--- title: "Armature reaction in synchronous generator" subject: "Electrical Machine" chapter: 6 key: "semester_4/electrical_machine:armature-reaction-sync-generator-loads" source: https://www.bctengineer.com/q/semester_4/electrical_machine/armature-reaction-sync-generator-loads site: BCT Engineering --- # Armature reaction in synchronous generator Electrical Machine · Chapter 6: Three Phase Synchronous Machines - Asked in: 2075 Baisakh, 8 marks; 2069 Bhadra (asked 2 times) - Video: [Armature Reaction of Alternators [Year - 3]](https://www.youtube.com/watch?v=8HFa_YjZIFc) — Mtutor & Mschool - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/armature-reaction-sync-generator-loads - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-6 ## Question What do you mean by armature reaction in a synchronous generator? Explain how resistive, inductive and capacitive loads affect the nature of armature reaction, with necessary diagrams. ## Short answer Armature reaction is the effect of stator armature mmf on the main field flux. Resistive load: cross-magnetizing (slight voltage drop). Inductive load: demagnetizing (large voltage drop). Capacitive load: magnetizing (voltage rise). ## Answer **Armature reaction** is the effect of the armature (stator) mmf, produced by the load current, on the main field flux of the machine. When load current flows in the three-phase stator winding, it creates its own mmf, which rotates at synchronous speed in step with the rotor poles and distorts, weakens or strengthens the main flux. Its effect depends on the angle ψ between the load current I_a and the induced emf E (the internal power-factor angle). The armature flux is in phase with the armature current, while E lags the main field flux by 90°. So when I_a is in phase with E, the armature flux is 90° (electrical) away from the main field flux. #### Resistive (unity pf) load - I_a is in phase with E. - The armature mmf is **at right angles to the main field**. Hence the armature reaction is **cross-magnetising**. - Effect: the field is distorted. In a generator, the flux density is reduced at the leading pole tip and increased at the trailing tip. With magnetic saturation the net flux falls slightly, so the terminal voltage drops a little (also because of the armature resistance drop). ```text Resistive load: Fa is 90° to Ff Ff (main field) ^ | --------+----> Fa (armature mmf) ``` #### Inductive (lagging pf) load - I_a lags E by up to 90°. - The armature mmf opposes the main field flux, so the armature reaction is **demagnetising** (purely so at zero pf lagging). - Effect: the net flux falls and the terminal voltage falls strongly, so the field excitation must be increased to keep the voltage constant. ```text Lagging load: Fa opposes Ff Ff ----> Fa <---- (Net flux = Ff - Fa, weakened) ``` #### Capacitive (leading pf) load - I_a leads E by up to 90°. - The armature mmf adds to the main field, so the armature reaction is **magnetising** (purely so at zero pf leading). - Effect: the net flux increases and the terminal voltage may rise above the no-load value (negative regulation). The excitation must be reduced. ```text Leading load: Fa aids Ff Ff ----> Fa ----> (Net flux = Ff + Fa, strengthened) ``` | Load | I_a vs E | Armature reaction | Effect on terminal voltage | |---|---|---|---| | Resistive | in phase | cross-magnetising | slight fall | | Inductive | lags | demagnetising | large fall | | Capacitive | leads | magnetising | rise | For a general pf, the armature mmf has a demagnetising or magnetising (direct-axis) component F_a sin ψ and a cross-magnetising (quadrature-axis) component F_a cos ψ. In calculations, the reaction is accounted for by the armature-reaction reactance X_a, and X_s = X_a + X_l is the synchronous reactance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is armature reaction? A. Effect of the armature mmf from load current on the main field flux B. Resistance drop in the armature C. Effect of field mmf on the armature emf D. Mechanical vibration of armature Answer: A. Load current creates a rotating mmf that distorts, weakens or strengthens main flux. (Key point: Definition) 2. What type of armature reaction occurs with resistive load? A. Demagnetising B. Magnetising C. Cross-magnetising D. None Answer: C. With Ia in phase with E, armature mmf is at 90° to the field. (Key point: Resistive load) 3. Lagging pf load gives what armature reaction and effect? A. Magnetising, voltage rises B. Demagnetising, voltage falls strongly C. Cross-magnetising, no change D. Magnetising, voltage falls Answer: B. Armature mmf opposes the main flux, so net flux and terminal voltage fall. (Key point: Inductive load) 4. Leading pf load makes terminal voltage... A. fall greatly B. stay exactly constant C. rise because the reaction is magnetising D. fall because it is cross-magnetising Answer: C. Armature mmf aids main flux; terminal voltage may exceed no-load value. 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