--- title: "DC generator construction, working, emf" subject: "Electrical Machine" chapter: 3 key: "semester_4/electrical_machine:dc-generator-construction-emf" source: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-generator-construction-emf site: BCT Engineering --- # DC generator construction, working, emf Electrical Machine · Chapter 3: DC Generator - Asked in: 2071 Bhadra, 8 marks - Video: [DC Machine: Construction And Working Principle (DC Motor & DC Generator)](https://www.youtube.com/watch?v=oI-O9FCDqmg) — Electrical4U - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-generator-construction-emf - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-3 ## Question Describe the construction and working principle of a dc generator with neat diagram. Also derive the emf equation of a dc generator. ## Short answer It consists of a yoke, field poles with windings, laminated armature core with slots and armature windings, a commutator and carbon brushes. The prime mover rotates the armature; conductors cut flux, inducing AC emf which the commutator rectifies to DC. The generated emf is E = PΦZN/(60A) volts, where A = P for lap and 2 for wave windings. ## Answer A DC generator converts mechanical energy into electrical energy by Faraday's law of induction (E = PΦZN/60A). The construction has a stationary field system and a rotating armature. #### Construction ```text +----------- YOKE (cast steel) ----------+ | [Pole] [Pole] | | [shoe] +-------------+ [shoe] | | | ARMATURE | | | | (core+slots| | | | winding) |--- shaft | | +-------------+ | +----------- commutator + brushes -------+ ``` 1. **Yoke**: outer frame of cast iron or cast steel. It gives mechanical support and carries the flux. 2. **Poles and pole shoes**: laminated steel, carry the field winding; shoes spread the flux evenly on the armature. 3. **Field winding**: copper coils on the poles which produce the main flux. 4. **Armature core**: laminated silicon steel with slots, to reduce eddy-current loss. 5. **Armature winding**: copper conductors in the slots (lap or wave connected). 6. **Commutator**: copper segments insulated by mica; converts the AC in the armature to DC. 7. **Brushes**: carbon blocks that collect current from the commutator. 8. Bearings, shaft and end covers. #### Working principle The prime mover turns the armature in the field of the poles. The conductors cut flux and an emf is induced (Fleming's right-hand rule). The emf in each coil alternates, but the commutator and brushes give a unidirectional voltage at the terminals. #### EMF equation Let Φ = flux per pole (Wb), P = poles, Z = total conductors, A = parallel paths, N = speed (rpm). ```text Flux cut by one conductor in 1 revolution = Φ x P (Wb) Time for 1 revolution = 60/N (s) Average emf per conductor = ΦP / (60/N) = ΦPN/60 (V) Conductors in series per path = Z/A E_g = (ΦPN/60) x (Z/A) ``` **E_g = PΦZN / (60A) volts** - Lap winding: A = P, so E_g = ΦZN/60 - Wave winding: A = 2, so E_g = PΦZN/120 Since E_g = (PZ/60A) Φ N = KΦN, the emf is proportional to flux and speed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are the functions of the yoke? A. Mechanical support for the machine and carrying the magnetic flux B. Holding the brushes only C. Converting AC to DC D. Producing the armature emf Answer: A. The yoke is the cast iron/steel outer frame that supports poles and carries flux. (Key point: Yoke) 2. Why is the armature core made of laminated silicon steel? A. To increase the copper loss B. To reduce eddy-current loss C. To insulate the commutator D. To increase the number of poles Answer: B. Lamination breaks up eddy-current paths. (Key point: Armature core) 3. In the derivation, what is the average emf in one conductor? A. ΦN/60P volts B. ΦPN volts C. ΦPN/60 volts, from flux cut ΦP in time 60/N seconds D. ΦP/N volts Answer: C. One revolution cuts ΦP Wb in 60/N s, so e = ΦP/(60/N) = ΦPN/60. (Key point: Emf per conductor) 4. What does the emf equation become for wave winding? A. E_g = ΦZN/60 B. E_g = PΦZN/60 C. E_g = ΦZN/120 D. E_g = PΦZN/120 Answer: D. Wave winding has A = 2, so PΦZN/(60 x 2) = PΦZN/120. (Key point: Winding cases) ## Related in Electrical Machine - [DC generator construction, working, emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-construction-emf) — Electrical Machines, chapter 3 - [DC generator working and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-principle-emf-equation) — Electrical Machine, chapter 3 - [DC generator principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-emf) — Electrical Machines, chapter 3, asked 4× - [Working principle of dc generator with diagram](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-principle-diagram) — Electrical Machines, chapter 3 - [EMF equation of a DC generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Adc-generator-emf-equation) — Electrical Machine, chapter 3 - [EMF equation of DC generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-generator-emf-equation) — Electrical Machines I, chapter 3 ## Related in other subjects - [Synchronous generator principle and EMF equation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Agenerator-principle-emf-equation) — Electric Machines II, chapter 1 - [Transformer operating principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-principle-emf-equation) — Electrical Machines I, chapter 2, asked 6× - [Ideal transformer principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aideal-transformer-principle-emf) — Electrical Machines, chapter 2 - [Transformer definition, principle, emf equation](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Atransformer-emf-equation) — Basic Electrical and Electronics Engineering, chapter 2