---
title: "Criteria for synchronizing two alternators"
subject: "Electrical Machines"
chapter: 6
key: "semester_4/electrical_machines:sync-alternators-synchronizing-criteria"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-alternators-synchronizing-criteria
site: BCT Engineering
---

# Criteria for synchronizing two alternators

Electrical Machines · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2068 Magh, 8 marks
- Video: [4.9.2 Requirements of parallel operation | EE501 |](https://www.youtube.com/watch?v=YLQeitfdAT4) — RGPV Official
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-alternators-synchronizing-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6

## Question

Write down the criteria for synchronizing two 3-phase alternators with the detail explanation.

## Short answer

The alternators must have equal voltage, equal frequency, same phase sequence, and matching phase angle at synchronising instant. These are checked with voltmeters, frequency meters, phase sequence indicator, and synchroscope or lamps.

## Answer

Synchronising (paralleling) is the process of connecting an incoming alternator to a live bus or to another running alternator. If this is done incorrectly, large circulating currents and mechanical shocks occur.

#### Conditions (criteria) to be satisfied
1. **Equal terminal voltage:** the rms voltage of the incoming machine must equal the bus voltage. It is adjusted by the field current (excitation).
2. **Equal frequency:** the frequency of the incoming alternator must equal the bus frequency. The prime-mover speed (governor) is adjusted until the difference is nearly zero.
3. **Same phase sequence:** the phase order (R-Y-B) must be the same as that of the bus. A wrong sequence causes a short circuit.
4. **Same phase angle (in phase):** corresponding phase voltages must be in phase at the instant the switch is closed.
5. **Same waveform** (sinusoidal for both) is also desirable.

#### Detailed explanation
- **Voltage:** if the voltages differ, a circulating current $I_c = \frac{E_1 - E_2}{Z_1 + Z_2}$ flows, which is mostly reactive. It is avoided by matching the voltages using a voltmeter.
- **Frequency:** if the frequencies differ, the phase difference changes continually and the machines go in and out of phase at the beat frequency. A synchroscope or lamps show the beat. The switch is closed when the beat is very slow.
- **Phase sequence:** checked once at installation with a phase sequence indicator, or with lamps (in the dark-lamp method all lamps should brighten and darken together). Interchanging any two lines corrects a wrong sequence.
- **Phase angle:** the switch is closed when the slowly rotating synchroscope pointer reaches the vertical (12 o'clock) position, or when the dark lamps are fully dark.

#### Methods
- Three dark lamps method
- Two bright, one dark lamp method
- Synchroscope (most common)

```text
 Bus ----+----+----+----
         |    |    |
       [S]--(Lamps / Synchroscope)--
              |
    Incoming alternator
```

## Check yourself

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

1. How is equal voltage obtained?
   A. By prime-mover speed
   B. By field current (excitation)
   C. By phase swap
   D. By load
   Answer: B. Excitation controls the terminal voltage. (Key point: Voltage adjustment)

2. How is frequency matched?
   A. Field current
   B. Prime-mover speed through the governor
   C. Transformer
   D. Switch
   Answer: B. Frequency follows speed. (Key point: Frequency adjustment)

3. What happens with wrong phase sequence?
   A. Nothing
   B. Short-circuit-like condition; corrected by interchanging two lines
   C. Lower voltage
   D. Higher efficiency
   Answer: B. Phases would never be in phase together; two lines must be swapped. (Key point: Wrong sequence)

4. When is the switch closed?
   A. When the synchroscope pointer is at 12 o'clock / lamps are dark
   B. When beat is fastest
   C. When voltage is zero
   D. Any time
   Answer: A. Same phase angle with a slow beat gives a smooth parallel connection. (Key point: Closing instant)

## Related in Electrical Machines

- [Synchronizing alternators by dark lamp method](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-dark-lamp) — Electrical Machines, chapter 6
- [Scott connection for three-phase to two-phase](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ascott-connection) — Electrical Machines I, chapter 2
- [Single phase motor not self-starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-starting-methods) — Electrical Machines, chapter 7

## Related in other subjects

- [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asynchronizing-two-alternators-criteria) — Electrical Machine, chapter 6
- [Fault current: two 500 kVA alternators](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Afault-two-500kva-alternators) — Power Plant Equipment, chapter 4