--- title: "Working of brushless and static excitation" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:brushless-static-working" source: https://www.bctengineer.com/q/semester_8/power_plant_design/brushless-static-working site: BCT Engineering --- # Working of brushless and static excitation Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2073 Magh, 5 marks - Video: [Brushless Excitation System For 500 MW Alternator in Power Plant [Hindi]](https://www.youtube.com/watch?v=q0G9eD7RVD8) — ElectricalTalks - 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-static-working - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Discuss the working of Brushless excitation system and static excitation system with proper schematic diagram. ## Short answer Brushless system: a PMG-powered AVR controls the stationary exciter field; rotating armature output is rectified by shaft-mounted diodes to feed the main field directly. Static system: a transformer-fed thyristor bridge adjusts field current through slip rings, with field flashing for startup. ## Answer Both are modern excitation systems for synchronous generators. They differ in **where the rectifier is** and **how DC reaches the field**. #### Brushless excitation system - A **PMG** (pilot exciter) on the shaft supplies the AVR. - The AVR feeds DC to the **stationary field** of the main AC exciter. - The **AC exciter armature rotates** with the shaft and produces 3-phase AC. - A **rotating diode bridge** on the shaft rectifies it and feeds the main field directly. No brushes are needed. - The AVR controls the exciter field, which in turn controls the main field. ```text PMG ---> AVR ---> Exciter field (stationary) : (air gap) [ROTOR] Exciter armature -> Diode bridge -> Main field : Main stator ---> Generator terminals ---> PT/CT to AVR ``` #### Static excitation system - An **excitation transformer** fed from the generator terminals supplies a **thyristor bridge**. - The AVR compares the terminal voltage with the set value and changes the firing angle of the thyristors. - The DC output goes to the rotor through a **field breaker** and **slip rings/brushes**. - At start, **field flashing** from the battery gives the initial field until the voltage builds up. ```text Gen terminals -> Exc. transformer -> Thyristor bridge | (alpha) AVR <- PT, CT | v Battery -> Field flashing -> Field CB -> Slip rings | Main field ``` | Item | Brushless | Static | |---|---|---| | Rectifier | Rotating diodes | Stationary thyristors | | Brushes | No | Yes | | Response | Slower | Very fast | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the key difference between the two systems? A. The grounding method B. The number of poles C. Where the rectifier is and how DC reaches the field D. The type of turbine Answer: C. Brushless uses rotating diodes feeding the field directly; static uses stationary thyristors and slip rings. (Key point: Difference) 2. In the brushless system, what does the AVR directly control? A. The main field slip rings B. The PMG magnets C. The turbine gates D. The stationary field of the AC exciter Answer: D. The AVR controls the exciter field, which in turn controls the main field. (Key point: AVR in brushless) 3. How does the AVR of the static system control the field voltage? A. By switching the field breaker B. By moving brushes C. By changing PMG speed D. By changing the firing angle of the thyristors Answer: D. The AVR compares terminal voltage with the setpoint and adjusts the firing angle. (Key point: Static control) 4. Why is field flashing needed in a static system at start? A. The diodes rotate B. To cool the thyristors C. The terminal voltage is zero, so initial field must come from a battery D. The PMG is too weak Answer: C. The excitation supply comes from the terminals, which have no voltage at standstill. (Key point: Field flashing) ## Related in Power Plant Design - [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× - [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 ## 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 - [TTL inverter schematic and working](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Attl-inverter) — Digital Logic, chapter 10 - [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× - [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3