---
title: "Synchronous motor features and lagging/leading operation"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:motor-features-lagging-leading"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-features-lagging-leading
site: BCT Engineering
---

# Synchronous motor features and lagging/leading operation

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2075 Chaitra, 7 marks
- Video: [SYNCHRONOUS MOTOR !! BASIC OPERATING PRINCIPLE !!](https://www.youtube.com/watch?v=C-HaMtfq1Wg) — AB Classes
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-features-lagging-leading
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

State the characteristic features of synchronous motor and explain how a synchronous motor can be operated to draw lagging current as well as leading current.

## Short answer

A synchronous motor runs at constant speed, is not self-starting, and its power factor is adjustable via field current. Under-excitation (Ef<V) makes armature current lagging; over-excitation (Ef>V) makes it leading. This occurs because Ia = (V - Ef)/jXs, so the resultant voltage E shifts, changing the current phase relative to terminal voltage.

## Answer

A synchronous motor runs at constant synchronous speed $N_s = 120f/P$ and, unlike an induction motor, its power factor can be set by the DC field current.

#### Characteristic features

1. Speed is constant at $N_s$ from no load to full load; it depends only on supply frequency and number of poles.
2. Not self-starting; started by damper winding, pony motor or variable frequency.
3. Operates at lagging, unity or leading pf depending on excitation.
4. Over-excited motor on no load acts as a **synchronous condenser**.
5. Breaks down (pulls out of step) if load exceeds the maximum (pull-out) torque.
6. Requires a separate DC excitation source.
7. Prone to **hunting** on sudden load changes.
8. High efficiency, especially for large, low-speed drives.

#### How it draws lagging or leading current

For a motor (with $R_a$ neglected): $\vec V = \vec E_f + jX_s\vec I_a$, so

$$
\vec I_a = \frac{\vec V - \vec E_f}{jX_s} = \frac{\vec E_r}{jX_s}
$$

The current lags the resultant voltage $E_r$ by $90^\circ$. Keeping the load (and so $I_a\cos\phi$ and $E_f\sin\delta$) constant, only $E_f$ is changed by the field current.

**1. Under-excitation, $E_f < V$ - lagging current**

```text
   V ---------------------->
    \  Er (from Ef tip to V tip)
     \ delta
      '-----> Ef (short)
   Ia = Er/jXs lags V   -> lagging pf
```

The resultant $E_r$ is nearly in phase with $V$, so $I_a$ (90° behind $E_r$) **lags** $V$. The motor takes magnetising (lagging) reactive power from the supply to make up for its weak field.

**2. Normal excitation - unity pf**

$E_f$ is such that $E_r$ is $90^\circ$ ahead of $V$; then $I_a$ is in phase with $V$, and current is minimum.

**3. Over-excitation, $E_f > V$ - leading current**

```text
   V ---------------------->
    \          Er points above V
     \ delta
      '-----------------> Ef (long)
   Ia = Er/jXs leads V  -> leading pf
```

$E_r$ swings ahead of $V$ by more than $90^\circ$, so $I_a$ **leads** $V$. The excess field means the motor supplies lagging reactive power to the network, behaving like a capacitor.

| Excitation | $E_f$ | Current | pf |
|---|---|---|---|
| Under | $< V$ | large | lagging |
| Normal | about $V$ | minimum | unity |
| Over | $> V$ | large | leading |

This control is the basis of the **V-curves** and of using synchronous motors for power factor correction.

## Check yourself

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

1. At what speed does a synchronous motor run from no load to full load?
   A. Slightly below Ns, falling with load
   B. Exactly Ns = 120f/P, independent of load
   C. Above Ns when over-excited
   D. A speed that rises with the DC field current
   Answer: B. Speed depends only on supply frequency and number of poles, so it stays at Ns at every load. (Key point: Constant speed)

2. What sets the power factor of a synchronous motor, unlike an induction motor?
   A. The number of poles
   B. The shaft load alone
   C. The DC field current (excitation)
   D. The damper winding
   Answer: C. Changing the DC field current changes Ef, and so the motor can run lagging, unity or leading. (Key point: Power factor control by excitation)

3. When Ef > V (over-excited), what does the armature current do?
   A. Lags V and the motor absorbs magnetising VARs
   B. Is exactly zero
   C. Leads V and the motor acts like a capacitor
   D. Stays in phase with V
   Answer: C. Er swings ahead of V by more than 90 degrees, so Ia = Er/jXs leads V. (Key point: Over-excitation gives leading current)

## Related in Electric Machines II

- [Synchronous motor drawing lagging and leading current](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-lagging-leading-current) — Electric Machines II, chapter 2, asked 4×
- [Leading kVAR supplied by synchronous motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Apf-correction-leading-kvar) — Electric Machines II, chapter 2
- [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

## 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×
- [Sync motor back emf at 0.5 pf leading](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-motor-3-3kv-back-emf) — Electrical Machine, chapter 6
- [Hunting; loaded operation of synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-hunting-loaded-operation) — Electrical Machines, chapter 6
- [3.3 kV synchronous motor back emf](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-3-3kv-back-emf) — Electrical Machines, chapter 6, asked 2×