Which statement correctly describes AC peak, RMS, and frequency and their relevance to motors?

• A. Peak is the maximum instantaneous voltage; RMS is the effective heating value; Frequency is cycles per second; Motors respond to RMS voltage.
• B. Peak is the RMS value; Frequency is cycles per minute; Motors respond to peak voltage.
• C. Peak is the maximum instantaneous voltage; RMS is the heating value; Frequency has no effect on motors.
• D. Frequency is always 60 Hz; RMS is greater than peak; Motors respond to peak current.

Answer: (A)

AC power is described by peak, RMS, and frequency. Peak is the maximum instantaneous voltage of the sine wave. RMS is the effective voltage that determines heating and actual power delivered; for a sine wave, Vrms = Vpeak/√2, so the RMS value is lower than the peak and what the motor windings “feel” in terms of heating. Frequency, measured in cycles per second (hertz), sets how fast the magnetic field rotates and, for induction motors, largely sets the motor speed (synchronous speed roughly equals 120f divided by the number of poles). Motors are rated and operated based on RMS voltage and supply frequency, which is why the RMS value is the relevant quantity.

That makes the statement correct: peak is the maximum instantaneous voltage; RMS is the heating value; frequency is cycles per second; motors respond to RMS voltage. The other descriptions mix up these definitions or misstate how motors respond or what frequency means.