1. *Sine Waves Out of Phase*. Figure 69 shows voltage wave E1

which is considered to start at 0 (time one). As voltage wave

E1 reaches its positive peak, voltage wave E2 starts its rise

(time two). Since these voltage waves do not go through their

maximum and minimum points at the same instant of time, a PHASE

DIFFERENCE exists between the two waves. The two waves are said

to be OUT OF PHASE. For the two waves in figure 69 the phase

difference is 90.

FIGURE 69. VOLTAGE WAVES 90 OUT OF PHASE.

To further describe the phase relationship between two sine

waves, the terms LEAD and LAG are used. The amount by which one

sine wave leads or lags another sine wave is measured in

degrees. Refer again to figure 69. Observe that wave E2 starts

90 later in time than does wave E1. You can also describe this

relationship by saying that wave E1 leads wave E2 by 90, or that

wave E2 lags wave E1 by 90. Either statement is correct; it is

the phase relationship between the two sine waves that is

important.

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