How is excitation provided in synchronous machines?

Short Answer:

Excitation in synchronous machines is provided by supplying direct current (DC) to the rotor winding to produce the necessary magnetic field. This magnetic field interacts with the stator’s rotating magnetic field and enables the machine to operate in synchronism.

The DC excitation can be provided through different methods such as external DC sources, brush and slip ring systems, or brushless excitation systems using rotating rectifiers. The type of excitation system used depends on the size, application, and design of the synchronous machine.

Detailed Explanation:

Excitation in synchronous machines

In a synchronous machine, the rotor must have a magnetic field to interact with the rotating magnetic field created by the three-phase AC supply in the stator. This rotor magnetic field is produced by exciting the rotor winding with DC current. This process is called excitation, and it is essential for both synchronous motors and generators.

Excitation enables the machine to develop torque in a motor or generate voltage in a generator. Without excitation, the rotor would not have its own magnetic field, and the synchronous machine would not function properly.

Methods of Providing Excitation:

There are several ways to provide the required DC excitation to the rotor:

  1. External DC Source (Battery or DC Generator):

In small synchronous machines or early designs, a separate DC source such as a battery or a small DC generator (exciter) is used to supply DC current to the rotor winding. The current is fed through slip rings and brushes.

  • Brushes: Conductive carbon pieces that maintain electrical contact with the rotating slip rings
  • Slip Rings: Metal rings mounted on the rotor shaft that receive DC from the brushes

This method is simple but requires regular maintenance due to brush wear and spark formation.

  1. Brushless Excitation System:

In modern large synchronous machines, brushless excitation is widely used. In this system, no slip rings or brushes are needed, making the setup more reliable and maintenance-free.

Working of Brushless Excitation:

  • A small AC exciter (an alternator) is mounted on the rotor shaft. It produces AC power when it rotates.
  • The output of this exciter is connected to a set of rotating rectifiers (diodes) also mounted on the rotor.
  • These rectifiers convert the AC from the exciter into DC.
  • The resulting DC is then fed directly into the main rotor winding.

Since all components rotate together, there is no need for sliding contacts, improving efficiency and reliability.

  1. Static Excitation System:

This method uses a power electronic converter to supply DC excitation directly from the main generator terminals. A transformer steps down the voltage, and a rectifier converts AC to DC. The DC is then controlled and fed to the rotor through slip rings.

This method allows fast and precise control of excitation using automatic voltage regulators (AVRs). It is mostly used in power plants where stable voltage control is essential.

Automatic Voltage Regulator (AVR):

In modern machines, the excitation is regulated by an AVR, which automatically adjusts the excitation current to keep the terminal voltage stable even when the load changes. This is very important in both generators and motors for efficient and reliable performance.

Importance of Excitation:

  • Maintains constant voltage in generators
  • Helps control power factor in motors
  • Provides reactive power support to the grid
  • Enables synchronizing with the power system
  • Essential for voltage stability and system reliability
Conclusion:

Excitation in synchronous machines is provided by supplying DC current to the rotor winding. This can be done through external DC sources with slip rings and brushes, or modern systems like brushless excitation and static excitation. Proper excitation ensures stable voltage, reliable performance, and efficient operation of synchronous motors and generators.