25. At the instant the circuit is energized or de-energized.
A generator produces an a-c voltage that varies at a sine-wave rate. One revolution of the generator rotor
produces one sine wave (cycle) of voltage. When two or more sine waves are compared, to observe if they
occur at the same time, their phase relationships are being compared. Shown below is a comparison of the
a-c voltage sine wave and the a-c current sine wave in a purely resistive circuit. (A circuit with only
The two sine waves are in phase, because their maximum and minimum points occur at the same time.
When the sine waves are in phase, any change (increase or decrease) in voltage causes a corresponding
change in current.