---
title: "Voltage build-up in induction generator"
subject: "Electrical Machines"
chapter: 5
key: "semester_4/electrical_machines:induction-generator-voltage-buildup"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/induction-generator-voltage-buildup
site: BCT Engineering
---

# Voltage build-up in induction generator

Electrical Machines · Chapter 5: Three Phase Induction Machines

- Asked in: 2071 Magh (old course), 8 marks
- Video: [EPM_Electrical testing M4 : Self-excited induction generator](https://www.youtube.com/watch?v=S-xyxHxnaMA) — Islam Alaa
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/induction-generator-voltage-buildup
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-5

## Question

How does voltage build up occur in an induction generator? Explain.

## Short answer

Residual rotor flux induces small stator emf, sending leading capacitor current that increases flux. This regeneratively builds voltage until the capacitor line crosses the saturated magnetisation curve. Conditions: residual magnetism, capacitance above critical value, sufficient speed, and light load.

## Answer

Voltage build-up in a self-excited induction generator is a regenerative process that starts from the **residual magnetism** of the rotor iron and is supported by a **capacitor bank** across the stator terminals.

#### Process
1. The rotor is driven above synchronous speed by a prime mover. The small residual flux cuts the stator conductors and induces a small emf $E_r$ (a few volts) at the terminals.
2. This emf drives a leading current $I_c = \frac{E_r}{X_c}$ through the capacitors.
3. The capacitor current flows in the stator winding as a **magnetising current** and increases the air-gap flux in the same direction as the residual flux.
4. The increased flux raises the terminal emf, which raises the capacitor current, which increases the flux further. The voltage thus builds up in steps.
5. The build-up stops at the point where the **capacitor line** ($V = I_c X_c$) intersects the **magnetisation (saturation) curve**. Beyond this point, saturation of the iron stops the rise of emf. This is the stable no-load voltage.

```text
 V
 |           /  capacitor line
 |       _.-*   <- stable point
 |    _-' /
 |  .'  /
 | / ->/
 |/ /  Er
 +------------------> Im
```

#### Conditions for build-up
- Residual magnetism must exist in the rotor iron (if lost, flash the field with a battery).
- Capacitance must be greater than the critical value, so the capacitor line is flatter than the linear part of the magnetisation curve.
- Speed must be high enough, because the emf is proportional to speed.
- Load must not be too heavy, otherwise the voltage collapses.

With a higher capacitance the line is flatter and the final voltage is higher. A larger load needs a larger capacitance to hold the voltage.

## Check yourself

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

1. What starts voltage build-up in a self-excited induction generator?
   A. A battery connected permanently
   B. Residual magnetism in the rotor iron
   C. The load current
   D. A sudden speed drop
   Answer: B. Residual flux induces a small emf which begins the regenerative process. (Key point: Starting point)

2. Why does the voltage rise step by step?
   A. Capacitor current acts as magnetising current, raising flux, emf and capacitor current again
   B. The load draws more current each step
   C. Saturation increases the flux
   D. Slip keeps decreasing
   Answer: A. The loop of emf, capacitor current and flux reinforces itself. (Key point: Regenerative process)

3. Where does the build-up stop?
   A. Where the capacitor line meets the magnetisation curve (saturation)
   B. When the emf reaches zero
   C. At the origin
   D. When speed equals N_s
   Answer: A. Saturation of the iron stops the rise at the intersection point. (Key point: Final voltage)

4. If a higher capacitance is used, the final no-load voltage is:
   A. Lower, as the line is steeper
   B. Unchanged
   C. Higher, as the line is flatter
   D. Zero
   Answer: C. A larger C gives a flatter line that crosses the saturation curve at a higher voltage. (Key point: Capacitance effect)

## Related in Electrical Machines

- [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
- [Shunt generator voltage build-up, critical resistance and speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-buildup-critical-define) — Electrical Machines I, chapter 3, asked 9×
- [Starting shunt generator on no load and voltage build-up](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-generator-no-load-buildup) — Electrical Machines I, chapter 3
- [Voltage build-up; series generator at no load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Avoltage-buildup-series-generator-no-load) — Electrical Machines, chapter 3
- [Shunt generator build-up conditions, critical speed and resistance](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-buildup-conditions-critical) — Electrical Machines I, chapter 3
- [Voltage build-up, critical resistance and speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Avoltage-buildup-critical-resistance-speed) — Electrical Machines, chapter 3, asked 3×

## Related in other subjects

- [Voltage build-up at line–cable junction](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aline-cable-junction-voltage) — High Voltage Engineering, chapter 3