---
title: "Brushless excitation and when to use it"
subject: "Power Plant Design"
chapter: 4
key: "semester_8/power_plant_design:brushless-excitation-when"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/brushless-excitation-when
site: BCT Engineering
---

# Brushless excitation and when to use it

Power Plant Design · Chapter 4: Electric System Design of a Power Plant

- Asked in: 2081 Chaitra, 6 marks; 2073 Magh, 3+2 marks; 2072 Asoj, 4 marks (asked 3 times)
- Video: [Brushless Excitation: Future of Generators 🚀](https://www.youtube.com/watch?v=0iprJKLtMhE) — Electrology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/brushless-excitation-when
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4

## Question

Discuss working of a brushless excitation with necessary schematic diagram. When would you recommend using such excitation system?

## Short answer

A shaft-mounted AC exciter with rotating diodes feeds DC directly to the main field, eliminating brushes. The AVR controls the exciter field. Recommended for small/medium hydro plants, remote unmanned stations, high-speed machines, and where low maintenance, carbon dust avoidance, or reliable excitation during faults is needed.

## Answer

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

```text
 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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a brushless system, the rotating diode bridge:
   A. Controls the AVR
   B. Converts exciter armature AC to DC fed directly to the main field
   C. Steps up the terminal voltage
   D. Provides the PMG supply
   Answer: B. The exciter armature and diodes rotate with the shaft, so DC goes to the rotor with no brushes. (Key point: Role of rotating diodes)

2. When load increases and terminal voltage falls, the AVR:
   A. Opens the field breaker
   B. Reduces the exciter field current
   C. Raises exciter field current, so exciter output and main field current rise
   D. Trips the generator
   Answer: C. More exciter field raises exciter output, main field current, and restores voltage. (Key point: AVR action)

3. Brushless excitation is recommended for:
   A. Large units on a stressed grid needing very fast response
   B. Small/medium, remote or unmanned plants where low maintenance matters
   C. Systems needing negative field forcing
   D. Plants wanting to measure rotor current directly
   Answer: B. It suits low-maintenance, remote plants; static excitation is preferred when fast response is critical. (Key point: When to recommend)

## Related in Power Plant Design

- [Working of brushless and static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-static-working) — Power Plant Design, chapter 4
- [Why recommend brushless over static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-over-static-logic) — Power Plant Design, chapter 4
- [Brushless versus static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-vs-static-excitation) — Power Plant Design, chapter 4, asked 5×
- [Brushless excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-pros-cons) — Power Plant Design, chapter 4
- [Schematic of a PROR hydropower plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apror-schematic-components) — Power Plant Design, chapter 3, asked 2×
- [PROR environmental impact and schematic](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apror-environmental-impact) — Power Plant Design, chapter 3

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