MM0704, Lesson 3
are similar to the voltages that would be generated by three single-phase AC generators whose voltages are out of phase
by angles of 120. The three phases are independent of each other.
Y Connection. Rather than have six leads come out of the three-phase AC generator one of the leads from each phase
may be connected to form a common junction. The stator is then Y connected. The common lead may not be brought
out of the machine. If it is brought out, it is called the neutral lead. The simplified schematic (fig 3-14B) shows a Y-
connected stator with the common lead not brought out. Each load is connected across two phases in series. Thus, Rab
is connected across phases A and B in series, and Rbc is connected across phases B and C in series. Thus, the voltage
across each load is larger than the voltage across a single phase. In a Y-connected AC generator, the three start ends of
each single-phase winding are connected together to a common neutral point, and the opposite, or finished, ends are
connected to the line terminals A, B, and C. These letters are always used to designate the three phases of a three-
phase system or the three line wires to which the AC generator phases connect.
Delta Connection. Another configuration for the stator of a three-phase AC generator is the delta connection. The
delta-connected stator is shown in figure 3-14C. Each load is connected across one phase in the delta connection, while
in the Y connection each load is across two phases. For this reason, the delta connection delivers less voltage to the
load than does the Y connection.
Figure 3-14. Three-Phase AC Generator.
Frequency. The frequency of the AC generator voltage depends upon the speed of rotation of the rotor and the number
of poles. The faster the speed, the higher the frequency; the lower the speed, the lower the frequency. The more poles
there are on the rotor, the higher the frequency for a given speed. When a rotor has rotated through an angle such that
two adjacent rotor poles (a north and a south pole) have passed one winding, the voltage induced in that winding will
have varied through one complete cycle.
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