How a Self-Excited Generator Gets Initial Field Flux to Start

The question of 'How does a self-excited generator supply its own field current and get initial field flux to start when it is first turned on?' can be intriguing and somewhat complex. This article aims to demystify the process, explaining the role of residual magnetism and the concept of hysteresis.

Role of Residual Magnetism

When a self-excited generator is first switched on, it has to overcome the challenge of generating an initial magnetic field without any active current supply. This is where residual magnetism plays a crucial role. Residual magnetism is the permanent magnetic field that remains in the core of the generator's field winding even after the magnetizing force has been removed. This residual magnetism can be enough to start the excitation process.

When the generator is first powered on, the residual magnetism in the field core can generate a small current in the field windings. This current, though weak, is sufficient to maintain the residual magnetism and gradually build up a stronger magnetic field. This process is akin to a domino effect, where the initial small current helps to reinforce more current, eventually leading to a fully functional magnetic field.

Magnetic Hysteresis and Core Flux

A key principle at play here is magnetic hysteresis. Magnetic hysteresis is the phenomenon where the magnetic flux density in a material does not instantly return to zero when the magnetizing force is removed. Instead, it follows a 'hysteresis loop' on a hysteresis curve. Some magnetic flux always remains in the core when the magnetizing force is removed, a property known as retentivity.

For a self-excited generator, this retentivity is critical. When the generator is initially powered on, the residual magnetism, with its help, can provide enough magnetic flux to create a current in the field windings. This current then helps to maintain and strengthen the magnetic field, making the generator self-sufficient in feeding its own field windings.

Residual Magnetism and 'Flashing'

In a typical self-excited generator, there is generally some level of residual magnetism in the rotor. However, in cases where the generator has been idle for an extended period, this residual magnetism might not be sufficient to start the generator. In such instances, a technique called 'flashing' can be used to initiate the process. Flashing involves briefly connecting a high-voltage battery to the field windings to create a strong magnetic field.

In brush-type generators, getting the polarity right is crucial. Incorrect polarity can lead to adverse effects, including sabotage of the generator's operation. However, the process of flashing is straightforward – a high current is passed through the windings for a very short time, enough to create the necessary magnetic field and initiate the excitation process.

It’s worth noting that in some generators, the field windings might be located in the stator, while the power windings are in the rotor. The process of 'flashing' remains the same, but the technical considerations may vary based on the generator's design.

Conclusion

In summary, the magnetism generated by a self-excited generator relies heavily on residual magnetism and the principle of magnetic retentivity. When the generator is first turned on, residual magnetism in the core can start the excitation process. For generators that lack sufficient residual magnetism, techniques like 'flashing' can be employed to provide the necessary initial field flux and ensure a smooth start-up.

Understanding these concepts is crucial for professionals working with self-excited generators, from maintenance and troubleshooting to design optimization.

Frequently Asked Questions

FAQ 1

If a self-excited generator supplies its own field current, how does it get initial field flux to start when it is first turned on?

The initial field flux in a self-excited generator is established through residual magnetism in the core, which helps to initiate the excitation process. If there is no residual magnetism, 'flashing' the generator with a high current can create the necessary initial magnetic field.

FAQ 2

What is magnetic hysteresis, and why is it important in self-excited generators?

Magnetic hysteresis is the phenomenon where the magnetic flux density in a material does not instantly return to zero and requires a stronger force to return to zero. This property is important in self-excited generators because it allows the residual magnetism to contribute to the initial field flux, ensuring a smooth start-up process.

FAQ 3

What happens if a self-excited generator does not have a magnetized rotor?

If a magnetized rotor is not available, the generator can be 'flashed' using a high-voltage battery to create a strong magnetic field. This initial magnetic field helps to initiate the excitation process and start the generator.