Which instrument indicates a difference in frequency between two AC sources?

• A. Thermistor
• B. Transformer
• C. Synchro System
• D. Synchroscope

Answer: (D)

Understanding how to tell if two AC sources are at different frequencies comes down to watching how the phase relationship between them behaves over time. A synchroscope is built to reveal exactly that: it shows the rate at which the phase angle between two sine-wave sources is changing. If the two frequencies differ, the phase difference slides steadily, and the synchroscope’s dial or rotor will rotate at a speed proportional to that difference. If the frequencies are the same, the phase angle stops changing, so the indicator comes to a halt, signaling that the sources are in sync and can be connected with minimal transient.

The direction the dial moves also tells you which source is ahead so you can adjust the incoming source accordingly to bring the frequencies into agreement. The other instruments don’t serve this purpose: a thermistor measures temperature, a transformer changes voltage levels, and a general synchro system indicates angular displacement between shafts rather than specifically conveying how much the frequencies differ.