Brush excitation system

An excitation system supplies and controls the DC current in the rotor field winding of a synchronous generator, so as to hold terminal voltage and control reactive power.

Brush (DC) excitation system

In a brush excitation system the field current is produced outside the rotor (by a DC exciter or by rectifiers) and is fed to the rotating field winding through brushes and slip rings. The classic form uses a shaft-mounted DC generator (main exciter), often with a smaller pilot exciter.

 Pilot     Main        Main alternator
 exciter   exciter    +-------------------+
 +----+    +----+     | rotor field       |
 | PE |--->| ME |=B/S=>| winding    stator |--> 3-ph
 +----+    +----+     |            wndg   |    out
   |   field  ^       +-------------------+
   |   rheo.  |                 |
   +----------+--<-- AVR <-- PT-+
 <====== common shaft (turbine) ======>
 B/S = brushes and slip rings

Components and working

1. Pilot exciter: small self-excited DC shunt generator on the same shaft. It supplies the field of the main exciter.
2. Main exciter: separately excited DC generator on the shaft. Its armature output (a few hundred volts DC) feeds the alternator field.
3. Brushes and slip rings: carry the main exciter output to the rotating field winding of the alternator.
4. Automatic voltage regulator (AVR): senses terminal voltage through a PT, compares it with the reference and adjusts the main exciter field current (through a rheostat, amplidyne or magnetic amplifier).
5. Field breaker and discharge resistor: isolate and de-excite the field quickly during internal faults.

When terminal voltage falls (more load or lagging pf), the AVR raises the main exciter field current, $E_{fd}$ rises, alternator field current rises and $V_t$ returns to the set value.

A modern variant of brush excitation is the static exciter, where thyristor rectifiers fed from the generator terminals supply the field through slip rings.

Merits and demerits