--- title: "Construction of synchronous machine and rotor types" subject: "Electric Machines II" chapter: 1 key: "semester_5/electric_machines_ii:synchronous-machine-construction-rotors" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/synchronous-machine-construction-rotors site: BCT Engineering --- # Construction of synchronous machine and rotor types Electric Machines II · Chapter 1: Three Phase Synchronous Generator - Asked in: 2083 Baisakh (new course), 3 marks; 2082 Bhadra (new course), 3 marks (asked 2 times) - Video: [Difference between Salient Pole and Cylindrical Pole Rotor](https://www.youtube.com/watch?v=OF2chMtJ36Q) — Come4Concepts - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/synchronous-machine-construction-rotors - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-1 ## Question With neat sketch, explain about constructional features of synchronous machine. Discuss different types of rotor and their applications. ## Short answer The stator core is laminated with three-phase distributed windings. The rotor carries the DC field. Salient-pole rotors have projecting poles, non-uniform air gap, many poles, and are used in low-speed hydro generators. Cylindrical (turbo) rotors are smooth forged steel, have uniform air gap, 2–4 poles, and are used in high-speed steam and gas turbine alternators. ## Answer A **synchronous machine** has a stationary armature (stator) and a rotating field system (rotor) excited by DC; the rotor runs at synchronous speed $N_s = 120f/P$. ```text +-------------------------+ | stator frame | | +-------------------+ | | | stator core with | | | | 3-ph winding slots| | | | +-------+ | | | | | rotor | <-- DC via slip rings | | | (N/S) | | | | | +-------+ | | | +-------------------+ | +-------------------------+ ``` #### Stator (armature) - **Frame** of cast iron or welded steel supports the core. - **Core** of thin silicon-steel laminations (to reduce eddy and hysteresis losses) with slots on the inner surface. - **Three-phase winding**, usually double-layer, short-pitched and distributed, placed in the slots; usually star connected. #### Rotor and field The field winding is fed with DC through **slip rings and brushes** (or a brushless exciter). There are two types of rotor: | Point | Salient-pole rotor | Cylindrical (non-salient) rotor | |---|---|---| | Shape | Projecting poles bolted on a spider | Smooth solid forged-steel cylinder with slots | | Diameter / length | Large diameter, short length | Small diameter, long length | | Poles | Many (4 to 60+) | 2 or 4 | | Speed | Low/medium (100–1000 rpm) | High (1500/3000 rpm) | | Air gap | Non-uniform | Uniform | | Damper winding | Fitted in pole faces | Not usually needed | | Prime mover | Hydraulic turbine, diesel engine | Steam/gas turbine (turbo-alternator) | | Application | Hydro power plants (e.g. most Nepali hydro plants) | Thermal and nuclear plants | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the stator core made of thin silicon-steel laminations? A. To reduce eddy-current and hysteresis losses B. To increase the copper loss C. To allow DC flux D. To reduce the air gap to zero Answer: A. Laminations limit eddy currents; silicon steel lowers hysteresis loss. (Key point: Core) 2. Which description fits the salient-pole rotor? A. Small diameter, long length, 2 or 4 poles, high speed B. Large diameter, short length, many poles, low speed, hydro turbines C. Uniform air gap, solid forged steel, steam turbines D. Small diameter, many poles, nuclear plants Answer: B. Salient-pole machines are slow, with many projecting poles, for hydro and diesel. (Key point: Salient pole) 3. Cylindrical rotors are used with A. hydraulic turbines at 100-300 rpm B. wind turbines at 20 rpm C. steam or gas turbines at 1500/3000 rpm D. diesel engines with 60 poles Answer: C. Non-salient rotors suit high speed turbo-alternators in thermal and nuclear plants. (Key point: Cylindrical rotor) ## Related in Electric Machines II - [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 - [BLDC motor construction and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Abldc-construction-applications) — Electric Machines II, chapter 3 - [Power angle characteristics of cylindrical rotor machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Apower-angle-cylindrical-rotor) — Electric Machines II, chapter 1, asked 2× - [Synchronous motor features and lagging/leading operation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-lagging-leading) — Electric Machines II, chapter 2 - [Construction and operation of synchronous generator](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-generator-construction-principle) — Electric Machines II, chapter 1 - [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3 ## Related in other subjects - [Constructional features of belted cable](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Abelted-cable-construction) — High Voltage Engineering, chapter 1 - [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 - [Swing equation of synchronous machine](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Aswing-equation-derivation) — Power System Analysis II, chapter 6 - [3-phase synchronous generator construction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-generator-construction-working) — Electrical Machine, chapter 6