Synchronization of Machines using dark lamp method:-
If phase sequence of running machine is RYB and incoming machine who is to be synchronized with the system is abc, than synchronization switch would be put across contacts a-R, b-Y, c-B respectively. Simultaneous closure of these switches causes synchronization of incoming machine with the running machine.
In dark lamp method of synchronization , There identical lamp’s L1, L2, L3 are contacted across a-R, b-Y, c-B respectively. Voltage rating of these lamps should be twice the rated line to neutral voltage of the machine. Correct phase sequence can be verified if all the three lamps become dark together and bright together with same intensity of brightness. If same doesn’t happens than by changing any 2 terminals of incoming machine correct phase sequence can be achieved.
Image below shows how to Synchronize machines using dark lamp method:-
If lamps never attain a dark phase but intensity of brightness of lamps changes with time then this indicates that there is high voltage difference. The excitation of incoming machine must be varied so that dark phase of all lamps is obtained.
Image above shows how various configurations during that method
For maximum brightness case rating of lamp selected should be 2 times the neutral voltage
If the lamps are dark or glow with the same intensity at all time than it means that frequency of incoming machine has become exactly equal to the frequency of running machine. In that case frequency of incoming machine should be adjusted in such a manner that interval between successive dark phases is maximized. That will ensure that frequency difference has become very low. Under these conditions, direction of dark period would become long enough to correctly anticipate the middle of dark period at which the synchronization switch should be closed to ensure proper synchronization of incoming machine with running machine.
If Magnitude difference between 2 phases is very high than lamp will not be dark
At minimum 30% of rated voltage , we can just see the brightness of lamp.
When frequency of phase difference between phases is same then we can see constant brightness of lamp.
For incoming machine Interval between successive darks must be maximum.