---
title: "Synchronizing alternators by dark lamp method"
subject: "Electrical Machines"
chapter: 6
key: "semester_4/electrical_machines:sync-alternators-dark-lamp"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/sync-alternators-dark-lamp
site: BCT Engineering
---

# Synchronizing alternators by dark lamp method

Electrical Machines · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2065 Chaitra (old course), 8 marks
- Video: [all dark lamp method of synchronization | two bright one dark lamp method | alternator generator](https://www.youtube.com/watch?v=3OO2HZ1UzKc) — Mind of Engineer 
- 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-dark-lamp
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-6

## Question

Explain the process of synchronizing two 3 phase alternators with dark lamp method.

## Short answer

Three lamps are connected across each phase of the synchronizing switch. Incoming alternator voltage, frequency and phase sequence are adjusted until all lamps flicker slowly together. The switch is closed at the middle of the dark period when voltage across each lamp is zero, indicating equal magnitude and phase alignment.

## Answer

#### Dark lamp method of synchronising
The dark lamp method uses three lamps to check the voltage, frequency, phase sequence and phase angle of the incoming alternator against the bus-bars.

#### Connections
Three lamps are connected across the switch contacts, one in each phase: lamp $L_1$ between R and R', $L_2$ between Y and Y', and $L_3$ between B and B'.

```text
 Bus      R ----+--------[S]-------- R'
          Y ----|--+-----[S]-------- Y'   Incoming
          B ----|--|--+--[S]-------- B'   alternator
                L1 L2 L3 (across switch)
```

#### Procedure
1. Start the incoming alternator with its prime mover and bring it to near rated speed.
2. Adjust its field current so that its terminal voltage equals the bus voltage (check with a voltmeter).
3. Observe the lamps:
   - If the **phase sequence is correct**, all three lamps flicker (grow bright and dark) **together**.
   - If the sequence is wrong, the lamps become bright and dark one after another (a rotating pattern). Interchange any two lines of the incoming alternator and repeat.
4. Adjust the speed of the prime mover (governor) until the flicker becomes very slow, which shows that the frequencies are almost equal.
5. When the lamps are **dark** (zero voltage across them), at the middle of a dark period, the incoming voltage is equal in magnitude and in phase with the bus voltage. **Close the switch** at that instant.
6. After closing, adjust the governor and the field to share the load.

#### Principle
The voltage across each lamp is the phasor difference between the bus and incoming voltages, $|\vec V_{bus} - \vec E_{inc}|$. It becomes zero only if both are equal and in phase, so a dark lamp indicates the correct instant.

#### Disadvantage
A lamp needs a minimum voltage to glow, so it stays dark even when an appreciable voltage (a sizeable phase difference) exists across it. The exact moment of synchronism is therefore hard to judge, and the method is not very accurate. A synchroscope or the two bright, one dark method gives better results.

## Check yourself

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

1. How are the lamps connected?
   A. Across the switch contacts, one per phase
   B. In series with load
   C. To ground
   D. Between phases
   Answer: A. Each lamp is across corresponding phase terminals R-R', Y-Y', B-B'. (Key point: Lamp connection)

2. If the sequence is correct, how do the lamps behave?
   A. Flicker together
   B. Flicker in rotation
   C. Stay dark
   D. Stay bright
   Answer: A. All three show the same phasor difference at the same time. (Key point: Correct sequence)

3. When is the switch closed?
   A. During brightest
   B. When lamps are dark in the middle of the dark period
   C. Any time
   D. When flicker is fast
   Answer: B. Dark means zero voltage difference: equal and in phase. (Key point: Closing time)

4. What is the disadvantage of the dark lamp method?
   A. Lamps stay dark with considerable voltage difference, so it is inaccurate
   B. Too costly
   C. Needs DC
   D. Cannot check sequence
   Answer: A. Lamps need a minimum voltage to glow. (Key point: Disadvantage)

## Related in Electrical Machines

- [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-synchronizing-criteria) — Electrical Machines, chapter 6

## Related in other subjects

- [Parallel operation conditions and dark lamp synchronizing](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-conditions-dark-lamp) — Electric Machines II, chapter 1, asked 3×
- [Need for parallel operation and dark lamp method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-operation-need-dark-lamp) — Electric Machines II, chapter 1
- [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asynchronizing-two-alternators-criteria) — Electrical Machine, chapter 6
- [Parallel operation conditions and three lamp method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-conditions-three-lamp) — Electric Machines II, chapter 1, asked 3×