A synchroscope is an instrument used to establish phase angle and frequency synchronization between alternating current (AC) power supplies. This is a critical safety measure when AC power networks or generator outputs are merged or connected together 
When combining AC power supplies, it is of critical importance to ensure both are the same voltage, frequency, and in perfect phase with each other. Connecting AC supplies when they are not balanced in this way may lead to severe damage to the network.
The voltage of AC power supplies are generally fairly stable and easy to indicate with conventional instrumentation. The phase angle and frequency relationship between supplies, however, is seldom balanced without some adjustment. When generator or network supplies are combined, a synchroscope indicates any differences between the two. If there are discrepancies, generator speeds can be adjusted until the synchroscope indicates a perfect balance. At that point, the supplies may be safely combined.
There are two basic types of synchroscope. The first is the electro-mechanical type which indicates phase and frequency relationships with a pointer and dial indicator. 


The synchroscope consists of a small motor with coils on the two poles connected across two phases. Let’s say it is connected in red and yellow phases of the incoming machine and armature windings supplied from red and yellow phases from the switchboard bus bars. 
The bus bar circuit consists of an inductance and resistance connected in parallel. 
The inductor circuit has the delaying current effect by 90 degrees relative to current in resistance. 
These dual currents are fed into the synchroscope with the help of slip rings to the armature windings which produces a rotating magnetic field. 
The polarity of the poles will change alternatively in north/south direction with changes in red and yellow
phases of the incoming machine. 
The rotating field will react with the poles by turning the rotor either in clockwise or anticlockwise direction. 
If the rotor is moving in clockwise direction this means that the incoming machine is running faster than the bus bar and slower when running in anticlockwise direction. 

•    Generally, it is preferred to adjust the alternator speed slightly higher, which will move the pointer on
      synchroscope is in clockwise direction. 
•    The breaker is closed just before the pointer reaches 12 o clock position, at which the incoming machine
      is in phase with the bus bar

The second type of instrument is the electronic synchroscope. This type utilizes a microprocessor designed to sample and compare two AC power supplies for phase and frequency characteristics. Any differences between the two are then calculated and indicated on either a digital liquid crystal display (LCD) or by means of LEDs. The latter type synchroscope typically has a circle of LEDs on its front panel which illuminate in a set pattern to indicate the presence and magnitude of phase and frequency differences. 

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 -
a. Using incandescent lamp

b. Using synchroscope.

(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.

The synchroscope consists of a two-phase stator.  The two stator windings are at right angles to one another, and by means of a phase-splitting network, the current in one phase leads the current of the other phase by 90°, thereby generating a rotating magnetic field. 

The stator windings are connected to the incoming generator, and a polarizing coil is connected to the running generator. The rotating element is unrestrained and is free to rotate through 360°.   It consists of two iron vanes  mounted  in  opposite  directions  on  a  shaft,  one  at  the  top  and  one  at  the  bottom,  and magnetized by the polarizing coil. 

If the frequencies of the incoming and running generators are different,  the synchroscope will rotate at a speed corresponding to the difference.   It is designed so that if incoming frequency is higher than running frequency, it will rotate in the clockwise direction; 

if incoming frequency is  less  than  running  frequency,  it  will  rotate  in  the  counterclockwise  direction.    When  the synchroscope indicates 0o phase difference, the pointer is at the "12 o’clock" position and the two AC generators are in phase

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