--- title: "Induction motor as generator in grid-connected mode" subject: "Electrical Machines I" chapter: 5 key: "semester_4/electrical_machines_i:induction-generator-grid-connected" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-generator-grid-connected site: BCT Engineering --- # Induction motor as generator in grid-connected mode Electrical Machines I · Chapter 5: Three-Phase Induction Machines - Asked in: 2076 Baisakh, 8 marks - 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-grid-connected - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-5 ## Question Explain how a 3-phase induction motor can be used as generator in grid-connected mode. ## Short answer An induction motor acts as a generator when its rotor is driven above synchronous speed, making slip negative. Active power flows from rotor to stator and into the grid, while magnetizing reactive power is drawn from the grid. ## Answer A 3-phase induction motor connected to the grid works as an **induction (asynchronous) generator** when its rotor is driven by a prime mover **above synchronous speed**. Slip becomes negative and the machine feeds active power into the grid. #### Principle Slip $s = \dfrac{N_s - N}{N_s}$. - As a motor, $N < N_s$, $s > 0$: the field moves faster than the rotor, rotor emf and current produce motoring torque. - If a turbine drives the rotor at $N > N_s$, the rotor conductors cut the field in the **opposite** direction. Rotor emf, rotor current and torque reverse; $s < 0$. - Air-gap power $P_{ag} = 3I_2^2 R_2/s$ becomes negative: power flows from the rotor across the air gap to the stator and out to the grid. The electromagnetic torque now opposes the prime mover. ```text T motoring (s>0) | .--. | .' '. |__/________\________________ N 0 Ns\ . \ .' '.__.' generating (s<0) ``` #### Grid-connected operation ```text Turbine/ +--------+ P (active) --> 3-phase wind =====| Ind. |================= grid N > Ns | machine| <-- Q (VAr) (V, f fixed) +--------+ ``` 1. **Starting:** the machine is started as a motor from the grid (or run up by the turbine) to near $N_s$, then the turbine input is increased so that $N$ goes slightly above $N_s$. 2. **Excitation from grid:** the machine has no field winding. It draws its **magnetizing (lagging reactive) current** from the grid, which acts as an infinite bus. Capacitors are not essential; some are added only for power-factor correction. 3. **Voltage and frequency** are fixed by the grid, independent of the exact rotor speed. No synchronizing equipment or exciter/governor for frequency is needed. 4. **Power output** rises roughly in proportion to the negative slip in the normal range (about 1–5% above $N_s$). If the turbine drives too fast (beyond the maximum generating torque), the machine runs away, so overspeed protection is needed. 5. **Power factor:** the machine delivers active power to the grid but absorbs reactive power from it, so in generator convention its power factor is leading. #### Advantages - Simple, rugged cage rotor; low cost and maintenance. - No synchronization or hunting problems; tolerates speed changes of the turbine. - Short circuit at terminals collapses the excitation, limiting fault current. #### Disadvantages - Needs reactive power from the grid, burdening it; poor power factor. - Cannot control terminal voltage; cannot work alone without capacitors. #### Applications Wind turbines (fixed-speed cage and doubly-fed induction generators), small and mini hydro plants connected to the national grid, and energy recovery in drives (e.g. regenerative braking of lifts and cranes when the load overhauls the motor). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When does a grid-connected induction machine act as a generator? A. When driven below synchronous speed B. When driven above synchronous speed so that slip is negative C. When its rotor is blocked D. When the supply is disconnected Answer: B. At N > Ns the slip is negative, rotor emf, current and torque reverse and power flows to the grid. (Key point: Condition for generation) 2. Where does a grid-connected induction generator get its magnetizing current? A. From a DC exciter on the shaft B. From residual magnetism alone C. From the grid, as lagging reactive power D. From the prime mover Answer: C. It has no field winding and absorbs magnetizing VAr from the grid, so it delivers P but absorbs Q. (Key point: Source of excitation) 3. What fixes voltage and frequency of a grid-connected induction generator? A. The turbine governor B. The rotor speed exactly C. The excitation capacitor D. The grid, which acts as an infinite bus Answer: D. Grid V and f are fixed, so no synchronizing equipment is needed and speed need only be slightly above Ns (about 1-5%). (Key point: Frequency and voltage control) 4. Which is a disadvantage of the induction generator? A. It draws reactive power from the grid and has poor power factor B. It needs precise synchronization C. It suffers from hunting D. It has a complex slip-ring rotor always Answer: A. It cannot control terminal voltage and burdens the grid with reactive power, though it needs no synchronizing and does not hunt. (Key point: Drawbacks) ## Close variants set in other exams - [Induction generator in isolated and grid-connected modes](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-isolated-grid) — 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 - [Construction and working of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-construction-working) — Electrical Machines I, chapter 5 - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — 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 - [Induction generator excitation capacitor value](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-generator-excitation-capacitor) — Electrical Machines I, chapter 5 - [Performance of 7.5 kW induction motor from equivalent circuit](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-400v-7p5kw-equivalent-circuit-slip2) — Electrical Machines I, chapter 5 ## Related in other subjects - [Output equation of three phase induction motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainduction-motor-output-equation) — Electric Machine Design, chapter 4, asked 6× - [Split phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asplit-phase-im) — Electrical Machine, chapter 7 - [Torque equation of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atorque-equation-induction-max-condition) — Electrical Machines, chapter 5 - [T/F: grid connected system more stable](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-grid-connected-more-stable) — Transmission and Distribution System Design, chapter 1