---
title: "Excitation capacitors in isolated and grid-connected induction generator"
subject: "Electrical Machines I"
chapter: 5
key: "semester_4/electrical_machines_i:induction-generator-capacitor-isolated-grid"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/induction-generator-capacitor-isolated-grid
site: BCT Engineering
---

# Excitation capacitors in isolated and grid-connected induction generator

Electrical Machines I · Chapter 5: Three-Phase Induction Machines

- Asked in: 2070 Bhadra, 8 marks
- Video: [SEM13 Isolated or Self Excited Induction Generator](https://www.youtube.com/watch?v=t1_0GZhxYEY) — Lectures in Electrical Engineering
- 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-capacitor-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 generator. Explain why excitation capacitors are required in isolated mode of operation. Explain why excitation capacitors are not required in grid connected mode of operation?

## Short answer

Driving rotor above synchronous speed yields negative slip and generation. In isolated mode, capacitors supply reactive power for self‑excitation to build voltage; grid‑connected mode draws magnetising current from the grid, so capacitors are only for power‑factor correction.

## Answer

An induction motor becomes an **induction generator** when a prime mover drives its rotor **above synchronous speed**. Slip becomes negative and the machine converts mechanical power into electrical power.

#### Motor used as generator

- Slip $s = (N_s - N)/N_s$. For $N > N_s$, $s < 0$.
- The rotor now moves faster than the rotating field, so the rotor conductors cut the field in the opposite direction. Rotor emf, rotor current and torque all reverse.
- The torque opposes the prime mover; air-gap power $P_{ag} = 3I_2^2R_2/s$ is negative, i.e. power flows from the rotor to the stator and to the load or supply.
- The useful range is a small negative slip (about −1% to −5%); beyond the maximum generating torque the machine overspeeds.

```text
 T
 |  motor (0<s<1)
 |    .--.
 |  .'    '.
 |_/________'.________ s
 |        0  '.   .'
 |             '-'  generator (s<0)
```

#### Why capacitors are needed in isolated mode

1. An induction machine has **no field winding**. Its magnetic field is created by a magnetizing current that lags the voltage by nearly 90°, i.e. it needs **reactive power** whether motoring or generating.
2. In isolated (stand-alone) operation there is no grid to supply this reactive power. A capacitor bank connected across the stator terminals draws a **leading** current, which is the same as supplying lagging reactive power to the machine.
3. **Self-excitation:** residual magnetism gives a small emf when the rotor is driven. This emf sends current through the capacitors, which increases the flux, which raises the emf further. The voltage builds up until the magnetization curve crosses the capacitor line $V = I_c/(\omega C)$.
4. Without capacitors (or with too small a capacitance, i.e. capacitor line steeper than the air-gap line) the voltage cannot build up.
5. The capacitors must also supply the reactive power of lagging loads; voltage and frequency vary with speed, load and capacitance.

```text
 Turbine ==[IG]==+=====> Load
                 |
                === C (3-phase)
```

#### Why capacitors are not required in grid-connected mode

1. The grid (infinite bus) has many synchronous generators that supply reactive power. The induction generator simply draws its magnetizing current from the grid.
2. The grid fixes the terminal **voltage and frequency**, so no voltage build-up process is needed and residual magnetism is irrelevant.
3. The machine delivers active power to the grid while absorbing reactive power from it.
4. Capacitors may still be installed, but only for **power-factor correction** to reduce the reactive burden on the grid — not for excitation.

| Point | Isolated mode | Grid-connected mode |
|---|---|---|
| Source of VAr | Capacitor bank | Grid |
| Voltage build-up | By self-excitation | Not needed |
| V and f | Set by speed, C, load | Fixed by grid |
| Capacitors | Essential | Optional (pf correction) |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. When does an induction motor act as a generator?
   A. When driven above synchronous speed, so slip is negative
   B. When driven below synchronous speed with s = 1
   C. When the supply frequency is zero
   D. When the rotor resistance is increased
   Answer: A. A prime mover pushes N above Ns; s < 0, rotor emf, current and torque reverse and power flows to the stator. (Key point: Generator operation)

2. Why does an induction generator need capacitors in isolated mode?
   A. The capacitors supply the active power to the load
   B. It has no field winding and needs lagging magnetising VAr, which capacitors supply
   C. The capacitors raise the speed above Ns
   D. The capacitors reduce rotor copper loss
   Answer: B. The magnetising current lags nearly 90 degrees, and a capacitor's leading current supplies that reactive power. (Key point: Need for reactive power)

3. What starts voltage build-up in an isolated induction generator?
   A. A DC field winding on the rotor
   B. The grid voltage
   C. Residual magnetism produces a small emf that drives current through the capacitors, raising flux further
   D. The load current
   Answer: C. Residual emf drives capacitor current, increasing flux and emf until the magnetisation curve meets the capacitor line. (Key point: Self-excitation)

4. Why are capacitors not needed for excitation when grid-connected?
   A. The generator produces no reactive power
   B. The rotor is short circuited
   C. The slip becomes zero
   D. The grid supplies the magnetising reactive power and fixes voltage and frequency
   Answer: D. The grid draws no build-up process; the machine takes magnetising current from it, and capacitors are optional only for power-factor correction. (Key point: Grid-connected mode)

## Related in Electrical Machines I

- [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×
- [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×
- [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
- [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

## Related in other subjects

- [Induction generator and excitation capacitor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Ainduction-generator-excitation-capacitor) — Electrical Machines, chapter 5
- [Line pf of 35 kW induction motor with capacitors](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ainduction-motor-35kw-capacitor-line-pf) — Utilization of Electrical Energy, chapter 6, asked 3×