Synchronization of Alternators by Lamp Method

Synchronization of Alternators by Lamp Method

In case of alternators, synchronization is done generally by lamp methods. It can be done by two ways :

a) Lamps dark method b) Lamps bright method.

1. Lamps Dark Method for Single Phase Alternators

In this method the lamps are arranged as shown in Fig. 3.2.1. The alternator to be synchronized (which is also called incoming alternator) consists of two lamps connected across the switch terminals of the same phase.

The voltage for the two alternators is measured with the help of a voltmeter. The lamps are connected in such a way that the polarity and the frequency for the two machines can be checked. No resultant voltage will appear across the switch terminals if the frequency of the two alternators is exactly same as well as their voltages are in exact phase opposition. Thus under this case lamps will not glow. The voltages for both the machines are having same maximum and r.m.s. values and are in exact phase opposition thus resultant voltage is zero in local circuit. This is represented in the Fig. 3.2.2

 It can be seen that with unequal frequencies of the two alternators, the two lamps will become alternately bright and dark. The light beat will be produced whose number is equal to the difference in frequencies for the two machines.

The resultant voltage appearing across the lamp will be difference of the two voltages at any instant resulting in a waveform shown in the Fig. 3.2.3. Since number of cycle completed by two machines in any given time are not same the light beat is produced which is shown in the Fig. 3.2.3.

Whenever the two voltages are in exact phase opposition (i.e. angle between them is 180°) then resultant voltage ER is zero. If the switch is not closed at this instant the voltage across lamp will go on rising and synchronization will not be proper. 

The alternate darkness and brightness of the lamp will not indicate whether the incoming alternator is running fast or slow. For the exact synchronization the speed of incoming alternator is adjusted in such a way that the light beats are produced at a very slow speed and the alternators are synchronized during the middle of the dark period where resultant voltage E_R will be zero. The word middle is used as the lamps will not glow even though there is sufficient voltage across it. So it becomes difficult to know the correct instant of zero voltage.

2. Lamps Bright Method

Since it is very difficult to judge the correct instant of zero voltage in Lamps dark method, this method is introduced which is shown in the Fig. 3.2.4.

The lamps remain maximum bright when there is no difference in voltages for the two machines. This is more sharp and accurate method of synchronization because the lamps are much more sensitive to changes in voltage at their maximum brightness than when they are dark.

3. Lamps Dark Method for Three Phase Alternators

In this method, three lamps are used, which are connected as shown in the Fig. 3.2.5.

When the voltages of busbar and incoming alternators are in phase opposition and frequencies of both are exactly same no voltage appears across the switch and lamps will not glow.

For unequal frequencies, the lamps L₁ L₂ and L₃ will flicker at a rate equal to difference in frequencies. If the phase sequence is correct, all the lamps will become dark and bright simultanenously. If this does not happen then two terminals of the incoming machine are interchanged to correct the phase sequence. The frequency of the incoming machine is so adjusted that lamp will flicker at very slow rate. The correct instant of closing the switch is the middle of the dark period of the lamps L₁ L₂ and L₃.

4. Disadvantages of Lamps Dark Method

1. When there is large difference between the two voltages, the lamps become dark as voltage across them is about half of their rated voltage. Hence faulty synchronization may be done in the dark period which may damage the machine.

2. This method does not given idea about how much the machine is slow or fast. 

3. Due to large voltage, lamps may burnout as lamp ratings are normally low. Hence extra trasnsformer is required to step down the voltages which increases the cost and complexity.

Hence generally lamps bright and dark method is used.

Review Questions

1. Explain the synchronizing of 3 phase alternator by lamps dark method.

2. Explain the synhronization of single phase alternators by lamps dark method and lamps bright method.

3. State the disadvantages of lamps dark method of synchronization.