--- title: "Voltage build-up in isolated induction generator" subject: "Electrical Machines I" chapter: 5 key: "semester_4/electrical_machines_i:induction-generator-isolated-buildup" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-generator-isolated-buildup site: BCT Engineering --- # Voltage build-up in isolated induction generator Electrical Machines I · Chapter 5: Three-Phase Induction Machines - Asked in: 2069 Poush, 8 marks - Video: [Isolated Induction Generator || Self Excited Induction Generator ||](https://www.youtube.com/watch?v=8uJhxLdPl58) — Edu Shifu - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-generator-isolated-buildup - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5 ## Question Explain the voltage build-up process of isolated three phase induction generator. ## Short answer Residual magnetism in the rotor induces a small voltage, which sends leading capacitor current that strengthens the field. This cumulative build-up continues until the magnetizing curve crosses the capacitor line V = Ic Xc, giving stable voltage. Requires sufficient capacitance (below critical value fails) and proper speed. Load reduces voltage and frequency. ## Answer An **isolated (self-excited) induction generator** is an induction machine driven above synchronous speed with a 3-phase capacitor bank across its stator terminals and no connection to the grid. The capacitors supply the reactive (magnetizing) power, and the terminal voltage builds up by itself from residual magnetism. ```text Prime mover +---------+ (N > Ns) =====| Ind. |===+====[S]===> Load | machine | | +---------+ === C per phase ``` #### Requirements for build-up 1. **Residual magnetism** in the rotor core. 2. Rotor driven at a suitable speed (near or above $N_s$). 3. **Sufficient capacitance**, so that the capacitor line is less steep than the air-gap (linear) part of the magnetization curve. 4. Load disconnected (or light) during build-up. #### Build-up process (step by step) 1. The prime mover drives the rotor. Residual flux in the rotor induces a small emf $V_r$ in the stator winding. 2. This small voltage drives a current $I_c = V_r\,\omega C$ through the capacitors. This current leads the voltage by 90°, which means it acts as a lagging **magnetizing current** for the machine. 3. The magnetizing current strengthens the air-gap flux, so the induced emf rises. 4. The higher emf drives a larger capacitor current, which raises the flux and emf further. 5. This cumulative action continues until the voltage reaches the point where the **magnetization curve** of the machine crosses the **capacitor line** $V = I_c X_c$. Beyond this point, saturation makes the machine need more current than the capacitors can provide, so the voltage stabilizes. ```text V | capacitor line | / V = Ic.Xc | __.----*-- magnetization | .-' / | curve | .' / | | / / | * = operating | / / | point |/ / | Vr+ / | |/______________|_____ I (magnetizing) ``` #### Factors affecting the voltage - **Capacitance:** larger C gives a flatter capacitor line and higher voltage. Below a **critical capacitance** (line steeper than the air-gap line) the voltage does not build up. - **Speed:** higher speed raises the magnetization curve (emf ∝ speed) and frequency, so voltage rises. - **Load:** a lagging load takes part of the capacitor VAr, and resistive load demagnetizes the machine; voltage and frequency fall. Too heavy a load collapses the voltage, which gives inherent short-circuit protection. #### Loss of excitation If the residual magnetism is lost, it can be restored by briefly connecting a battery (DC) across a stator phase or running the machine as a motor from a supply for a moment. Self-excited induction generators are widely used in micro-hydro and small wind plants in remote areas because they are cheap, rugged and need little maintenance; an electronic load controller keeps voltage and frequency steady. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What starts voltage build-up in an isolated induction generator? A. Residual magnetism in the rotor induces a small emf in the stator B. The battery in the load C. The capacitor stores initial charge D. The grid provides the first voltage Answer: A. Residual flux, when the rotor is driven, induces the small voltage Vr that begins the process. (Key point: Start of build-up) 2. Why does the capacitor current Ic = Vr w C help build up the voltage? A. It supplies active power to the rotor B. It leads the voltage by 90 degrees and acts as a lagging magnetising current, strengthening the flux C. It reduces the speed D. It cancels the residual flux Answer: B. The leading capacitor current magnetises the machine, increasing flux, emf and capacitor current cumulatively. (Key point: Role of capacitor current) 3. Where does the voltage stop rising? A. At the supply voltage B. When the capacitor is fully charged C. Where the magnetisation curve crosses the capacitor line V = Ic Xc D. At the point where speed equals Ns Answer: C. Saturation makes the machine need more current than the capacitor line provides beyond the intersection, so the voltage stabilises. (Key point: Final voltage) 4. What happens if the capacitance is too small? A. The voltage builds up too high and burns the machine B. The frequency becomes zero C. The machine behaves as a motor D. The capacitor line is steeper than the air-gap line, and voltage does not build up Answer: D. Below the critical capacitance the capacitor line never meets the magnetisation curve above the residual point. 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