Excitation System of a Synchronous Machine

1. Excitation System of a
Synchronous Machine
Excitation
The word Excitation means the production of in the field
winding.
By
Halar Zameer

2. How excited field winding??
• By provide DC current on field winding
Excitation System
• The arrangement or the system used for the excitation of the synchronous machine
is known as Excitation System
• Why excited with help of DC why not AC???
• Why is necessary to excited a synchronous generator????
• It is necessary if never excited field winding means no flux ,than no change in flux
and no output voltage provide by generator therefore it is necessary.

3. Types of
Excitation
System:
DC Excitation System
AC Excitation System
Static Excitation System

4. DC Excitation System:
In this system two exciter are used (Pilot exciter) and (Main exciter)
• Pilot exciter is nothing it is dc shunt generator which output provide to a main exciter and main exciter with
help of brush and slip ring excite a field winding
• Output of pilot exciter control by AVR in order to maintain terminal voltage of generator
• so what is output of Pilot exciter it decide by AVR
• Applications: This method used in small synchronous motor and generator
• Disadvantage of this method
• Maintance cost is high because continuous check brushes and may voltage drop across brushes , power loss
in machine
• Cost is very high
• Cooling problem also occur

5. Static Excitation System:
In Static Excitation System, field winding energized from the out put voltage of synchronous generator
A transformer knows as Excitation Transformer, is connected to the output terminals of Generator to
step down the voltage to required voltage level usually 415 V AC. As we need DC supply, therefore
Transformer output is connected to a Thyristor Full Bridge Rectifier. Figure below shows simplified block
diagram of Static Excitation System.
The firing angle of Thyristor Full Bridge Rectifier is controlled by a Regulator so that required field
excitation may be provided. Secondary terminal of CT and PT connected to Generator output
terminals is fed to regulator. On the basis of Generator terminal voltage, the regulator adjusts its firing
angle.

6. Static
Excitation
System

7. AC Excitation System:
• The AC excitation system consists of an alternator and thyristor rectifier bridge directly connected to the main alternator shaft
• AC excitation has type
• Brushless excitation system
• This method used in large generator
• What is brushless exciter??
• It is special type of ac generator whose field winding mounted on stator and armature winding mounted on shaft of rotor
• The output of brushless exciter is 3 phase ac, hence it rectifies and apply on main rotor of synchronous generator in order to excited
• Why called brushless exciter???
• Because without using brushes and slip ring energized a main field
• This method has several advantages
• Less Maintaince cost
• Power loss reduce
• Fast response
• Suitable for high rating generator

8. AC Excitation
System

9. How energize rotor of brushless exciter???
Mainly using pilot exciter
• Pilot exciter small ac generator with permanent magnetic mounted on shaft of
main generator and armature winding on rotor of brushless exciter or stator of
synchronous generator
• Due to pilot exciter this become independent external source of supply…