There is a specific
procedure of connecting synchronous machine to infinite bus bars.
Infinite bus bar is one which keeps constant voltage and frequency
although load varies. The Fig. 1 shows a synchronous machine which is to
be connected to the bus bars with the help of switch K.
If the synchronous machine is running as a generator then its phase
sequence should be some as that of bus bars. The machine speed and field
current is adjusted in such a way so as to have the machine voltage
same as that of bus bar voltage. The machine frequency should be nearly
equal to bus bar frequency so that the machine speed is nearer to
conditions are satisfied, the instant of switching for synchronization
should be determined. This can be determined by lamps dark method, Lamps
bright and dark method or by using synchroscope.
Once switch K is closed, the stator and rotor fields of the machine
lock into each other and the machine then runs at synchronous speed. The
real power exchange with the mains will be now governed by the loading
conditions on the shaft while the reactive power exchange will be
determined by field excitation.
The same procedure is to be followed for synchronizing the synchronous
motor to the infinite bus bars. The motor is run by an auxiliary device
such as small dc or induction motor initially and then synchronized to
the bus bars.
we know that the synchronous motors are not self starting hence if
switch K is closed when rotor is stationary, the average torque will be
zero as the two fields run at synchronous speed relative to each other
so the motor fails to start. They are made self starting by providing
short circuited bars on the rotor which produce torque as produced in
case of induction motors.