During synchronization, which instrument is commonly used to visually compare the rotor position with the bus voltage to decide when to close the breaker?

• A. Ammeter
• B. Voltmeter
• C. Synchroscope
• D. Wattmeter

Answer: (C)

To connect a generator to a running bus, you must match voltage magnitude, frequency, and especially phase angle. The instrument that provides a visual readout of the relative phase between the generator’s terminal voltage and the bus voltage is the synchroscope. It shows whether the generator is leading or lagging and how far out of phase it is. As the generator comes into step with the bus, the difference in phase shrinks and the synchroscope needle or pointer slows and eventually aligns with the bus voltage reference. Closing the breaker at that moment places the two sources in phase, minimizing transient surge and ensuring a smooth parallel operation.

Voltmeter only indicates voltage level, ammeter shows current, and wattmeter measures power; none of them reveal the instantaneous phase relationship needed for proper synchronization.