--- title: "Brush excitation system" subject: "Power Plant Equipment" chapter: 3 key: "semester_7/power_plant_equipment:brush-excitation-system" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/brush-excitation-system site: BCT Engineering --- # Brush excitation system Power Plant Equipment · Chapter 3: Var/Voltage Control in Hydrogenerating Systems - Asked in: 2071 Shrawan, 8 marks - Video: [lesson 11: Generator Excitation System](https://www.youtube.com/watch?v=7MvjbC4f5Aw) — International Engineering Training - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/brush-excitation-system - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-3 ## Question What is excitation system? Explain the brush excitation system with neat diagram. ## Short answer An excitation system supplies controlled DC field current. In the brush system, a shaft-driven DC main exciter (with a pilot exciter) supplies field current through brushes and slip rings. The AVR adjusts the exciter field to hold terminal voltage. It is simple but requires brush and commutator maintenance. ## Answer An **excitation system** supplies and controls the DC current in the rotor field winding of a synchronous generator, so as to hold terminal voltage and control reactive power. #### Brush (DC) excitation system In a brush excitation system the field current is produced outside the rotor (by a DC exciter or by rectifiers) and is fed to the rotating field winding through **brushes and slip rings**. The classic form uses a shaft-mounted **DC generator (main exciter)**, often with a smaller **pilot exciter**. ```text Pilot Main Main alternator exciter exciter +-------------------+ +----+ +----+ | rotor field | | PE |--->| ME |=B/S=>| winding stator |--> 3-ph +----+ +----+ | wndg | out | field ^ +-------------------+ | rheo. | | +----------+--<-- AVR <-- PT-+ <====== common shaft (turbine) ======> B/S = brushes and slip rings ``` #### Components and working 1. **Pilot exciter:** small self-excited DC shunt generator on the same shaft. It supplies the field of the main exciter. 2. **Main exciter:** separately excited DC generator on the shaft. Its armature output (a few hundred volts DC) feeds the alternator field. 3. **Brushes and slip rings:** carry the main exciter output to the rotating field winding of the alternator. 4. **Automatic voltage regulator (AVR):** senses terminal voltage through a PT, compares it with the reference and adjusts the main exciter field current (through a rheostat, amplidyne or magnetic amplifier). 5. **Field breaker and discharge resistor:** isolate and de-excite the field quickly during internal faults. When terminal voltage falls (more load or lagging pf), the AVR raises the main exciter field current, $E_{fd}$ rises, alternator field current rises and $V_t$ returns to the set value. A modern variant of brush excitation is the **static exciter**, where thyristor rectifiers fed from the generator terminals supply the field through slip rings. #### Merits and demerits | Merits | Demerits | |---|---| | Simple, proven design | Brush and commutator wear; regular maintenance | | Field can be de-excited quickly through slip rings | Sparking, carbon dust; fire risk | | Easy field current measurement | Commutation limits exciter rating (large units) | | Low cost for small and medium units | Slow response (large $T_E$) for DC exciters | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What defines a brush excitation system? A. Field current produced outside the rotor and fed through brushes and slip rings B. A rotating rectifier on the shaft C. No exciter at all D. An AC exciter with no rectifier Answer: A. Field current comes from a DC exciter or rectifiers and is fed to the rotor via brushes and slip rings. (Key point: Brush excitation) 2. What does the pilot exciter supply? A. The alternator field directly B. The field of the main exciter C. The stator D. The AVR Answer: B. The self-excited pilot exciter feeds the main exciter's field. (Key point: Pilot exciter) 3. When terminal voltage falls, what does the AVR do? A. Reduces main exciter field current B. Raises main exciter field current so Efd and alternator field current rise C. Opens the field breaker D. Reduces turbine input Answer: B. Increasing the exciter field raises Efd and field current, restoring Vt. (Key point: AVR action) 4. Which is a demerit of the brush system? A. Brush and commutator wear, sparking and slow response B. Cannot be de-excited C. Needs no maintenance D. Very fast response Answer: A. Brushes, commutators wear and spark, and large T_E makes DC exciters slow. (Key point: Demerits) ## Related in Power Plant Equipment - [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 - [Excitation system with stabilizing transformer: transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-stabilizing-transformer-model) — Power Plant Equipment, chapter 3, asked 13× - [Dynamic response of excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-dynamic-response) — Power Plant Equipment, chapter 3, asked 3× - [Function of excitation system and its transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-function-tf) — Power Plant Equipment, chapter 3, asked 2× - [Static excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Astatic-excitation-system) — Power Plant Equipment, chapter 3 - [Need for excitation and improving dynamic response](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-dynamic-improvement) — Power Plant Equipment, chapter 3 ## Related in other subjects - [Excitation system and changing excitation at constant load](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aexcitation-system-constant-load) — Electric Machines II, chapter 2 - [SR flip-flop diagram, excitation table, equation](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Asr-flipflop-diagram-equation) — Digital Logic, chapter 6 - [Block diagram of digital communication system](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ablock-diagram-digital-comm) — Data Communication, chapter 1, asked 8× - [Communication system and block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acomm-system-block-diagram) — Basic Electronics Engineering, chapter 5, asked 7×