Synchronization of alternator

Synchronization of alternator means connecting an alternator into grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency. Many alternators and loads are connected into a grid, and all the alternators in grid are having same output voltage and frequency (whatever may be the power). It is also said that the alternator is connected to infinite bus-bar.
A stationary alternator is never connected to live bus-bars, because it will result in short circuit in the stator winding (since there is no generated emf yet). Before connecting an alternator into grid, following conditions must be satisfied:
  1. Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.
  2. Similar frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
  3. Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or bus-bars.
  4. Phase angle: The phase angle between the generated voltage and the voltage of grid must be zero.
The first condition of voltage equality can be satisfied by a voltmeter. To satisfy the conditions of equal frequency and identical phases, one of the following two methods can be used:
(i) Synchronization using incandescent lamp
(ii) Synchronization using synchroscope.

Synchronization of alternator using incandescent lamp

 synchronization of alternator using incandescent lamps

Let, alternator 2 is to be synchronized in a grid and the alternator 1 is already in the grid as shown in above figure. The alternator 2 is connected to grid through three synchronizing lamps (L1, L2 and L3) as shown in above figure.  If the speed of the alternator 2 is not such that the frequency of output voltage is equal to the frequency of the grid, there will also be a phase difference in the voltages, and in this case the lamps will flicker. Three lamps are connected asymmetrically, because if they were connected symmetrically, they would glow or dark out simultaneously (if the phase rotation is same as that of bus-bars). Asymmetrically connected lamps indicate whether the incoming machine is running slower or faster. If the alternator 2 is running slower, the phase rotation of alternator 2 will appear to be clockwise relative to the phase rotation of the grid and the lamps will light up in the order 3,2,1;3,2,1 .... If the alternator 2 is running faster, the phase rotation of alternator 2 will appear to be anticlockwise relative to the phase rotation of the grid and the lamps will light up in the order 1,2,3;1,2,3....
When the speed of the alternator 2 reaches so that, the frequency and phase rotation of output voltage is similar to that of the grid voltage, lamp L1 will go dark and lamps L2 and L3 will dimly but equally glow (as they are connected between different phases and due to this there will be phase difference of 120 degree). The synchronization is done at this very moment. This method of synchronization is sometimes also known as 'two bright and one dark method'.

Drawbacks of 'synchronization using incandescent lamps' method are:
  • Synchronization by using incandescent lamps depends on the correct judgement of the operator.
  • This method does not tell how slow or fast the machine is.
  • To use this method for high voltage alternators, extra step down transformers need to be added as ratings of lamps are normally low.

Synchronization of alternator using synchroscope

synchronization of alternator using synchroscope
A synchroscope is a device which shows the correct instant of closing the synchronizing switch. Synchroscope has a pointer which rotates on the dial. The pointer rotates anticlockwise if the machine is running slower or it rotates clockwise if the machine is running fast. The correct instant of closing syncronizing switch is when the pointer is straight upwards.


  1. good ,information in an easy way

  2. If at the instance of 2 bright and 1 dark same phase angle occuer between incoming generator and bus bar, how????

  3. Please add dark lamp method too.

  4. thank you very much sir.