A post from many years ago, Generator Excitation 101, remains highly popular year after year here on the blog.

For synchronous generators, excitation control is what controls the output voltage under changing electrical load conditions by adjusting the direct current (DC) to the generator’s field windings.

Since a 101 post is so popular, let’s go back to the basics. Hopefully, most of you remember your grade school days when you wrapped wire around a metal nail and connected a battery to each end of the wire, turning the nail into an electromagnet that could attract other metal objects.

The opposite also works. If you move a magnet through a coil of wire, you can induce electrons to flow depending on which way the magnetic field moves in relation to the coil. When the magnetic field is either on the generator’s rotor coils or stationary on the generator’s stator coils, it induces alternating voltage and current.

This 6-minute YouTube video from The Engineering Mindset, AC Electrical Generator Basics – How electricity is generated, does an excellent job of explaining how this works.

Most synchronous generators used in commercial utilities use electromagnetic excitation (wound-field synchronous generators) rather than permanent magnets (permanent magnet generators—PMGs) in their design.

These generators use an electromagnet in the rotor, powered by DC through a field winding, to create the magnetic field required for electricity generation. This design is preferred because it enables precise control of the output voltage and reactive power by adjusting the field current, which is crucial for maintaining grid stability and meeting varying demand. They’re also well-suited for high-power applications (hundreds of Megawatts) due to their scalability and efficiency.

Excitation systems are classified into two general categories: rotating exciters and static exciters. Rotating exciters include brushless and brushed types and typically have the following characteristics:

• Usually mounted to the end of the generator shaft
• Uses an excitation current typically less than 150 amperes
• Have a small voltage regulator, 1 or 2 cabinets

Static exciters include compound sources and potential sources and have different characteristics than rotating exciters:

• Connects directly to the generator rotor using brushes and slip rings
• Has a capacity from 300 to 10,000 amperes
• Has a large voltage regulator and multiple large cabinets

The excitation system builds voltage upon startup and stabilizes it at the desired level. It adjusts the strength of the magnetic field by changing the DC. The automatic voltage regulator (AVR) in this system continuously monitors voltage levels in real time and adjusts the direct current accordingly.

The Ovation platform offers a native generator exciter and excitation control system with capabilities such as high-speed oscillography, dynamic bridge balancing, and an independent field current regulator for all synchronous generators. Integrated power system stabilizer software and a full suite of limiting and protection functions are also available.

The platform’s common HMIs, network, historian, alarm system, engineering tools, diagnostic data, and security functions enable better decision-making and improve operator efficiency. Integrated power system stabilizer software and a full suite of limiting and protection functions can be added based on specific plant needs.

Visit the Excitation System section on Emerson.com for more information on this exciter solution and integrated generator excitation control systems designed for synchronous generators.