DARK LAMP METHOD:
Since the phasor will be in phase (fig). If the phase sequence is wrong, one lamp will be dark and the other two will be bright and vice versa
- In this method, the lamps are directly connected across synchronizing switch as shown is fig.
- The voltages and frequencies of both the systems are made equal.
- The incoming machine frequency is slightly altered deliberately. Assuming the phase sequences are the same the three lamps will be dark or bright together.
- The lamp will be dark below a certain voltage. If the dark period is judged and switch is closed at the middle of the dark period, the machines are synchronized.
- In that case, any two leads are interchanged.
- In this method, the lamps are connected asymmetrically as shown (fig).
- If the phase sequence is correct, the lamps brighten and grow dim in sequence.
- Two sets of star vectors will rotate at unequal speeds if the frequencies of the two machines are different (fig.).
- Synchronization is done at the moment the uncrossed lamp L1 is in the middle of the dark period. When the uncrossed lamp L1 is dark, then other two crossed lamps L2&L3 are dimly but equally bright.