to rotate in a direction that three-phase voltages are

generated in the following order: Ea, Eb, and Ec.

The voltage in phase b, or Eb, lags the voltage in

phase a, or Ea, by 120°. Likewise, Ec lags Eb by 120°,

and Ea lags Ec by 120°. In figure 3-26, the arrows, Ea,

Eb, and Ec, represent the positive direction of generated

voltage in the wye-connected ac generator. The arrows,

I1, I2, and I3, represent the positive direction of phase

and line currents supplied to balance unit power-factor

loads connected in wye. The three voltmeters

connected between lines 1 and 2, 2 and 3, and 3 and 1

indicate effective values of line voltage. The line

voltage is greater than the voltage of a phase in the

w ye-connected circuit because there are two phases

connected in series between each pair of line leads, and

their voltages combine. However, line voltage is not

twice the value of phase voltage because the phase

can remember this by the memory hint ELI **--voltage**

voltages are out of phase with each other.

is leading current, the vector diagram shows voltage in

The relationship between the phase and the line

a counterclockwise or positive direction from current.

voltages is shown in the vector diagram (fig. 3-27).

Effective values of phase voltage are indicated by

Now, look at figure 3-25, which shows the current

vectors Ea, Eb, and Ec. Effective values of line and

(I) leading the voltage (E) by degrees. Using the

phase current are indicated by vectors Ia, Ib, and Ic.

memory hint ICE-- **current leads voltage in a**

Because there is only one path for the current between

any given phase and the line lead to which it is

Since current leads voltage by degrees, the vector

connected, the phase current is equal to the line current.

representing voltage is in a clockwise or negative

The respective phase currents have equal values because

direction from the vector representing current.

the load is assumed to be balanced. For the same reason,

the respective line currents have equal values. When the

load has unity power factor, the phase currents are in

The phase relationships in a three- wire, three-phase,

phase with their respective phase voltages.

wye-connected system are shown in figure 3-26. In

constructing vector diagrams of three-phase circuits, a

In combining ac voltages, it is important to know

counterclockwise rotation is assumed in order to

the direction in which the positive maximum values of

maintain the correct phase relation between line

the voltages act in the circuit as well as the magnitudes

voltages and currents. Thus, the ac generator is assumed

of the voltages. **(NOTE: **As you read this section, look