What is measured to indicate the engine RPM with a three-phase AC electric tachometer?

• A. Frequency of the AC produced by the tachometer generator
• B. Voltage of the generator
• C. Amplitude of the AC signal
• D. Phase angle

Answer: (A)

Engine speed shows up as how fast the tach generator’s electrical cycles occur. In a three-phase AC tachometer, as the engine spins faster, the generator produces more cycles per second, so the output frequency increases. The instrument is calibrated so that that frequency directly translates to RPM, with the conversion depending on the generator’s pole count. Voltage, amplitude, or phase angle can vary with load, excitation, or wiring and don’t track speed consistently, so they aren’t reliable indicators of RPM. That’s why the frequency of the AC output is the correct measure.