Short Answer:

Excitation systems are used to supply the required DC current to the rotor winding of synchronous machines to produce the magnetic field. There are several types of excitation systems, mainly classified as DC excitation systems, AC excitation systems (brushless), and static excitation systems.

Each type has its own method of supplying and controlling the field current. DC systems use mechanical generators, AC systems use rotating rectifiers, and static systems use power electronics. The choice depends on the machine size, reliability needed, and maintenance preferences.

Detailed Explanation:

Types of excitation systems

An excitation system in a synchronous machine is responsible for supplying the direct current (DC) required to excite the rotor winding and produce the magnetic field necessary for generating or receiving electrical energy. Depending on the machine’s design, application, and size, different excitation systems are used. The system must also regulate the voltage output and respond quickly to load changes.

Excitation systems are mainly divided into three types:

1. DC Excitation System:

This is the traditional and oldest method, where a separate DC generator (also called an exciter) is mounted on the same shaft as the main synchronous machine. The DC generator supplies the field current to the rotor winding via brushes and slip rings.

Working:

• A small DC generator produces DC power
• The output is fed to the rotor winding of the main machine
• Voltage is controlled manually or through voltage regulators

Advantages:

• Simple to understand and operate
  Disadvantages:
• Requires frequent maintenance due to brushes
• Not suitable for large modern machines

2. AC Excitation System (Brushless Excitation):

In this system, a small AC generator (exciter) is used along with rotating rectifiers. The exciter is also mounted on the same shaft as the main rotor. The AC output of the exciter is converted into DC using rotating diodes and then fed directly into the rotor winding—no brushes or slip rings are used.

Working:

• A permanent magnet generator (PMG) supplies power to the exciter
• The exciter generates AC
• The AC is rectified by rotating diodes mounted on the shaft
• The DC output goes directly to the rotor winding

Advantages:

• Brushless system → no wear and tear
• Reliable and low maintenance
• Widely used in large power plants

Disadvantages:

• Slightly complex in design
• Diode failure can affect performance

3. Static Excitation System:

In this system, no rotating exciter is used. The DC excitation is taken from the output terminals of the synchronous generator itself, converted from AC to DC using power electronic converters like thyristors or rectifiers.

Working:

• AC is drawn from the generator terminals
• It is stepped down using a transformer
• Converted to DC using rectifiers
• Supplied to rotor via slip rings

Advantages:

• Fast response
• Precise voltage regulation
• Easy to automate using microcontrollers or AVR

Disadvantages:

• Requires slip rings
• Sensitive to harmonics and requires protection circuits

Comparison and Selection:

• DC Excitation: Best for small or old machines
• AC Brushless Excitation: Ideal for large modern machines needing low maintenance
• Static Excitation: Preferred where fast and precise voltage control is critical

Selection depends on:

• Size of the generator/motor
• Application (power plant, industrial use)
• Desired maintenance level
• Cost and control requirement

Conclusion:

Excitation systems in synchronous machines are of three main types: DC excitation, AC brushless excitation, and static excitation. Each type uses a different method to provide DC to the rotor and has its own advantages and limitations. The choice of system depends on the machine’s application, control needs, and maintenance preferences. Reliable excitation is essential for stable machine operation and power system performance.