What is the relationship between peak rms and average values of an ac voltage?

Peak voltage of an AC voltage is the value at its highest or lowest point.

RMS (Root Mean Square) voltage of an AC voltage is a mathematical derivation involving the square root of the average value of the squares of samples of the voltage as the sample interval approaches zero.

Average voltage is simply that - the average or mean voltage.

For a true sine wave, RMS and average are equivalent, but they are not equivalent when the wave is distorted, or has some other shape such as triangular.

RMS is the best way to measure an AC voltage, as it gives you a true reading of the amount of power that the voltage can deliver.

One issue with non-RMS AC meters is that they typically measure the rectified, filtered peak value and then compensate by dividing by 1.4. This is not correct unless the voltage is a sine wave.

The peak value of an a.c. voltage or current is the amplitude of that voltage or current waveform -i.e. the maximum value of voltage or current in either the positive or the negative sense.

The root-mean-square (rms) value of an a.c. voltage or current. For a sinusoidal waveform, the rms value is 0.707 times the peak value (amplitude). A.C. voltages or currents are always quoted in rms values unless otherwise specified.

The average value of an a.c. voltage or current is zero over one complete cycle so, when used, it applies only over one half cycle. Therefore, the average value for one-half cycle of a sine wave is 0.637 times the peak value. Average values are of little relevance to a.c. calculations.