DC excitation systems
In DC excitation system, the field of the main synchronous generator is fed from a DC generator, called exciter. That is dc generator acts as source of excitation power. It provide steady current to the field winding of synchronous machine. DC excitation systems has two exciters
1) Main exciter
– separately excited dc generator
– provide field current to alternator
2) Pilot exciter
– self excited dc generator
– provide field current to main exciter
Since the field of the synchronous generator is in the rotor, the required field current is supplied to it through slip rings and brushes. The DC generator is driven by either a motor or the same turbine shaft as the generator itself. One form of simple DC excitation system is shown in figure.
Automatic Voltage Regulator
– It regulate the output of pilot exciter
1) To control terminal voltage by regulating output of pilot exciter
2) To limit the alternator current during faults
Field discharge resistor
– To dissipate energy as field breaker open.
This type of DC excitation system has slow response. Normally for 10 MVA synchronous generator, the exciter power rating should be 20 to 35 KW for which we require huge the DC generator. For these reasons, DC excitation systems are gradually disappearing.
DC excitation system with amplidyne voltage regulator
The dc exciter, self-excited or separately-excited, supplies current to the main generator field through slip rings. The exciter field is controlled by an amplidyne which provides incremental changes to the field. The exciter output provides rest of its own field by self-excitation or separately-excitation. If amplidyne is out of service the exciter is on “manual control”.
Disadvantages: Large size, voltage regulation was complex. Very slow response. This system is backdated (1920-1960).
Note that amplidyne is a dc machine with special construction to produces large power amplification. It has two sets of brushes: One set on d-axis & another on q-axis. Since q-axis brushes are shorted, so at very little field power it produce a large current. Thus, more flux along q-axis and more output power.