generator properly, and controlling environmental temperature and humidity, demagnetization can be effectively prevented. When demagnetization does happen, applying the solutions outlined above will restore the generator's functionality, ensuring the long-term stability of the diesel generator set.
