--- title: "Induction generator in isolated and grid-connected modes" subject: "Electrical Machines I" chapter: 5 key: "semester_4/electrical_machines_i:induction-generator-isolated-grid" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-generator-isolated-grid site: BCT Engineering --- # Induction generator in isolated and grid-connected modes Electrical Machines I · Chapter 5: Three-Phase Induction Machines - Asked in: 2079 Chaitra, 8 marks; 2078 Baisakh, 8 marks; 2070 Magh, 8 marks; 2069 Bhadra, 8 marks (asked 4 times) - Video: [induction generator working principle | induction generator in hindi | animation | construction](https://www.youtube.com/watch?v=2eTLpzuozLA) — 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_machines_i/induction-generator-isolated-grid - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5 ## Question Explain how an induction motor can be used as an induction generator. Explain its operation in isolated and grid connected mode. ## Short answer An induction generator operates with negative slip (rotor speed above synchronous speed). It requires reactive power to magnetize. In grid-connected mode, the grid supplies reactive power and fixes voltage and frequency. In isolated mode, a capacitor bank provides reactive power; voltage builds up from residual magnetism and depends on speed, capacitance and load. ## Answer An **induction generator** is an ordinary induction machine driven by a prime mover above synchronous speed, so that slip becomes negative and the machine converts mechanical power into electrical power. #### Motor to generator action Slip $s = (N_s - N)/N_s$. - $N < N_s$: $s$ positive, rotor conductors cut the field in one direction, machine works as a **motor**. - $N = N_s$: $s = 0$, no emf in rotor, no torque. - $N > N_s$: $s$ negative. The rotor now cuts the rotating field in the opposite direction, so rotor emf, rotor current and torque reverse. The torque opposes the prime mover, and the air-gap power $P_{ag} = 3I_2^2R_2/s$ becomes negative: power flows from rotor to stator to the supply. The machine is a **generator**. ```text T | motoring | .--. | .' '. |___/________'.______ N | Ns (s<0) | '. .' generating | '' (T negative) ``` The induction generator cannot produce its own magnetizing current. It always needs **reactive power** (lagging VAr) to set up the rotating field. Where this comes from decides the two modes. #### Grid-connected mode ```text Prime +--------+ 3-phase grid mover ===| IG |=====(fixes V and f, N > Ns +--------+ supplies VAr) ``` 1. The stator is connected to the grid (infinite bus). The grid fixes voltage and frequency and supplies the magnetizing (reactive) current. 2. The machine is first run up near $N_s$ (or started as a motor), then driven above $N_s$ by the turbine. 3. Active power delivered rises with negative slip (typically 1–5% above $N_s$). Frequency stays equal to grid frequency even if turbine speed varies a little, so no synchronizing is needed. 4. Used in wind farms and small hydro plants connected to the grid. #### Isolated (self-excited) mode ```text Prime +--------+ mover ===| IG |===+=====> Load N > Ns +--------+ | ===== C (delta/star capacitor bank) ``` 1. There is no grid, so a **capacitor bank** across the stator terminals supplies the lagging VAr (capacitors take leading current = machine's lagging magnetizing current). 2. **Voltage build-up:** residual magnetism in the rotor gives a small emf when it is driven. This drives a small current through the capacitors, which increases the flux, which raises the emf further. The process continues until the magnetizing curve of the machine meets the capacitor line $V = I_c X_c$ — the operating voltage. 3. Voltage and frequency depend on speed, capacitance and load. Higher speed or larger C raises voltage; a load needing VAr lowers it. Regulation is poor, so voltage/frequency controllers or electronic load controllers are used. 4. Used in micro-hydro and stand-alone wind systems in remote areas. | Point | Grid-connected | Isolated | |---|---|---| | VAr source | Grid | Capacitor bank | | Voltage, frequency | Fixed by grid | Depend on speed, C, load | | Synchronizing | Not needed | Not applicable | | Residual magnetism | Not needed | Needed for build-up | | Typical use | Wind farms | Micro-hydro, remote sites | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What makes an induction machine act as a generator? A. Driven below synchronous speed B. Driven above synchronous speed, so slip is negative C. Driven at synchronous speed D. Supplied with DC Answer: B. At N > Ns slip is negative, rotor torque reverses and power flows to the stator. (Key point: Generator condition) 2. What does an induction generator always need? A. Reactive (magnetizing) power from a grid or capacitors B. DC excitation C. Synchronizing equipment D. A field winding on the rotor Answer: A. It cannot produce its own magnetizing current. (Key point: Reactive power) 3. In grid-connected mode, what fixes the voltage and frequency? A. Prime mover speed B. The grid, which also supplies the VAr C. Capacitor bank D. Residual magnetism Answer: B. The grid is an infinite bus, so no synchronizing is needed. (Key point: Grid mode) 4. How does voltage build up in an isolated generator? A. Residual magnetism gives a small emf, which drives current in the capacitors, strengthening flux until the magnetizing curve meets the capacitor line B. The load supplies the current C. A battery excites the rotor D. It needs no residual magnetism Answer: A. The cumulative process stops at the intersection of the magnetization curve and V = Ic Xc. (Key point: Isolated mode build-up) ## Close variants set in other exams - [Induction motor as generator in grid-connected mode](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-grid-connected) — Electrical Machines I, chapter 5, asked 5× ## Related in Electrical Machines I - [Excitation capacitors in isolated and grid-connected induction generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-capacitor-isolated-grid) — Electrical Machines I, chapter 5 - [Isolated induction generator and renewable energy role](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-isolated-renewable) — Electrical Machines I, chapter 5 - [Voltage build-up in isolated induction generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-isolated-buildup) — Electrical Machines I, chapter 5