--- title: "3-phase synchronous generator construction" subject: "Electrical Machine" chapter: 6 key: "semester_4/electrical_machine:sync-generator-construction-working" source: https://www.bctengineer.com/q/semester_4/electrical_machine/sync-generator-construction-working site: BCT Engineering --- # 3-phase synchronous generator construction Electrical Machine · Chapter 6: Three Phase Synchronous Machines - Asked in: 2074 Bhadra, 8 marks - Video: [alternator working principle | synchronous generator working principle | synchronous generator](https://www.youtube.com/watch?v=AhJh8nEGovI) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/sync-generator-construction-working - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-6 ## Question Explain about constructional details and working principle of three phase synchronous generator. ## Short answer A three-phase synchronous generator has a stationary armature (stator) with a three-phase winding and a rotating field (rotor) with dc-excited salient or cylindrical poles. The prime mover rotates the rotor, and the rotating flux induces balanced three-phase emf. Frequency f = PN/120, and generated emf E = 4.44 f Φ T_ph K_w. ## Answer A three-phase synchronous generator (alternator) converts mechanical energy into three-phase a.c. electrical energy. The frequency is tied to the speed: f = PN/120. #### Constructional details ```text Stator (armature) Rotor (field) +----------------+ Salient pole Cylindrical | slots with 3ph | (hydro, low (turbo, high | winding on | speed) speed) | laminated core | ____ ______ +----------------+ / N \ | N | Frame / end covers | S | |__S___| ``` 1. **Stator (armature):** a hollow cylindrical frame carrying a laminated silicon-steel core (to reduce eddy-current loss) with slots. A three-phase winding, placed 120° apart in the slots, delivers the output at high voltage. The armature is stationary so that high voltages (11 kV or more) can be taken out without slip rings, and it is easier to insulate and cool. 2. **Rotor (field system):** carries the d.c. excited field winding and has two types: - **Salient-pole:** projecting poles, many poles, large diameter and short axial length; speed 100 to 1500 rpm; used with hydro and diesel turbines; has damper winding. - **Cylindrical (non-salient, turbo):** smooth cylinder with distributed slotted winding, 2 or 4 poles, small diameter and long axial length; speed 1500 to 3000 rpm; used with steam and gas turbines. 3. **Exciter:** a small d.c. generator on the same shaft (or a static rectifier system) supplies the field current through two slip rings and brushes. 4. **Other parts:** bearings, cooling system (air or hydrogen), and a prime mover (turbine) to rotate the shaft. #### Working principle Faraday's law of electromagnetic induction is used. 1. The field winding is excited by d.c., creating alternate N and S poles on the rotor. 2. The prime mover rotates the rotor at synchronous speed N_s. The rotating flux cuts the stationary armature conductors. 3. An emf is induced in each phase winding. Because the three phase windings are 120° apart in space, the three emfs are 120° apart in time: balanced three-phase voltage. 4. Frequency is f = PN_s/120 Hz. For 50 Hz, a 4-pole machine runs at 1500 rpm, and a 2-pole machine at 3000 rpm. 5. The rms emf per phase is **E = 4.44 f Φ T_ph K_w** volts (Φ = flux per pole, T_ph = turns per phase, K_w = winding factor). 6. When a load is connected, armature current flows and creates its own rotating mmf at the same speed. This armature reaction and the winding impedance reduce the terminal voltage below the induced emf: V = E − I_a (R_a + jX_s). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the armature kept stationary in a large alternator? A. To allow high voltage output without slip rings and easier insulation B. To reduce frequency C. To avoid d.c. excitation D. Because rotor needs more copper Answer: A. Output at 11 kV or more is taken directly, while slip rings carry only the small field current. (Key point: Stator and rotor roles) 2. Which rotor is used with steam turbines? A. Salient-pole, many poles, low speed B. Cylindrical, 2 or 4 poles, high speed C. Squirrel cage D. Wound rotor with short-circuited ring Answer: B. Turbo-alternators use smooth cylindrical rotors at 1500 to 3000 rpm. (Key point: Rotor types) 3. What is the frequency relation? A. f = 120N/P B. f = PN/120 C. f = N/P D. f = 4.44 N Answer: B. For 50 Hz, a 4-pole machine runs at 1500 rpm. (Key point: Frequency) 4. Why are the three emfs 120° apart in time? A. Phase windings are 120° apart in space B. Rotor has three poles C. Exciter has three phases D. Load is three-phase Answer: A. The rotating flux cuts windings spaced 120° apart in turn. 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