Synchronizing alternators by dark lamp method

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'.

 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.