---
title: "Excitation system and changing excitation at constant load"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:excitation-system-constant-load"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/excitation-system-constant-load
site: BCT Engineering
---

# Excitation system and changing excitation at constant load

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2082 Bhadra (new course), 3 marks
- Video: [Lecture 18: Effect of varying load and field excitation on power factor of synchronous motors](https://www.youtube.com/watch?v=FjVeN2H4zYY) — Electrical Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/excitation-system-constant-load
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

Define excitation system in synchronous motor. Explain briefly with neat phasor diagram about the effect of changing excitation with constant load in synchronous motor.

## Short answer

Excitation system supplies dc field current, determining back emf and power factor. At constant load, changing excitation alters only reactive power: underexcitation gives lagging pf, normal excitation gives unity pf and minimum current, overexcitation gives leading pf, following V-curves.

## Answer

The **excitation system** of a synchronous motor is the DC source and its control (DC exciter, static rectifier or brushless exciter, with field regulator) that supplies the **field current** to the rotor poles; it decides the back emf $E_f$ and hence the motor's power factor.

#### Effect of changing excitation at constant load

With constant load, $VI_a\cos\phi$ and $E_f\sin\delta$ are constant. Only the reactive part of $I_a$ changes.

```text
         Ia3 (over: lead)
           \        Ia2 (normal: upf)
            \       |
  -----------\------+--------> V
              \     |
          Ia1 (under: lag)
   tip of Ia moves on a line normal to V
```

| Excitation | $E_f$ | $I_a$ | pf |
|---|---|---|---|
| Under | $< V$ | large | lagging |
| Normal | $\approx V$ | minimum | unity |
| Over | $> V$ | large | leading |

The phasor diagram for the voltages shows the same effect. Since $E_f\sin\delta$ is constant, the tip of $E_f$ moves on a line parallel to $V$ as excitation changes:

```text
  Ef1 (under)  Ef2 (normal)  Ef3 (over)
   .-----------.-------------.   <- locus of Ef tip (parallel to V)
   |           |             |
   O----------------------------> V
   Er = V - Ef joins the tip of Ef to the tip of V;
   Ia = Er / jXs lags Er by 90 deg
```

As field current rises, $I_a$ first falls to a minimum (unity pf) and then rises with leading pf, giving the **V-curve**.

## Check yourself

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

1. What does the excitation system of a synchronous motor supply?
   A. Stator three-phase current
   B. DC field current to the rotor poles, deciding Ef and the pf
   C. Cooling air
   D. Starting torque
   Answer: B. It is the DC source and control (exciter, rectifier, regulator) that sets the field current. (Key point: Excitation system)

2. At constant load, which quantities stay constant when excitation is changed?
   A. Ia and pf
   B. V Ia cos(phi) and Ef sin(delta)
   C. Only Ef
   D. Reactive power
   Answer: B. Only the reactive part of Ia changes; the active part stays fixed. (Key point: Constant quantities)

3. As field current rises from low values, how does Ia change?
   A. It rises continuously
   B. It falls to a minimum at unity pf, then rises with leading pf
   C. It stays constant
   D. It rises then falls to zero
   Answer: B. Under-excited lagging current falls to a minimum at unity pf, then rises again as leading current, giving the V-curve. (Key point: V-curve behaviour)

## Related in Electric Machines II

- [Synchronous motor features, applications and excitation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-applications-excitation) — Electric Machines II, chapter 2
- [Motor adjustment to load and excitation effects](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-adjust-load-excitation) — Electric Machines II, chapter 2
- [Effect of excitation on armature current and pf](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aexcitation-armature-current-pf) — Electric Machines II, chapter 2, asked 3×
- [Synchronous motor adjusting to increasing load](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-adjust-increasing-load) — Electric Machines II, chapter 2
- [Salient pole generator phasor diagram in per unit](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asalient-generator-phasor-pu) — Electric Machines II, chapter 1
- [Excitation emf and maximum power of 4 kVA motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asalient-motor-4kva-max-power) — Electric Machines II, chapter 2

## Related in other subjects

- [Effect of excitation on synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-motor-excitation-effect) — Electrical Machine, chapter 6, asked 6×
- [Effect of excitation on synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-excitation-effect-pf) — Electrical Machines, chapter 6, asked 5×
- [Loaded transformer and phasor diagram for inductive load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-loaded-operation-phasor-inductive) — Electrical Machines I, chapter 2
- [Effect of excitation on steady state stability](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Aexcitation-effect-steady-state) — Power System Analysis II, chapter 6