Which description matches a three-phase AC tachometer's output signal?

Which description matches a three-phase AC tachometer's output signal?

• A. A DC voltage proportional to RPM
• B. An AC signal whose frequency is proportional to RPM
• C. A variable resistance
• D. A digital pulse train

Answer: (B)

The main idea is that this tachometer uses frequency to indicate speed. A three-phase AC tachometer generates an AC waveform as the shaft turns, and the faster the shaft spins, the higher the frequency of that AC signal. The RPM is read by measuring how often the electrical cycles occur per second, so the output is an AC signal with a frequency proportional to RPM. A DC voltage, a variable resistance, or a digital pulse train describe other signaling methods that aren’t how a three-phase AC tachometer reports speed.

The main idea is that this tachometer uses frequency to indicate speed. A three-phase AC tachometer generates an AC waveform as the shaft turns, and the faster the shaft spins, the higher the frequency of that AC signal. The RPM is read by measuring how often the electrical cycles occur per second, so the output is an AC signal with a frequency proportional to RPM. A DC voltage, a variable resistance, or a digital pulse train describe other signaling methods that aren’t how a three-phase AC tachometer reports speed.