Theory of Experiment- 1

Before synchronization, following conditions must be satisfied:


The terminal voltage of both the systems i.e. the incoming alternator and the bus bar voltage or other alternator must be same.


The phase sequence of both the systems must be same.


The frequency of both the systems must be same. The condition (1) can be checked with the help of voltmeter and the condition (2) and (3) by any synchronizing method.
There are two synchronizing methods -

  1. Using incandescent lamp
  2. Using synchroscope.

  3. Now we discuss in detail about these methods.

(a) Using Incandescent lamp

Let machine G2 be synchronized with machine G1 which is already connected with the bus bar, using three lamps (L1, L2 and L3) method. These lamps are known as synchronizing lamps connected as shown in Fig.1

If the speed of machine 2 is not brought upto that of machine 1 then its frequency will also be different, hence there will be a phase difference between their voltages as shown in Fig.2. Due to difference in frequencies the resultant voltage will under go changes similar to the frequency changes of beats produced when two sound sources of nearly equal frequencies are sounded together.

Fig. 1 Synchronization using three lamp method

The resultant voltage is sometimes maximum and sometimes minimum. Hence, the lamps will flicker, sometimes dark and sometimes bright. Synchronization is done at the middle of the dark period. This method of synchronizing is known as dark lamp method.

Fig. 2 Waveforms when two systems operating at different frequencies

Lamp L1 is connected between A1 and A2, L2 between B1 and C2 and L3 between C1 and B2. These three lamps slowly brighten and darken in cyclic successor in a direction depending upon whether incoming machine 2 is fast or slow. The synchronizing switch will be closed at the moment when lamp L1 will be completely dark. This transposition of two lamps suggested by Siemens and Aalske helps to indicate whether the incoming machine 2 is running too slow or too fast. If lamps were connected symmetrically, they would dark out or glow up simultaneously (if phase rotation is same.).

This method has following drawbacks:

  1. The lamps become dark at about one third of the rated voltage. Hence, faulty synchronizing may be done in dark period.
  2. Using this method it is not possible to find out that how much the  machine is slow or fast.
  3. This method is not applicable for high voltage alternators, because lamp ratings are normally low. For such situations we need an extra transformer to step down the voltage.


Synchroscope is a device that shows the correct instant of closing the synchronizing switch with the help of a pointer which will rotate on the dial. The rotation of pointer also indicates whether the incoming machine is running too slow or too fast. If incoming machine is slow then pointer rotates in anticlockwise direction and if machine is fast then pointer rotates in clockwise direction.

Fig. 3 Synchronizing by Synchroscope