---
title: "Armature reaction in synchronous generator"
subject: "Electrical Machines"
chapter: 6
key: "semester_4/electrical_machines:armature-reaction-sync-gen-loads"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/armature-reaction-sync-gen-loads
site: BCT Engineering
---

# Armature reaction in synchronous generator

Electrical Machines · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2069 Bhadra
- Video: [Armature Reaction in Synchronous Generator](https://www.youtube.com/watch?v=8lQJo5e2hRA) — Rahul K A
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/armature-reaction-sync-gen-loads
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6

## Question

Explain the armature reaction in a synchronous generator for resistive, inductive and capacitive loading with necessary diagram.

## Short answer

For resistive load, armature reaction is cross-magnetising, distorting flux. Inductive load causes demagnetising reaction, reducing flux. Capacitive load causes magnetising reaction, increasing flux and raising terminal voltage.

## Answer

Armature reaction in a synchronous generator is the effect of the armature mmf $F_a$ (produced by load current) on the main field mmf $F_f$. Armature current in the three-phase winding produces a field that rotates at synchronous speed, so it is stationary with respect to the rotor poles. The result depends on the **angle $\psi$ between the armature current and the induced emf $E$**.

#### 1. Resistive load (unity pf)
Current $I$ is in phase with $E$. The armature mmf $F_a$ lags $F_f$ by $90^\circ$, so it acts across the pole axis.
- **Cross-magnetising** armature reaction.
- The flux is distorted: the trailing pole tip is strengthened, the leading tip weakened; the net flux is roughly the same (slightly reduced by saturation).
- Terminal voltage drops slightly.

```text
  Ff -->   Fa (down, 90 deg)   Result: Fr = Ff + Fa
   N-----S      |                 (shifted)
```

#### 2. Inductive load (lagging pf)
Current $I$ lags $E$ by up to $90^\circ$. For zero-pf lagging, $F_a$ is directly opposite to $F_f$.
- **Demagnetising** (partly cross-magnetising for general lagging).
- Net flux is reduced, so the induced emf falls and $V$ drops significantly.
- To keep $V$ constant the field current must be increased.

```text
  Ff -->        <-- Fa (zero pf lag)
  Fr = Ff - Fa
```

#### 3. Capacitive load (leading pf)
Current $I$ leads $E$ by up to $90^\circ$. For zero-pf leading, $F_a$ is in the same direction as $F_f$.
- **Magnetising** (partly cross-magnetising for general leading).
- Net flux increases, the induced emf rises, so $V$ may exceed $E$.
- The field current must be decreased to hold voltage.

```text
  Ff -->   --> Fa (zero pf lead)
  Fr = Ff + Fa
```

| Load | Armature reaction | Effect on flux | Terminal voltage |
|---|---|---|---|
| Resistive | Cross-magnetising | Distorted | Slightly lower |
| Inductive | Demagnetising | Reduced | Lower |
| Capacitive | Magnetising | Increased | Higher |

## Check yourself

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

1. What determines the type of armature reaction?
   A. Angle between armature current and induced emf E
   B. Speed
   C. Number of poles
   D. Frame size
   Answer: A. The angle psi between I and E fixes how Fa lies relative to Ff. (Key point: Deciding factor)

2. What is the armature reaction for a resistive load?
   A. Cross-magnetising
   B. Demagnetising
   C. Magnetising
   D. Zero
   Answer: A. Fa lags Ff by 90 deg, acting across the pole axis. (Key point: Resistive load)

3. Effect of an inductive load on net flux and V?
   A. Flux reduced, V falls
   B. Flux increased, V rises
   C. Flux unchanged
   D. V doubles
   Answer: A. Fa opposes Ff so net flux falls; field current must be increased to restore V. (Key point: Inductive load)

4. For a capacitive load, what is the effect?
   A. Demagnetising, V falls
   B. Cross-magnetising only
   C. Magnetising, flux increases and V may exceed E
   D. None
   Answer: C. Fa aids Ff, so flux and emf rise. (Key point: Capacitive load)

## Related in Electrical Machines

- [Transformer at resistive, inductive, capacitive loads](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atransformer-loading-conditions-phasor) — Electrical Machines, chapter 2
- [Synchronous generator working; armature reaction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-working-armature-reaction) — Electrical Machines, chapter 6
- [Synchronous generator load characteristics, capacitive load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-gen-load-characteristics-capacitive) — Electrical Machines, chapter 6
- [Armature reaction in DC machines and its remedies](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aarmature-reaction-effects-remedies) — Electrical Machines I, chapter 3
- [Short note: Armature reaction in dc machine](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Anote-armature-reaction-dc) — Electrical Machines, chapter 3
- [DC generator types; compound loading characteristics](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-generator-types-compound-characteristics) — Electrical Machines, chapter 3

## Related in other subjects

- [Armature reaction in synchronous generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aarmature-reaction-sync-generator-loads) — Electrical Machine, chapter 6, asked 2×
- [Armature reaction at different load power factors](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aarmature-reaction-power-factor-effect) — Electric Machines II, chapter 1, asked 7×
- [Terminal voltage above emf and armature reaction](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aterminal-voltage-greater-ar-effect) — Electric Machines II, chapter 1
- [MCB types for resistive and inductive loads](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amcb-types-resistive-inductive) — Switchgear and Protection, chapter 3