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
upon the speed
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.