--- title: "Brushless excitation pros and cons" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:brushless-pros-cons" source: https://www.bctengineer.com/q/semester_8/power_plant_design/brushless-pros-cons site: BCT Engineering --- # Brushless excitation pros and cons Power Plant Design Β· Chapter 4: Electric System Design of a Power Plant - Asked in: 2080 Chaitra, 6 marks - Video: [Brushless Excitation: Future of Generators πŸš€](https://www.youtube.com/watch?v=0iprJKLtMhE) β€” 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-pros-cons - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Discuss brushless excitation system. Also mention its pros and cons in comparison to its best possible alternative. ## Short answer Brushless excitation supplies the main field via a shaft-mounted AC exciter and rotating diode bridge, eliminating brushes. Compared to static excitation, pros are low maintenance and sustained field during close-in faults; cons are slower response and inability to rapidly de-excite the field. ## Answer A **brushless excitation system** supplies the synchronous generator field from a shaft-mounted AC exciter whose output is rectified by a **rotating diode bridge**, so no brushes or slip rings are needed between the exciter and the main field. #### Working 1. A small **permanent magnet generator (PMG)** or station supply feeds the AVR. 2. The AVR controls a thyristor bridge that feeds the **stationary field** of the AC exciter. 3. The AC exciter has a **rotating armature** on the main shaft. Its 3-phase output goes to a **rotating diode rectifier** mounted on the same shaft. 4. The rectified DC goes directly to the main generator field winding. ```text PMG --> AVR --> Exciter field (stator) | ===== shaft ========|===================== Exciter armature -> Rotating diodes -> Main field (rotating) (rotating) (rotor) | Main stator --> Gen terminals ``` #### Best alternative The best alternative is the **static (thyristor) excitation system**, where the field is fed from the generator terminals through an excitation transformer and thyristor bridge, using slip rings. #### Pros of brushless excitation (compared with static) - No brushes or slip rings: **low maintenance**, no carbon dust, no brush sparking; good for remote, unmanned plants and hazardous areas. - Excitation power is taken from the shaft, so it is **independent of generator terminal voltage**; it can supply field current during close-in faults (good for fault current support and relay operation). - Self-contained with a PMG: no need for an external supply or **field flashing** at start. - Fewer external power components (no large excitation transformer or high-current thyristor cubicle). #### Cons of brushless excitation (compared with static) - **Slow response**: the exciter field time constant (about 0.5–1 s) adds delay, so transient stability support is poorer than static excitation (response in tens of ms). - **No fast field de-excitation**: the main field cannot be forced negative or shorted quickly; field suppression after a fault is slow because the decay depends on the rotor time constant. - Rotating diodes and fuses cannot be inspected while running; diode failure needs special monitoring. - Longer shaft and extra rotating mass; higher cost for very large machines. - Rotor field current and voltage cannot be measured directly. | Feature | Brushless | Static | |---|---|---| | Brushes/slip rings | None | Needed | | Response | Slow | Very fast | | De-excitation | Slow | Fast (inverting) | | Maintenance | Low | Brush upkeep | | Fault-time supply | Maintained | Drops with $V_t$ | | Typical use | Small–medium hydro, diesel | Large hydro, thermal | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a brushless system, what rectifies the exciter armature output? A. A field breaker B. Slip-ring thyristors C. A rotating diode bridge on the shaft D. A stationary diode bridge Answer: C. The AC exciter armature rotates with the shaft and feeds a rotating diode bridge connected directly to the main field. (Key point: Working) 2. What is the best alternative to brushless excitation? A. Static (thyristor) excitation with slip rings B. Solid grounding C. DC generator exciter with commutator only D. Manual rheostat control Answer: A. The static system feeds the field from the terminals via a transformer and thyristor bridge through slip rings. (Key point: Best alternative) 3. How does brushless compare with static during a close-in fault? A. Brushless keeps supplying field current, since it is independent of terminal voltage B. Both lose excitation C. Brushless excitation collapses with Vt D. Static is better because it uses a PMG Answer: A. Static excitation drops with terminal voltage, but brushless keeps excitation. (Key point: Fault performance) 4. Why is de-excitation slow in a brushless system? A. It has a field discharge resistor B. Field cannot be forced negative; decay depends on the rotor time constant C. The thyristor inverts D. The brushes are worn Answer: B. There is no direct access to the rotor, so field suppression relies on slow natural decay. (Key point: Cons) ## Related in Power Plant Design - [Static excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Astatic-excitation-pros-cons) β€” 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) β€” Power Plant Design, chapter 4, asked 5Γ— - [Brushless excitation and when to use it](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-excitation-when) β€” Power Plant Design, chapter 4, asked 3Γ— - [Why recommend brushless over static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-over-static-logic) β€” Power Plant Design, chapter 4 - [Working of brushless and static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-static-working) β€” Power Plant Design, chapter 4 ## Related in other subjects - [Distribution layouts, pros/cons, improvements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-layouts-improvements) β€” Water Supply Engineering, chapter 7, asked 4Γ— - [Collective intelligence pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Acollective-intelligence-pros-cons) β€” Information System, chapter 4 - [Phase shift method of SSB generation: pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-phase-shift-pros-cons) β€” Communication Systems, chapter 2, asked 2Γ— - [Roof coverings in Terai, pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aroof-covering-pros-cons) β€” Building Technology, chapter 4