Excitation System
Definition: The system which is used for providing the necessary field current to the rotor winding of the synchronous machine, such type of system is called an excitation system. In other words, excitation system is defined as the system which is used for the production of the flux by passing current in the field winding. The main requirement of an excitation system is reliability under all conditions of service, a simplicity of control, ease of maintenance, stability and fast transient response.
The amount of excitation required depends on the load current, load power factor and speed of the machine. The more excitation is needed in the system when the load current is large, the speed is less, and the power factor of the system becomes lagging.
The excitation system is the single unit in which the each alternator has its exciter in the form of generator. The centralised excitation system has two or more exciter which feeds the bus-bar. The centralised system is very cheap, but the fault in the system adversely affects the alternators in the power plant.
The excitation system is mainly classified into three types. They are
1. DC Excitation System
2. AC Excitation System
o Rotor Excitation System
o Brushless Excitation System
3. Static Excitation System
Their types are explained below in details.