A synchroscope is used to determine the correct instant for closing the switch connects an alternator to the generating station busbars. This process of connecting at the correct instant or synchronizing is necessary when an unloaded incoming alternator is to be connected to the busbars in order to share the load.

The operation on connecting an alternator in parallel with another alternator or with common bus-bars is called synchronizing.

The correct instant of synchronizing the incoming alternator to bus-bars is when :

  1. The magnitude of terminal voltage of the incoming alternator is same as that of the bus-bar voltage.
  2. The terminal voltage of the incoming alternator must be the same as bus-bar voltage.
  3. The speed of the incoming machine must be such that its frequency equals bus-bar frequency.
  4. The phase of the alternator voltage must be indentical with the phase of the bus-bar voltage. It means that the switch must be closed the instant the two voltages have correct phase relationship.

Types of Synchroscope

  • Electro-dynamometer (KILESTON) type synchroscope
  • Moving iron type synchroscope