--- title: "Brushless versus static excitation" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:brushless-vs-static-excitation" source: https://www.bctengineer.com/q/semester_8/power_plant_design/brushless-vs-static-excitation site: BCT Engineering --- # Brushless versus static excitation Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2082 Shrawan, 6 marks; 2079 Jestha, 8 marks; 2075 Bhadra, 6 marks; 2074 Bhadra, 6 marks; 2071 Bhadra, 6 marks (asked 5 times) - Video: [How Static Excitation POWERS Massive Generators ⚡](https://www.youtube.com/watch?v=fCONBrP2zyo) — 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-vs-static-excitation - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Compare Brushless Excitation System with Static Excitation System for a synchronous generator on the basis of schematic diagram, operating range, accessories required, cost and so on. ## Short answer Brushless excitation uses a shaft-mounted AC exciter with rotating diodes, no brushes, slower response, low maintenance, typical for small/medium units. Static excitation uses a thyristor bridge fed from generator terminals via transformer, requires slip rings and brushes, provides fast response and high ceiling voltage, preferred for large units needing transient stability. ## Answer Both systems supply DC to the rotor field of a synchronous generator. A **brushless** system uses a shaft-mounted AC exciter with rotating diodes, so no brushes are needed. A **static** system takes power from the generator terminals through a transformer and a **thyristor** bridge, and feeds the field through **slip rings and brushes**. #### Schematic diagrams ```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 ``` ```text STATIC EXCITATION Gen terminals --+---------------> GSU --> Grid | Excitation transformer | Thyristor bridge <-- AVR | Field breaker | Brushes + slip rings | Main field (rotor) ``` #### Comparison | Basis | Brushless excitation | Static excitation | |---|---|---| | Power source | AC exciter on shaft (with PMG or aux supply to AVR) | Generator terminals via excitation transformer | | Rectifier | Rotating diode bridge | Stationary thyristor bridge | | Brushes/slip rings | None | Required | | Control | AVR controls exciter field; acts through exciter time constant | AVR fires thyristors directly on main field | | Response speed | Slower (about 0.5–1 s exciter time constant) | Very fast (tens of ms), high ceiling voltage | | Negative field forcing / fast de-excitation | Not possible (diodes) | Possible (inverter mode, field breaker) | | Operating range | Good for steady loads; limited transient support | Wide; best for stability, PSS, large swings | | Behaviour during close-in fault | Unaffected if PMG-fed | Ceiling drops with terminal voltage; needs field flashing for start | | Accessories | Rotating diodes, diode fuses, diode-failure and rotor earth-fault monitoring (telemetry), PMG | Excitation transformer, thyristor bridge with cooling, field breaker, de-excitation resistor, field flashing (battery), brush gear | | Maintenance | Very low; no carbon dust | Brush and slip ring maintenance; carbon dust | | Field measurement | Rotor current and temperature not directly measurable | Directly measurable | | Cost | Economical for small and medium machines; becomes costly for large ratings | Economical for large machines; extra cost of transformer and brushes | | Typical use | Small/medium hydro, diesel and remote plants | Large hydro and thermal units on strong grid | **Conclusion:** brushless is chosen where low maintenance and reliability matter (small to medium, remote plants); static is chosen for large units where fast response and transient stability matter. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a static excitation system, field power is taken from: A. A shaft-mounted PMG B. A rotating diode bridge on the shaft C. A battery bank only D. The generator terminals through an excitation transformer and thyristor bridge Answer: D. Static excitation uses a transformer and thyristor bridge fed from the terminals, with slip rings and brushes. (Key point: Power source) 2. Which statement on response is correct? A. Brushless is faster because it has no brushes B. Both have the same response C. Brushless is slower (exciter time constant 0.5-1 s); static is very fast with high ceiling voltage D. Static is slower because of the thyristors Answer: C. The brushless AVR acts through the exciter time constant; the static AVR fires thyristors directly on the main field. (Key point: Response speed) 3. Which accessories/maintenance fit the brushless system? A. Slip rings, brushes and carbon dust cleaning B. Excitation transformer and field breaker C. Rotating diodes, diode fuses, diode failure monitoring and PMG; very low maintenance D. Field flashing battery Answer: C. Brushless needs rotating diodes and monitoring but no brush gear. (Key point: Accessories and maintenance) 4. Which choice is typical? A. Brushless for large units on a strong grid, static for small remote plants B. Brushless for small/medium remote plants, static for large units needing fast response C. Static for all small diesel sets D. Both only for thermal plants Answer: B. Brushless is economical and low-maintenance for small/medium machines; static suits large units and stability. (Key point: Typical use and cost) ## 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 excitation and when to use it](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-excitation-when) — Power Plant Design, chapter 4, asked 3× - [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 - [Static excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Astatic-excitation-pros-cons) — Power Plant Design, chapter 4 - [Solar PV versus hydro and their combination](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolar-pv-hydro-combination) — Power Plant Design, chapter 1 ## Related in other subjects - [Static excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Astatic-excitation-system) — Power Plant Equipment, chapter 3 - [Synchronous generator constructional diagram](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Asynchronous-generator-diagram) — Basic Electrical and Electronics Engineering, chapter 2 - [Synchronous motor features, applications and excitation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-applications-excitation) — Electric Machines II, chapter 2 - [Schematic diagram of rural gravity scheme](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arural-gravity-scheme-diagram) — Water Supply Engineering, chapter 1, asked 5×