Page 138 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

1 The three lamps will light and go out in unison at a rate which depends on the difference in frequency between
the two alternators.
2 The three lamps will light and go out at a rate which depends on the difference in frequency between the two
machines, but not in unison. In this case, the machines are not connected in the proper phase sequence and
are said to be out of phase. To correct this, it’s necessary to interchange any two leads to alternator 1. The
machines are not paralleled until all lamps light and go out in unison. The lamp method is shown for greater
simplicity of operation.
By making slight adjustments in the speed of alternator 1 the frequency of the machines can be equalized so
that the synchronizing lamps will light and go out at the lowest possible rate. When the three lamps are out, the
instantaneous electrical polarity of the three leads from 1 is the same as that of 2 At this instant, the voltage of 1 is
equal to and in phase with that of 2 Now the paralleling switch can be closed at the middle period of the darkness
of the lamps so that both alternators supply power to the load. The two alternators are in synchronism, according
to the three dark method.
The three dark method has certain disadvantages and is seldom used. A large voltage may be present across an
incandescent lamp even though it’s dark (burned out). As a result, it’s possible to close the paralleling connection
while there is still a large voltage and phase difference between the machines. For small capacity machines
operating at low speed, the phase difference may not affect the operation of the machines. However, when large
capacity units having low armature reactance operate at high speed, a considerable amount of damage may
result if there is a large phase difference and an attempt is made to parallel the units.
Two bright, one dark method (Dark and bright lamp method) : Another method of synchronizing alternators is
the two bright, one dark method. In this method, any two connections from the synchronizing lamps are crossed
after the alternators are connected and tested for the proper conditions for paralleling phase rotation. (The
alternators are tested by the three dark method.) Fig 2 shows the connections for establishing the proper phase
rotation by the three dark method. Fig 2 shows the lamp connections required to synchronize the alternator by the
two bright, one dark method.

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CITS : Power - Electrician & Wireman - Lesson 76-85