--- title: "Load characteristics and capacitive load voltage rise" subject: "Electric Machines II" chapter: 1 key: "semester_5/electric_machines_ii:load-characteristics-capacitive-load" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/load-characteristics-capacitive-load site: BCT Engineering --- # Load characteristics and capacitive load voltage rise Electric Machines II · Chapter 1: Three Phase Synchronous Generator - Asked in: 2075 Asoj, 8 marks - Video: [Experiment to plot load characteristics of Alternator.](https://www.youtube.com/watch?v=n7B74T-dulM) — Shweta Tiwari - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/load-characteristics-capacitive-load - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-1 ## Question Explain load characteristics of synchronous generator. Why terminal voltage of a synchronous generator is greater than internal generated emf (E) in case of capacitive load? Explain with the help of armature reaction and phasor diagram. ## Short answer Load characteristics show terminal voltage vs. armature current at constant field current. Lagging pf causes a drop, unity pf a slight drop, and leading pf a rise. With a capacitive load, armature reaction is magnetizing, increasing the air-gap flux, so terminal voltage becomes greater than the generated emf E. ## Answer #### Load (external) characteristics The **load or external characteristic** of an alternator is the curve of terminal voltage $V$ against load (armature) current $I_a$, at constant speed and constant field current, for a given load power factor. ```text Vt | ___.-- leading pf | ___.--' |E ------'----------.___ unity pf | `--. | `--.__ lagging pf | `--.__ +--------------------------------> Ia 0 rated Ia ``` - All curves start at $V = E$ (no load). - **Lagging pf**: voltage falls sharply because of the demagnetising armature reaction and the large $IX_s$ drop in phase with $V$. - **Unity pf**: voltage falls slightly (cross-magnetising effect, $IR_a$ and small quadrature drop). - **Leading pf**: voltage rises with load because armature reaction is magnetising. - Hence a lower pf (lagging) gives poorer regulation; an automatic voltage regulator changes the field current to hold $V$ constant. #### Why $V > E$ for capacitive load Per phase, $\bar E = \bar V + \bar I(R_a + jX_s)$ and $$ E = \sqrt{(V\cos\phi + IR_a)^2 + (V\sin\phi - IX_s)^2}\quad\text{(leading pf)} $$ The reactive term $V\sin\phi - IX_s$ is reduced by the reactance drop, so $E$ can be smaller than $V$. **Armature reaction view:** with a capacitive load the current leads the emf (by 90° for a pure capacitor). The armature mmf then acts **along** the axis of the field poles and **aids** the field flux (magnetising effect). The air-gap flux increases, so the voltage at the terminals is higher than the open-circuit emf $E$ produced by the field current alone. ```text Leading pf phasor diagram I E \ /'. \ / '. j I Xs \/ '. O----------+ V (E < V) ``` **Consequence:** voltage regulation is negative; less excitation is needed at leading pf, and over-voltage can occur when a lightly loaded long line or capacitor bank is fed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The load (external) characteristic of an alternator is the plot of: A. E against field current at no load B. Terminal voltage against armature current at constant speed and field current for given pf C. Armature current against field current on short circuit D. Terminal voltage against speed at constant load Answer: B. It is the V versus Ia curve at constant speed and field current, for a given load power factor. (Key point: Definition of load characteristic) 2. How does terminal voltage vary with load current for a leading pf load? A. It falls sharply B. It falls slightly C. It stays exactly constant D. It rises with load Answer: D. Leading pf gives magnetising armature reaction, so V rises with load, while lagging pf makes it fall sharply. (Key point: Shape for different pf) 3. With a capacitive load, how does the armature mmf act relative to the field poles? A. Along the pole axis, aiding the field flux B. Along the pole axis, opposing the field flux C. At 90 degrees to the pole axis D. It is zero Answer: A. The leading current produces an armature mmf along the field axis that aids the main flux, increasing air-gap flux and terminal voltage. (Key point: Armature reaction at capacitive load) 4. What is a consequence of V > E in capacitive loading? A. Positive regulation and more excitation needed B. Negative regulation and less excitation needed C. Zero regulation always D. Alternator must stop Answer: B. Since V exceeds E, regulation is negative; less excitation is needed and over-voltage may occur on lightly loaded long lines. (Key point: Consequence of V > E) ## Close variants set in other exams - [Why terminal voltage exceeds emf on capacitive load](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-greater-capacitive) — Electric Machines II, chapter 1, asked 3× ## Related in Electric Machines II - [Terminal voltage above emf and armature reaction](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-greater-ar-effect) — Electric Machines II, chapter 1 - [When terminal voltage is less or more than emf](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-less-or-more) — Electric Machines II, chapter 1 - [Internal emf and armature reaction reactance](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ainternal-emf-armature-reactance) — Electric Machines II, chapter 1 - [Armature reaction at different load power factors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aarmature-reaction-power-factor-effect) — Electric Machines II, chapter 1, asked 7× - [Salient pole generator phasor diagram in per unit](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asalient-generator-phasor-pu) — Electric Machines II, chapter 1 - [Internal emf, no-load emf and regulation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ainternal-noload-emf-50kva) — Electric Machines II, chapter 1 ## Related in other subjects - [Synchronous generator load characteristics, capacitive load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-load-characteristics-capacitive) — Electrical Machines, chapter 6 - [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aarmature-reaction-sync-gen-loads) — Electrical Machines, chapter 6 - [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aarmature-reaction-sync-generator-loads) — Electrical Machine, chapter 6, asked 2× - [Synchronous generator working; armature reaction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-working-armature-reaction) — Electrical Machines, chapter 6