Brushless excitation and when to use it

A brushless excitation system supplies DC to the rotor of a synchronous generator without brushes or slip rings. A small AC exciter and a diode rectifier are mounted on the generator shaft, so the rectified current goes straight into the field winding.

Schematic

 BRUSHLESS EXCITATION
        stationary | rotating (on shaft)
 PMG/aux --> AVR   |
 supply     |      |
            v      |
   Exciter field   |  Exciter armature (3-ph)
   (stator) ~~~~~~~|~~~~> |
                   |   Rotating diode bridge
                   |      |
                   |   Main field (rotor)
                   |      |
               Main generator stator --> Grid

Working

1. A permanent magnet generator (PMG) on the same shaft (or an auxiliary supply) feeds the AVR.
2. The AVR compares the generator terminal voltage with the set value and controls the DC current in the stationary field of the AC exciter.
3. The exciter armature rotates with the shaft and produces three-phase AC.
4. A rotating diode bridge on the shaft converts this AC into DC.
5. This DC flows directly into the main field winding on the rotor; the main generator then produces its terminal voltage.
6. If the voltage falls (load increase), the AVR raises the exciter field current → exciter output rises → main field current rises → terminal voltage is restored.
7. Fuses protect the diodes. Since there are no slip rings, rotor earth-fault protection uses telemetry or a special injection device.

When to recommend brushless excitation

• Small and medium hydro plants, especially remote or unmanned stations in Nepal, where low maintenance is important.
• Where carbon dust from brushes is a problem (enclosed or humid powerhouses) or in hazardous atmospheres.
• High-speed machines (horizontal Pelton/Francis units, turbo generators) where slip ring wear is high.
• Where the generator feeds a weak or isolated system and a PMG-fed AVR keeps excitation available during faults.
• When very fast excitation response is not required for system stability (for large units on a stressed grid, static excitation is preferred).