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The impact of high frequency on a generator is significant. The frequency should not exceed 52.5 Hz, which is 5% above the rated value. High-frequency operation is mainly limited by the mechanical strength of the rotating components. When the frequency increases, the motor's rotational speed rises, leading to higher centrifugal forces on the rotor. This can cause damage to critical rotor parts over time.
On the other hand, low frequency has several negative effects on the generator. First, it reduces the rotor's rotational speed, which in turn lowers the airflow from the cooling fans at both ends. This results in poor cooling and increased temperatures throughout the generator. Additionally, lower frequency causes the rotor coil temperature to rise, potentially reducing the output power. In extreme cases, it may even lead to blade failure in turbines.
When the frequency drops, maintaining a constant terminal voltage requires an increase in magnetic flux. This can cause saturation in the stator core, leading to magnetic leakage and localized overheating. It may also affect structural components of the frame, even causing damage in severe cases. Moreover, low frequency slows down the motor in the factory, reducing overall output and negatively impacting user safety, product quality, and operational efficiency.
Another consequence of low frequency is reduced voltage. This is because the induced voltage is directly related to the rotational speed. Additionally, the generator’s exciter output decreases with slower speed, affecting reactive power generation.
16. What should the operator pay attention to when the generator enters phase?
In theory, a generator can enter phase operation, which means the power factor becomes leading. At this point, the current leads the terminal voltage. While the generator still supplies active power to the system, it absorbs reactive power. This condition occurs under low excitation, and operators must be cautious about two key issues: (1) reduced static stability, and (2) increased temperature at the stator end due to end leakage.
17. Explain the principle, advantages, and disadvantages of a 600MW generator brushless excitation system.
The brushless excitation system works by using a high-frequency 400Hz power supply generated by a permanent magnet machine's stator. This power is supplied to the main exciter's winding through two sets of full-controlled rectifier bridges. The intermediate frequency 200Hz power from the main exciter's armature is then fed into a rotary rectifier, whose DC output provides the excitation power for the generator. This power is delivered to the rotor's field winding via the central bore of the rotor.
Advantages of the brushless excitation system include eliminating large current collector rings and carbon brushes, which helps prevent sparking. It also offers a compact design and reduces maintenance requirements.
However, there are some disadvantages. The excitation control system has a certain time lag, so to improve its response, hard and negative feedback from the rotor voltage is added to the regulator circuit. This reduces the time constant but increases the exciter's capacity and voltage level.
GW MB10F 0.8A 1000V Rectifier Bridge
Probe for current and voltage, pin 420*4450, head diameter 5.0, overcurrent and voltage pin
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