Synchroscope is an instrument, which shows the phase relationship of emf of the incoming alternator and at the same time it also indicates whether it is running slow or fast. This instrument works on the principle of rotating magnetic fields. It consists of a small motor with rotor and stator. Both wound for two phase. A potential transformer connected to two of the main bus-bar give supply to the stator ‘A’ winding and another potential transformer of same type connected to the corresponding terminals on the incoming machine supply to the stator ‘B’ winding. The rotor rotates if the stator resultant flux in the ‘A’ and ‘B’ is different and the exact time of synchronizing is the stand still position of the rotor. That means the both the voltages in winding ‘A’ and ‘B’ are same and there is no resultant flux to rotate the rotor. The speed of the rotor depends on the frequency of the alternator and is too fast when alternator (incoming machine) speed is more and less when alternator is too slow.