What is an excitation system on an AC alternator?
The excitation system on a AC alternator refers to the way the alternators voltage is initially built when rotated and controlled while in use. The excitation system is responsible for supplying the field current to the main rotor. The requirements of an excitation system include reliability under all conditions of service, a simplicity of control, ease of maintenance, stability and fast transient response.
There are three main types for standard brushless self excited machines. Shunt or self excited, Auxiliary wound and Permanent Magnet (PMG) . You may also see transformer controlled alternators. Each system has advantages and disadvantages, meaning the preferred option is based on the applications requirements and budget.
What are the main components of the excitation system?
The components of a self excited excitation system are:
- The automatic voltage regulator (AVR)
- The exciter stator
- The exciter rotor
- The diodes
- The main rotor
An auxiliary wound AC alternator in addition has a auxiliary winding embedded in the main stator. A PMG excited machine has a PMG rotor and PMG stator fitted to the non drive end.
Why does the excitation system matter?
Without the excitation system the AC alternator would have no way of building its voltage as it starts to rotate, nor would not be able to regulate its voltage to the pre-set nominal level while running at its rated speed.
So, without an excitation system, an AC alternator would be useless for its purpose.
How does it effect the voltage stability?
The three main types shunt or self excited, Auxiliary wound and Permanent Magnet (PMG) all use an automatic voltage regulator (AVR) to maintain the voltage at a pre-set level. The AVR adjusts its output voltage up and down which is fed to the exciter stator. This in turn powers the exciter rotor. The main stator is then powered via the rectification diodes.
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